RELATION OF THE WISSAHICKON J\1ICA GNEISS TO THE SHENANDOAH LIMESTONE AND OCTORARO SCHIST OF THE DOE RUN 

AND AVONDALE REGION, CHESTER COUNTY, PENNSYLVANIA. 

By ELEANORA F. BLrss and ANNA I. JoNAS. 

AREA DISCUSSED. southern half, the Avondale region (see fig. 5, 

The region discussed in this paper lies in p. 17), has been studied by Anna I. Jonas in 

Chester County, Pa., and is included in the order to determine the relation of the Wissaeastern half of the Coatesville quadrangle. hickon mica gneiss to the Shenandoah lime-

(See fig. 3.) It is within the belt of crystal- stone. 

FIGURE 3.-Index map showing location of Doe Run' and Avondale region, Chester County, Pa. 

line schists and gneisses . of the Piedmont 

Plateau. The northern half of the area, which 

will be called the Doe Run region, from the 

village of that name (see fig. 4, p. 15), has been 

surveyed by Eleanora F. Bliss in connection 

·with the problem of the relation of the Wissahickon mica gneiss to the Octoraro schist. The 

The field work was done during parts of the 

summers of 1906 and 1909 in connection with 

a thesis problem at Bryn Mawr College, under 

the direction of Miss Florence Bascom, professor of geology at Bryn Mawr, to whom 

thanks are due for criticism and assistance in 

the preparation of the results for publication. 

9 
10 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916. 

DESCRIPTIVE GEOLOGY. 

GENERAL RELATIONS. 

All the rocks of this region have a like geologic history. The formations consist of metamorphic rocks, ranging in age from pre-Cambrian to Ordovician, and all of them are cut 

by fresh diabase dikes of the Triassic period. 

The high degree of metamorphism shown by 

the rocks of the region is characteristic of the 

formations of the southeastern part of the 

Piedmont Plateau as contrasted with those of 

the northwestern part, where dynam,ic action 

was less effective than in the, southeast. 

The succession of formations in the Coatesville quadrangle is as follows: 

Metamorphic rocks: 

Ordovician: Octoraro schist. 

Cambrian and Ordovician: Shenandoah limestone. 

Cambrian: Chickies formation. 

Pre-Cambrian: 

Wissahickon mica gneiss. 

Baltimore gneiss. 

Igneous rocks: 

Triassic: Diabase. 

Pre-Silurian or younger: 

Pegmatite. 

Serpentine. 

Early Cambrian or older: Metagabbro. 

The Baltimore gneiss is a part of the preCambrian floor upon which the Paleozoic sediments were laid down. The Lower Cambrian 

represents coarse arenaceous sediments that 

were irregularly deposited on this floor. After 

the accumulation of these deposits the calcareous materials of the Cambrian and Ordovician periods were laid down and subsequently 

covered by clays of Ordovician age. These sediments have been altered in character, structure, 

and extent by compression, folding, uplift, and 

erosion. During Triassic time they were partly 

submerged, and gravel, sand, and clay were 

spread over the old eroded surface; these materials, in turn, have suffered partial removal 

by erosion, which cut through the Paleozoic 

rocks to the pre-Cambrian gneiss of the original floor. 

The accompanying table shows the correlation of these formations with the rocks of New 

England, ea_stern New York, Maryland, Virginia, and the southern Appalachian regwn. 

The distribution of these formations in the 

Doe Run and Avondale region is shown on 

Plate I. 

It may be of interest to note the work of 

earlier investigators in the Philadelphia region. 

Conclusions on these formations published by 

the First and Second geological surveys of 

Pennsylvania are somewhat similar to each 

other. 

H. D. Rogers,1 of the First Geological Survey, 

included all the gneisses (the Baltimore gneiss, 

the Wissahickon mica gneiss, and the granite, 

gabbro, and hornblende gneisses) in the older 

crystalline gneissic strata, which he grouped in 

three belts exposed in section on Schuylkill 

River from Grays Ferry, in Philadelphia, 

northward to Spring Mill. The first and 

second gneiss belts include the Wissahickon 

mica gneiss and an intrusive granite gneiss; the 

third belt represents the Baltimore gneiss. He 

called the Chickies formation "Primal white 

sandstone," which he made equivalent to the 

Potsdam or Upper Cambrian of New York; the 

Shenandoah limestone he called "Auroral 

magnesian limestone," · which he correlated 

. with the Chazy and Black River limestones of 

New York; tlie Octoraro schist he called 

"Primal upper slate" and placed over the 

''Primal sandstone.'' 

C. E. Hall,2 of the Second Geological Survey, 

substituted the terms" Eozoic," '.'Azoic gneiss," 

and "Laurentian syenite" for the third b1elt 

and separated the first and second belts, 

beginning at the base, into "Philadelphia 

schists and gneisses" and "Chestnut Hill 

garnetiferous schists." The quartzite he called 

"Potsdam, formation No. 1"; the Shenandoah 

limestone, "formation No. 2," described as 

representing the "Calciferous and Trenton 

groups"; the Octoraro schist is his "formation 

No. 3," described as representing the "Hudson 

River group." The Octoraro schist is described 

by T. D. Rand 3 under the term "hydro-mica 

schist." 

t Pennsylvania First Geol. Survey Rept., vol. 1, pp. 66-82, 122-183, 

1858. 

z Pennsylvania Second Geol. Survey Rept., C6, pp. 1-47, 1881. 

3 Rand, T. D., Notes on the geology of Radnor Township, in Delaware 

County, Pa., and of the townships adjacent: Pennsylvania Geol. Survey 

Ann. Rept., 1886, pt. 4, p. 1580, 1887. 
Approximau correlation of sedimentary formation& of Doe Run and Avondale region, Pennsylvania. 

Southern Appalachian region. New Jersey. 

- District of Co- Doe Run and Avondale Philadelphia region: New York OifrJ and 

Geologic age. Keith. Ho.rpers Ferry r~ion: lumbia: Keith rrn!on, Pennsylvania: Bascom (Philadelphia vicinig: Merri I (New New England : PumpeUh, Wolff, and Dale (MonoDarton (Franklin, Mon- Keith (Harpers erry (Washington B sa and Jonas (this folio). Spencer ami Kiimmel Darton and Bayley (Pas- York 'ty folio). grap 23). 

Campbell (Bristol folio). tcrcy, anti Staunton folio}. iolio). paper). Bascom (Trenton folio). (Franklin Furnace saic folio). 

Nantahala folio. Roan Mountain folio. folios). folio). 

- ------- -· 

Rowe schist. 

Athens shale. Athens shale. Martinsburg shale. Martinsburg shale. Absent. Octoraro schist. 0C"toraro Ech ist. Absent. :\hrtinsburg shale. Hudson schist. Hudson schist. Grrrclock schist. 

Be owspipe limestone. Hoosac schist. .. Ordovician. Berkshire schist. 

- - ---

Absent. Chickamauga limestone. Jacksonburg limestone. 

Knox dolomite. Knox dolomite. 

Nolichucky shale. Nolichucky shale. 

·- Shenandoah limestone. Shenandoah limestone. Absent. Shenandoah limestone. Shenandoah limestone. Shenandoah limestone. Kittatinny limestone. Absent. Stockbridge dolomite. Stockbridge limestone. 

Honaker limestone. Honaker limestone. 

Nottely quartzite. Watauga shale. Russell formation. Andrews schist. 

C'nmbrian. Murphy II.UI.l'ble. Shady limestone. Absent. 

V alleytown formation. Erwin quartzite. Absent. Antietam sandstone. 

Brnsstown schist. Absent. 

Tusquitee ~uartzite. 

Nantaha.la s ate. 

Hampton shalo. Hampton shale. Harpers shale. Absent. Chickies formation. Chickies quartzite. Chickies quartzite. Hardyston quartzite. Absent. Poughquag quartzite. Vermont quartzite. 

·-- . -- - --

Great Smoky conglomerate. Unicoi formation. Unicoi formation. Weverton sandstone. Hiwassee slate. Loudoun formation. 

U nt·utl.fonni ty 

Franklin limestone. Franklin limestone. Franklin limestone. 

. Wi.ssahickon mica gneiss. W issa.hickon mica. gneiss . 

Wissahickon mica gneiss. 

Pre-Cam brian. Carol.ina gneiss. Carolina. gneiss. Carolina. gneiss. Baltimore gneiss. Baltimore gneiss. Baltimore gneiss. Garnetiferous graphite Fordham gneiss. schist. I 

30830"-17. (To face page 10.) 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 96 PLATE I 

lj 

I 

I 

~ 

S cale ~ 

UEOLO<D C )fAP OF D )E IU 'X ~ D A\.,. !'fDA L.E REGION, lll e l • TY PA. 

LEGEND 

META M OR P HIC 

EOIMENT ARY ROCKS Z 

' r 

I t 

I 

Oc

~ to ra r o sch ist 

Shenandoah 

l imesto ne 

Ch1ck ies 

f ormat. ion 

/.INCONFORM ITY 

W•ssa hic kon 

mica gneiss 

IGNEOUS ROCKS 

~ Dia base 

[!1] s 

Serpent•ne 

[li] 

Metagabbro 

L:'_j 

~ 

}t 

~~ <1: > -o O:o 

(!) a: 

0 u o 

z lf 

j5 

z 

~ 

a: 

(!) 

~ 

<5 

I 

w 

Q: 

Q 

H 

) 

~ 

a: zw 

<{" 

- z '.:?::> 

>o 8>-

O: Q: 

00 

z 

<{ 

ii 

(OQ: 

::tw 

csg 

)cO 

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S+rike and dip of bed 

0 

Quarry 
WISSAHICKON MICA GNEISS OF DOE RUN AND AVONDALE REGION, PA. 11 

METAMORPHIC ROCKS. 

PRE-CAMBRIAN. 

BALTIMORE GNEISS. 

Distribution.-In the Doe Run and Avondale 

region the Baltimore gneiss has a wide distribution at the surface: it occurs in three areas, 

one of which extends into both parts of the 

region. The most northerly area is north of 

Chester Valley, where gneiss underlies the 

northwest corner of the region and extends 

parallel to the strike from a point about a mile 

north of Caln southwestward to the western 

boundary of the area shown on the map. The 

second area, which is somewhat ellipsoidal in 

shape with its longest diameter extending in a 

northeasterly direction, lies in the central part 

of the Doe Run and Avondale region. Its 

northeast end is half a mile north of Upland, 

and its southwest end is a mile southwest of 

Chatham. The southernmost area extends 

south of the towns of Avondale and Westgrove to the southern boundary of the quadrangle. In this southern area it has been 

found difficult to make a satisfactory lithologic 

separation between the Baltimore gneiss and 

the _\Vissahickon mica gneiss and it may be 

eventually found that some of the rock mapped 

as Baltimore gneiss may prove to be Wissahickon gneiss. 

Field relations.-The boundaries of the 

Baltimore gneiss are formed by younger formations-the pre-Cambrian Wissahickon mica 

gneiss, the Cambrian Chickies formation, or 

the Shenandoah limestone. The formation is 

penetrated by dikes of pegmatite and diabase, 

the latter intruded parallel to the strike of the 

country rock. The average strike of the 

Baltimore gneiss is N. 60° E., and the dip of the 

schistosity 30° SE. Its structure can be 

observed in stream gorges, in road cuts, and 

in the railroad cuts about Landenberg. The 

Baltim~re gneiss produces a fertile soil, though 

it is made stony by the presence of slabs of the 

gneiss, which are colored a yellowish brown by 

iron oxide. South of Avondale, owing to a 

change in the character of the gneiss, the soil 

is sandy, dark colored, and full of black biotite. 

Lithologic character.-The rock is a mediumgrained crystalline, massive or banded gneiss 

30830°-17-2 

composed of quartz, feldspar, biotite, and 

hornblende. The relative amount of each 

constituent varies widely; biotite may be 

abundant or scanty; hornblende may be present with the biotite or may replace it. The 

more or less partial segregation of the leucocratic and melanocratic constituents in alter..;. 

nating layers gives the rock the gneissic structure. In many exposures the rock is entirely 

disintegrated but has the gneissic structure still 

preserved; the kaolinized feldspar shows in 

white bands defined by the dark constituents 

of the rock. 

The Baltimore gneiss is composed of an 

igneous and a sedimentary facies which it has 

been found impossible to separate in mapping. The igneous facies occurs in the center 

of the Baltimore gneiss area north of Coatesville and in a few scattered bowlders a mile 

southeast of Springdell and about a mile south 

of Clonmell. This rock weathers into spheroidal bowlders and lacks the banding which is 

characteristic of the main mass of the Baltimore gneiss in this region. The sedimentary 

facies is a light-gray banded gneiss containing 

considerable quartz and a varying amount of 

melanocratic constituents; a micaceous variety 

occurs in which. the mica is abundantly developed along planes of schistosity. Thin sections 

of the sedimentary Baltimore gneiss show the 

accessory minerals apatite, zircon, and magnetite in slightly rounded or waterworn forms. 

Quartz in a hornstone structure emphasized 

by iron oxide stain forms a groundmass in 

which are small blades of brown biotite whose 

longer axes have a generally parallel orientation. Feldspar occurs in fresh xenomorphic 

individuals of orthoclase, microcline, and oligoclase. 

Analyses.-The following analyses were made 

on specimens of the Baltimore gneiss outside of the Avondale region-No. 1 representing samples obtained at six localities in the 

Philadelphia region and Nos. 2 and 3 the 

Baltimore gneiss of the Germantown quadrangle, where only the igneous facies is found. 

The analyses with low alumina, lime greater 

than magnesia, and soda greater than potassa 

indicate a rock of igneous character. 
12 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916. 

Analyses of Baltimore gneiss. 

SiO~- ... -------- Al2U3 ... _ ...... . Fe20 3 ••••••••••• 

FeO ............ . MgO ........... . CaO. -~ ........ . 

~:o~ _-- -_ ~ ~ ~ ~ ~ ~ ~ : : 

H20+ .......... . H~O- ......... . 

cu~---··········· Tiu2 •••••••••••• 

Zr02 ••••.••••••• 

P20s ........... . CL ....... · ...... . F ••••.......... :. s ............... . NiO ........... . MnO ........... . BaO ~ .......... . 

SrO .. -----------

1 

70.21 

13. 95 

1.05 

3.08 

1.26 

3.10 

3.27 

2.69 

.48 

.19 

.11 

. 52 

Trace. 

.10 

(b) 

(b) 

.09 

Trace. 

.11 

. 09 

Trace. 

100.30 

a Not determined. 

2 3 

72.99 63. 93 

10.90 12.02 . 55 2.40 

2.50 2.95 

1. 07 2.44 

1. 88 4.00 

3.34 3. 15 

1. 20 . 84 

1. 47 1. 30 

c 1.13 . 51 . 84 1. 04 

.. ............. --······--·· · FeS2 1. 61 Fe7SR 4. 04 . . . . . . . . . . . . Trace. . . . . . . . . . . . . Trace. ............ CuO Trace. 

99.66 98.62 

b Not estimated. 

1. Analysis by W. F. Hillebrand, U. S. Geological Survey. Bascom, Florence, Piedmont district of Pennsylvania: Geol. Soc. America Bull., vol. 16, p. 295, 1905. 

2 and 3. Analyses by F. A. Genth, jr., Pennsylvania 

Second Geol. Survey Rept. 06, pp. 116, 105, 1881. 

The following norm is obtained by recalculation of analysis 1 : 

Norm of Baltimore gneiss. 

Corundum. . . . . . . . . . 0. 20 Ilmenite............ . 1. 06 

Quartz .•••........... 30. 48 Pyrite............... . 18 

Orthoclase .......... 16. 68 Magnetite........... 1. 62 

Albite.......... . . . . . 27. 77 H20. . . . . . . . . . . . . . . . . 67 

Anorthite ........... 14. 46 002 •••••••••• ."..... • 11 

Hypersthene........ 6. 90 

Apatite............. . 34 100. 47 

The rock falls into Class I, order 4, rang 3, 

subrang 4-that is, the salic constituents are 

preponderant over the fernie, the feldspar is 

dominant over the quartz, the potassa-soda 

percentage exceeds the lime, and finally, of the 

alkalies, soda is only a little in excess of the 

potassa. The rock is a biotitic hornblende 

granoyellowstonose. 

The analysis of the Baltimore gneiss in its 

massive facies shows a rock of igneous origin. 

This rock is a remnant of the ancient central 

portion of the formation, which is found in the 

Baltimore gneiss of the area north of Coatesville. The Baltin1ore gneiss northwest of London Grove is thought to be of sedimentary 

origin, although derived from igneous material 

with little reworking. In pre-Cambrian time, 

by secular disintegration, such as takes place 

readily in a moist c\imate and in a country of 

slight topographic diversity, a soil was formed 

over the old igneous Baltimore gneiss land 

mass. By transgression of the sea the soil and 

underlying disintegrated rock were removed, reworked, and subsequently covered by other deposits. The rock formed by the consolidation 

and recrystallization of this _material resembles 

the origmal rock in its constituents but possesses 

sedimentary rather than igneous characteristics. Folding has united the original and derived rocks into a complex that is difficult to 

separate into well-defined areas of igneous and 

sedimentary rocks. A formation similarly developed among the pre-Cambrian gneisses of 

Hoosac Mountain has been described b_y Pumpelly.1 

Th~ckness.-As the Baltimore gneiss represents the remnant of the pre-Cambrian gneiss 

floor upon which sediments of later age were 

laid down, its thickness can not be estimated. 

Age and correlation.-Stratigraphic relations 

determine the pre-Cambrian age of the Baltimore gneiss; the formation is overlain by 

Cambrian sediments, from which it is everywhere separated by an unconformity. 

The gneiss has been called Baltimore because 

of its correlation with gneiss exposed at Baltimore, Md., and has been recognized as the 

partial equivalent of the Fordham gneiss of 

New York, the Stamford granite gneiss of New 

England, and the Carolina gneiss of the South. 

WISSARICXON MICA GNEISS. 

Distribution.-The Wissahickon mica gneiss 

covers the western and central portions of the 

Doe Run and Avondale region. Its northern 

boundary is the contact with the Octoraro 

schist, a line which runs northeastward from 

Timicula in a direction roughly parallel to the 

trend of the South Valley Hills. In the central 

and eastern parts of the Doe Run area of the 

formation it surrounds several bodies of limestone and quartzite, and north of Upland it 

occurs in two lenses bounded by limestone and 

quartzite. From the northwestern part of the 

Avondale district, west of the Baltimore gneiss 

and the Paleozoic formations, the Wissahickon is 

exposed continuously southward to the southern boundary of the Coatesville quadrangle. 

1 Pumpelly, Raphael, The relation of secular rock disintegration to 

certain transitional crystalline schists: Geol. Soc. America Bull., vol 2, 

pp. 209-224, 1891. 
WISSAHICKON MICA GNEISS OF DOE RUN AND AVONDALE REGIONJ PA. 13 

Field relations.-The usual strike of the mica 

gneiss lies between N. 60° E. and N. 75° E. 

The isoclinal dip of the cleavage and of the 

bedding is usually southeast, the prevailing dip 

of the rocks of the eastern Piedmont, and the 

average angle of dip is 35°. Stratification is a 

pronounced feature of the formation, and the 

strata are usually crumpled. In addition to 

cleavage planes transverse to the fine crumpling 

of the strata, the mica gneiss has four systems 

of joint planes, parallel to which the rock breaks 

readily; thus it never weathers into bowlders 

but into angular fragments, producing a loamy 

clay soil full of rock fragrnents, of mica, and, in 

many localities, of garnets. From Jennersville 

southward through Kelton to the border of the 

quadrangle and eastward to Wickerton the soil 

is red, sandy, and sparkling with muscovite. 

Although the Wissahickon gneiss crops out 

in many places, it weathers so readily as to 

afford few fresh exposures. It is generally exposed along stream beds:, in road cuts, or in 

weathered cliffs projeoting from hillsides that 

are covered by a mantle of heavy soil. It 

breaks readily into splintery fragments, which 

have been compared in appearance to halfrotted wood. 

Lithologic character.-The Wissahickon mica 

gneiss is a silvery-gray to greenish medium to 

coarse grained schistose rock characterized by 

abundant muscovite and a varying amount of 

quartz and feldspar. Muscovite is developed 

either in minute crystals or in large glistening 

crystals which give the rock a spangled appearance. Garnets are a common constituent, and 

tourmaline is abundant. 

The mica gneiss is intruded by dikes of metagabbro and pegmatite and, in some localities, 

is so thoroughly injectedl by these intrusives 

as to lose its peculiar characteristics. Where 

free from such injections the Wissahickon mica 

gneiss has the appearance: of a metamorphosed 

sediment. A change in mineral constitution 

exists parallel to and is a means of recognizing 

the primary structure or bedding; where the 

bedding planes are finely plicated conspicuous 

unplicated transverse cleavage planes render 

the rock schistose and obscure the primary 

structure. 

By microscopic study the feldspar is found 

to be orthoclase and andesine. The rock fails 

to show the sequence of erystallization of constituents which is peculiar to igneous rocks, 

and the accessory minerals, apatite and zircon, 

are present ill rounded grains-both facts that 

point to a sedimentary origin for the rock. 

The absence of pressure effects in the quartz 

and feldspar also indicates that the rock is a 

recrystallized sediment. 

This typical gneissic facies weathers to · a 

greenish-brown rock in which the micaceous 

character is very conspicuous. It disintegrates 

readily, forming an argillaceous sandy soil that 

glistens with mica and is speckled with garnets. 

With local variations this gneiss, which is well 

exposed near Springdell in a road cut, is generally characteristic of the southern half of the 

main Wissahickon area. 

Farther north, toward the schist boundary, 

the mica gneiss changes considerably in character. The fresh rock is fine grained, with 

bedding planes so thin that the rock assumes a 

schistose appearance, and the mica occurs in 

very fine crystals, evenly disseminated throughon t the rock to such an extent as to obscure 

the other constituents. It weathers to a glistening green rock of schistose character, in 

which only the mica is visible on bedding 

planes, but on planes normal to the bedding 

the quartz and feldspar can be seen in very 

fine bands or in lenses wrapped about by the 

micaceous layers. Garnets are present in some 

of the rock. The weathered facies resembles 

the Octoraro schist, and where the two are 

adjacent th~y are difficult of separation. It 

differs from the schist, however, both in the 

presence of a characteristic crumpled bedding 

and also in the mode of occurrence, for it crops 

o·ut along the roads and stream banks as high 

cliffs or strews the surface of the country in 

large slabs broken parallel to joint planes and 

contrasting with the small silvery fragments of 

the schist. A green slaty variety found at the 

northern boundary of the Wissahickon gneiss 

differs from the typical rock in its fine grain, 

extremely quartzose character, scanty feldspar, 

and abundant chlorite. 

Analyses.-The best analysis obtainable of 

the mica gneiss was made by W. F. Hillebrand, of the United States Geological Survey, from four samples obtained at different 

localities in the Philadelphia region.1 It is 

I Bascom, Florence, Piedmont district of Pennsylvania: Geol. Soc. 

America Bull., vol. 16, p. 403, 1905. 
14 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916. 

given in col~ 1 of the subjoined table. 

Analysis 2 was made by F. A. Genth, jr., from 

a specimen taken along N eshaminy Creek. 1 

Analyses of Wissahickon mica gneiss. 

Si02 ••••••••• _______________ • ____ _ 

t:~~:-: :-.~:-.·-:-.~-~-~-~- ~- ~ ~::::::::::: : : ::

MgO.·-----------------------·----

CaO ............................. . 

Na20 ..... ·---------------··--·--- K20 ............................ . . . 

H2 0+---······-···-·-·--------- · -· H 20-. -------------------------· Ti02 ••••••••••••. ___________ -----· 

PzOs··········--- ---------- ----·-- 8 ............................... . Cr02 •••••••••••••••••••••••• __ •••• 

MnO ............ -----·------··---· BaO ........ ----------------·-··--

1 

66. 13 

15. 11 

2.52 

3. 19 

2.42 

1.87 

2.71 

2.86 

1.55 

. 24 

. 82 

. 22 

.03 

None. . 20 

Trace. 

2 

56.40 

19.76 

4.35 

4.40 

3. 11 

. 09 

5.82 

1.27 

3.37 ................ 1.05 

. 37 ................. 

Trace. 

Trace . Trace. 

The analyses of the mica gneiss show such 

characters as have been described-alumina is 

high, magnesia is greater than lime, and potassa 

is greater than soda. This arkosic mud, by 

crystallization and metasomatism, has been 

metamorphosed into a mica gneiss. The feldspathic material has produced feldspar; the 

ferromagnesian material, muscovite; the sand 

has furnished the quartz, and a gneiss is the 

result. 

Its more gneissic facies is due to injection of 

material parallel to the bedding. Pressure has 

caused reorientation of the old minerals with 

their longest axes in a plane normal to the 

pressure and the production of the new minerals with similar orientation. The muscovite, 

because of its perfect basal cleavage, is the 

mineral to which is due mainly the production 

of rock cleavage and fissility. SrO .............................. . Trace. 

99.87 

Trace. Thickness.-The thickness of the W!ssa99. 99 hickon mica gneiss can not be exactly de- ___________ _:__ ___ :__ ___ termined. Its isoclinal dip with repeated 

Origin.-The Wissahickon mica gneiss is considered to be a sedimentary rock whose origin 

has been obscured by extreme metamorphism. 

The rock has an or~ginal stratification, more 

or less perfectly preserved, which is more apparent where the Cleavage, usually parallel, is 

discordant to the bedding. Other proofs of 

its sedimentary origin brought out by microscopic study have been mentioned. 

That the Wissahickon mica gneiss has been 

derived from an arkosic argillite is to be inferred from its chemical composition. In the 

belt of weathering the underground and superficial waters carry materials in solution and 

suspension, and the materials that are easily 

soluble are not likely to be present in mechanical sediments, which represent the residual 

materials after· solution has done its work more 

or less thoroughly and which therefore consist 

of members of the kaolin group, and serpentinetalc group, of quartz, aluminum oxide minerals, and acidic feldspars. An argillite, therefore, would be deficient in the alkalies and 

alkaline earths and more deficient in sodium 

and calcium than in potassium and magnesium, 

the percentage of alumina would be large, and 

after metamorphism the rock would retain its 

original chemical composition. 

I Pennsylvania Second Geol. Survey Rept. C6, p. 109,1881. 

folding gives the impression of great thickness, 

but the actual thickness is difficult to estimate 

because the formation is an ancient preCambrian gneiss bounded above by an unconformity. Its estimated thickness has been 

placed between 1,000 and 2,000 feet. 

Age and correlation.-In the absence of fossil 

content the determination of the age of the 

Wissahickon gneiss must depend upon the 

stratigraphic relations, which have been much 

obscured in the eastern Piedmont region; these 

relations, which are discussed at length elsewhere (pp. 31-3'4), point to a pre-Cambrian age 

for the Wissahickon gneiss of the Doe Run and 

Avondale region. The Wissahickon mica gneiss 

is correlated with portions of the Carolina 

gneiSS. 

Name.-The name Wissahickon was given to 

this formation on account of its fine exposures 

along the banks of Wissahickon Creek, near 

Philadelphia. 

CAMBRIAN. 

CRICKIES FORMATION. 

Distribution.-The Chickies formation, of 

which quartzite is the most conspicuous member, associated with gneissic and schistose 

bands, forms the steep ridge of the North 

Valley Hills, extending across the north-
WISSAHICKON MICA GNEISS OF DOE RUN AND AVONDALE REGION, PA. 15 

western part of the Doe Run district (fig. 4) in 

a straight band of almost uniform width, 

whose general direction is parallel to the trend 

of Chester Valley. On the south the quartzite 

0 

Contour interval 20 f'eet 

Patum is mean sea level 

gneiss is brought to the surface north of 

Coatesville. 

In the neighborhood of Doe Run the quartzite comes to the surface as a band of varying 

LEGEND 

METAMORPHIC 

SEDIMENTARY ROCKS 

..S:c( 

zi} 

::5> 

} 

~z 

Shenandoah ffi g Jimestone ~ a:: 

} 

i<~o Chicki.es ""' 

£ormation 

UNCONFORMITY 

I GNE.OUS ROCKS 

1/71 

Diabase 

II II 

Metagabbro 

Thrust 

~ fault 

,<;.Wo I 

Strike and dip of bed 

~ Quarry 

}j 

~0:: CDLIJ 

:::EO 

..s:...J 

uo 

>-a:: ...JO 

r:r. 

< 

.w 

FIGURE 4.-Geologic map of Doe Run valley, Chester County, Pa. 

passes conformably under the Shenandoah 

limestone of Chester Valley; on the north it 

overlies the Baltimore gneiss. The limestone 

and the quartzite together form the southern 

limb of an anticline whose core of pre-Cambrian 

thickness which overlies the northeastern, 

northern, western, and southern flanks of the 

Baltimore gneiss. 

North of Avondale and Toughkenamon another area of the Chickies formation is bounded 
16 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916. 

by limestone. The formation also forms a 

ridge north of W estgrove. 

Field relations.-The hardness and chemical 

stability of the Chickies formation make its 

outcrop a prominent topographic feature. 

The rock is well exposed in several quarries in 

the North Valley Hills; on the road going 

southwest from London Grove, where two 

quarries are cut in the northern face of the 

hill; and in the quarries of the hill north of 

the State road from A von dale to Toughkenamon. The general strike of the formation is 

N. 60° E. and the dips are in all observed exposures to the southeast with an average inclination of 20°. At the contacts of quartzite 

and limestone the limestone appears to underlie the quartzite, a relation which is the result 

of overturned folding. 

All these facies occur in the Chickies formation 

as exposed on the western and southern flanks 

of the Baltimore gneiss anticline. The Chickies northeast of Avondale and W estgrove is the 

mica gneiss facies, in which abundant mica is 

disseminated throughout the rock. 

Microscopic study shows that the feldspar 

is largely microcline, but orthoclase, andesine, 

and bytownite are also present. Magnetite, 

zircon, pyrite, titanite, perofskite, and tourmaline are accessory constituents. 

Analysis.-The analysis of a specimen of 

quartzite from the ridge south of Langhorne, 

Pa., is given below. 1 It shows high silica content and sufficient alumina and potassa to form 

muscovite and feldspar. 

Analysis of quartzite of Chickiesformation. 

Lithologic character.-The Chickies formation Si02.............. 87.87 

is composed of quartzite, mica schist, and Al20a. · · · · · · · · · · · · 6· 61 

mica gneiss. The quartzite is a cream-white ~~g~ ~ ~: ~ ~ _· ~:::::: rr:~c~~ to gray fine-grained crystalline thin-bedded cao.............. . 24 

Ignition ........ . .. . 

Ti02 ••••••••••••• • 

P205 .. ············ 

1.20 

. 38 

. 06 

M:nO .. . .......... . . 13 

rock. Sericite is developed parallel to the Na20.. . .......... .19 100.80 

bedding planes and causes the rock to split K20. · · . · · ..... · · · · 1. 73 

easily into flat slabs. The mica gneiss member Thickness.-The close isoclinal character of 

of the formation is a gray fine-grained crystal- the folding makes the determination of the 

line, slightly banded gneiss, whose constituents thickness of this formatio~ difficult. Its estiare quartz, biotite, and muscovite abundantly mated thickness in Chester Valley is 1,300 feet,Z 

present and feldspar sparingly so. but in the Doe Run district a thickness of 1,500 

With an increase in the amount of mica the feet seems probable. Thesectionexposedon the 

rock becomes a mica schist, a pinkish or grayish- DoeRunroad represents adepositof about 1,000 

green rock in which small sparkling plates of feet. (See Pl. II, B.) Within the Avondale 

mica produce a frosted effect. This Cambrian area (fig. 5) the thickness of the Chickies formamica schist resembles the Wissahickon mica tion varies from 700 to 800 feet. The variabilgneiss, but the mica in the schist is usually ity in thickness of the formation in the region 

present in smaller flakes than it is in the gneiss, south of Chester Valley is presumably due to 

and the Cambrian rock, in distinction from the· original irregularity of deposition in the estuWissahickon, has straight bedding planes. aries of the Cambrian sea. 

The two rocks. are further distinguished_ b_y Age and correlation.-The quartzite of the 

structural r_ela~wns and by a c~aractensti,~ I North Valley Hills forms a continuous band 

aspect that Is difficult to define. Stretched which extends from the western edge of Bucks 

tourmaline-th~t is,_ broken _and linearly .dis- County to the vicinity of Gap, in Lancaster 

placed tourm~lin~-Is foun~ 1n all three mem- County. Borings of Scolithus linearis have 

bers of the C~1Ck1es formatiOn. . . been found at several places in the quartzite 

The quartz:te of the North Val:ey Hills 1~ a of the North Valley Hills, which is in direct 

pure hard white _compact fine-grained, heaVIly stratigraphic continuity with the quartzite 

bedded rock, With abundant small flakes of near Coatesville. In 1896 Walcott s deterglistening sericite developed along the bedding planes. In the neighborhood of Doe Run 

the Cambrian formation is represented by 

the quartzose and schistose members. Near 

Springdell it is represented by the mica gneiss. 

1 Pennsylvania Rept. Second Geol. Survey C6, p. 117, 1881. Analysis by F. A. Genth, jr. 

2 Bascom, Florence, U.S. Geol. Survey Geol. Atlas, Philadelphia folio 

(No. 162), p. 4, 1909. 

a walcott, C. D., The Cambdan rocks of Pennsylvania: U.S. Geol. 

Survey Bull. 134, p. 17, 1896. 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 98 PLATE II 

A. QUARRY IN SHENANDOAH LIMESTONE, H:l MILES SOUTHEAST OF 

DOE RUN, CHESTER COUNTY, PA. 

B. SECONDARY FOLDS ON LIMB OF ANTICLINE IN CHICKIES FORMATION 

ON DOE RUN ROAD, CHESTER COUNTY, PA. 

WISSAHICKON MICA GNEISS. OF DOE RUN AND AVONDALE REGION, PA. 17 

mined the position of the Scolithus-bearing 

quartzite as underlying shales and calcareous 

sandstone which contain fragments of Olenellus. 

This determination establishes the age of the 

Scolithus-bearing rock as Lower Cambrian. 

The rock south of the North Valley Hills does 

Name.-The name Chickies (formerly spelled 

Chikis) was first used to describe the rock exposed on Susquehanna River near Columbia, 

Lancaster County. Later the spelling was determined as Chickies by the United States 

Geographic Board. 

Contour interval 20 teet 

IJatum is meaTl sea level 

CAMBRIAN AND 

~ 

ORDOVICIAN 

tm 

Shenandoah 

limestone 

LEGEND METAMORPHIC SEDIMENTARY ROCKS 

~~ 

CAMBRIAN ~ 

Cbickies 

~I v £ormation. ~ 

Tlu--ust fau1t 

PRE- CAMBRI A.N 

I ~350 I 

Strike and dip of bed 

IGNEOUS ROCKS 

ORDOVICIAN 

~ 

OR YOUNGER 

Pegmatite 

~ 

0 

Quarry 

EARLY CAMBRIAN 

~ 

OR OLDER 

Metagabbro 

~ 

FIGURE 5.-Geologic map of Avondale Valley, Chester County, Pa. 

not contain Scolithus linearis, but from its 

lithologic character and structural relations 

may be correlated with the Chickies quartzite 

of the North Valley Hills. The formation has 

been correlated with the Cheshire ("Poughquag") quartzite of New York, the Hardyston quartzite of New Jersey, and the Setters 

quartzite of Maryland. 

CAMBRIAN AND ORDOVICIAN. 

SHENANDOAH LIMESTONE. 

Distribution and field relations.-In the 

northern part of the Doe Run district (see 

Pl. I, p. 10) the outcrop of Shenandoah limestone corresponds perfectly to the floor of 

Chester Valley, forming a narrow belt, which 
18 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, ]916. 

varies in width from one-fourth to threefourths of a mile. On the north side of the 

valley the limestone overlies the quartzite, 

which is exposed on the southern limb of an 

anticline. Toward the south the limestone 

passes conformably under the Octoraro schist 

of the South Valley Hills. 

In the central and southern parts of the 

Doe Run district limestone occurs in four distinct areas of irregular conformation. The 

largest area extends from Buck Run almost 

due south to Doe Run village, where the outcrop is pierced by an anticlinal hill of resistant 

quartzite. From Doe Run southwest to Green 

Lawn there is a continuous outcrop of limestone, and from Springdell directly south 

limestone joining the western flank of the 

quartzite occurs in a long valley that extends 

to the southern edge of the Doe Run district. 

On the northern and northeastern boundary 

of the quartzite the limestone occurs in two 

small but distinct outcrops at Guest's and 

Logan's quarries. (See Pl. II, A.) The fourth 

occurrence is a narrow and irregular outcrop 

along a tributary of Brandywine Creek, about 

2. miles northeast of Logan's quarry and 1 

mile north of Upland. 

Except where it borders the quartzite the 

limestone of these areas is surrounded by the 

Wissahickon mica gneiss, thereby producing a 

radial and finger-like arrangement of the surface outcrops which is particularly striking. 

(See fig. 4.) 

In the A von dale district (see fig. 5) the 

limestone occupies the valley of White Clay 

Creek about A von dale and to the north, and 

the valley of West Branch of White Clay Creek 

and its tributaries near Landenberg. The 

areas are not more than half a mile wide and 

are very irregular. The northern area extends southwestward for about 5 miles from 

the center of the eastern border of the Coatesville quadrangle; then it turns south and 

curves. about a hill of mica gneiss which lies 

north of Westgrove; from that point it extends southward through Baker and southeastward to Avondale, where it connects with 

two belts of the formation-one from the east 

along Trout Run and the other from the north. 

(See fig. 5.) About a mile south of Landenberg there is a small area of limestone extending southwestward along Broad Run and 

aoross White Clay Creek almost to West 

Branch. It is not more than a mile wide anywhere and does not appear at the surface except at Nevin's quarries. 

As there are no natural outcrops of the limestone, its extent is best determined by the soil 

it makes-a red clay such as is formed by the 

weathering of a magnesian lime rock. It is 

exposed for the most part in abandoned quarries which are partly filled with water; the 

rock forming the walls may be fresh or partly 

disintegrated into calcareous sand or almost 

entirely covered by a talus of soil. In the 

region of Baker and Avondale there are large 

quarries which are now being operated in this 

rock and which furnish valuable marble and 

.building stone. 

The strike of the formation ranges between 

N. 60° E. and N. 70° E. The direction of dip is 

generally southeast, the only prominent exception being a northwest dip found in a small 

quarry just north of Westgrove. The angle of 

dip is low, varying from 20° to 25°. 

Lithologic character.-Three varieties of the 

Shenandoah limestone are recognizable in the 

Doe Run and Avondale region. One occurs in 

Chester Valley, where the fresh rock is a massive, highly crystalline blue magnesian limestone, which breaks readily along three joint 

planes. Few accessory minerals are found in 

this massive variety. The upper member of 

the formation in this area is a highly micaceous and thinly bedded schistose rock, in 

which quartz and mica are so abundant that 

the rock may be called a calcareous schist. 

This member grades into the overlying Octoraro schist. 

The third variety occurs in the central belt 

of limestone, where it is well exposed in Guest's 

and Logan's quarries, and in the limestone of 

the Avondale district. Where pure the fresh 

rock is a medium-grained, ·highly crystalline 

lustrous white saccharoidal marble, characterized by the abundant development of various 

accessory minerals. Phlogopite, in glistening 

amber-brown scales, is in some places so plentiful that, on planes parallel to the bedding, the 

rock may appear to be composed almost entirely of mica. Where the phlogopite occurs in 

smaller flakes it gives the rock a variegated 

appearance, which has won for it the name 

"bastard granite." Biotite, muscovite, tour-
WISSAHICKON MICA GNEISS OF DOE RUN AND AVONDALE REGION, PA. 19 

maline in large black crystals, magnetite, 

apatite, and pyrite are common accessory 

• minerals. 

Evidences of pressure seen in the thin section are twinning of the calcite crystals, granulation, and undulatory extinction of the quartz, 

which occurs in interlocking areas forming a 

mosaic with- the calcite. The dolomite of 

Chester Valley is finer grained than the lime"" 

stone of the southern area, which in Guest's 

quarry is almost pure c~lcite. 

Analyses.-Some analyses of the limestone 

are giv9n below: 

Analyses of Shenandoah limestone . 

1 2 3 4 

--------1---------- --

Ca003 •••••..• • •..••• 54. 750 40. 27 54. 071 

Mg003 ••••••••••••••• 44. 204 31. 24 43. 309 

~~~o-3·--_-_-_-_-_ ~::::::::: }----~~;- { 2i: ~~ } . 621 

Al20 3 •••••••••••• • _ • • • 1. 12 

57.125 

12.498 

1.235 

3. 770 

11g0--------- ----- - - - - - - - - - - - . 11 ---- - - . - - - - - - - - - CaO ............ -. . . . . . . . . . . . . 55 ........ . ...... . 

~6~_-_:_·_-_-_-_-_::: :: :::: ::::::::::::::::::::::::} 25.372 

s- ------------------ . 011 -- - - - - - - - 013 - - - - - - -- p- -- - ---- -- --------- . 010 - - - - - - - - . 003 - - - - - - - - Alkalies._ ... _ . ...... _ . . _. . . . . 1. 42 . _ . _ ........... _ 

Insoluble............. . 436 1. 950 

99. 928 100.00 99.959 100.000 

1. Haldeman South quarry, near Chickies. Best blue 

limestone, fine grained, hard. Analyst, A. S. McCreath, 

Pennsylvania Second Geol. Survey Rept. CO, page 271, 

1874. 

2. West Conshohocken, Montgomery County, Pa. Analyst, F. A. Genth, jr., Pennsylvania Second Geol. Surv.ey 

Rept. C6, page 126, 1881. 

3. Benjamin Swayne quarry, 2 miles north of Avondale. 

Analyst, F. A. Geuth, jr., idem, page 127. 

4. Pennsylvania Marble & Granite Co.'s quarry, Baker. 

Analysts, Booth, Garrett & Blair, Philadelphia. 

Thickness.-In the Avondale district the 

thickness of the limestone, which does not exceed 1,000 feet, represents only part of the 

original thickness; the remainder has been removed by erosion. In Chester Valley the formation is probably not over 1,500 feet thick. 

Age and correlation.-In the central p,art of 

the region, at Guest's quarry, and in the northern part, in Chester Valley, the limestone conformably overlies the Chickies formation. On 

account of the highly metamorphosed character of the rock, fossils are rare in the Shenandoah limestone and none have been found in 

the Doe Run and A von dale region, but at 

Henderson station, Montgomery County, Pa., 

fossils of Ordovician age have been found in 

the limestone of Chester Valley.1 

That the limestone of the Doe Run and 

Avondale region is to be correlated with the 

Cambrian and Ordovician Shenandoah limestone of Montgomery and Delaware counties is 

proved by the stratigraphic continuity of the 

limestone in Chester Valley, by marked lithologic resemblance between the limestone of the 

Doe Run and A von dale region and the rock in 

Chester Valley, and by the position of the 

limestone south of Chester Valley in approxiInately the same line of strike as the narrow 

belt of Shenandoah limestone in Cream Valley 

near Conshohocken, Pa. 

Name.-The name Shenandoah was adopted 

by Darton 2 in 1892 to designate the limestone 

formation of Cambrian and Ordovician age 

which occurs in the Appalachian region of Virginia. Subsequently the United -States Geological Survey accepted the name because of its 

priority and applied it to the "Chester Valley" 

limestone and to the outcrops southeast of 

Chester Valley which had been correlated with 

the Shenandoah. 

ORDOVICIAN. 

OCTORARO SCHIST. 

Distribut'ion and field relations.-The north 

face of the South Valley Hills forms the northern boundary of the outcrop of the Octoraro 

schist; the southern boundary is a line passing 

about half a mile north of the village of Timicula, through Ercildoun, a quarter of a mile 

west of Y oungsburg, north to Modena, where 

it runs in a southeasterly direction to the eastern boundary of the Coatesville quadrangle. 

The schist conformably overlies the Shenandoah limestone on the north, grading imperceptibly into the limestone through a calcareous lower n1ember; on the south it. adjoins the 

Wissahickon mica gneiss, the line of contact 

representing a fault which extends across the 

country in a roughly northeast direction. The 

average strike of the formation js N. 65° E. 

and the dips are about 60° SE. 

1 Bascom, Florence, U . S. Geol. Survey Geol. Atlas, Philadelphia 

folio (No. 162), p. 5, 1909. 

2 Darton, N. H., U. S. Geol. Survey Geol. Atlas, Staunton folio 

(No. 14), 1894. 
20 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916. 

In topographic features the country underlain by the Octoraro schist is very similar to 

that formed by the erosion of the Wissahickon 

mica gneiss. The schist, like the gneiss, yields 

readily to the mechanical processes of weathering and forms a deep micaceous soil, which 

differs from the gneiss soil in that it does not 

contain individual flakes of mica, but rather 

small fragments of the rock that have not been 

disintegrated into the individual constituents. 

The soil produced by the disintegration of 

the schist is dull and sandy, nongarnetiferous, and full of green, shiny fragments that 

have a soapy feel, but the gneiss soil is glistening and sparkling with large flakes of mica, 

abundantly garnetiferous, and less likely to be 

red. Another marked difference in the two formations is the method of disintegration. The 

Octoraro schist does not have the three wellmarked joint planes which cause the Wissahickon gneiss to break readily into parallelepipeds; but, having an easy cleavage along the 

planes of schistosity, it splits into thin laminated fragments. 

Lithologic character.-The Octoraro schist 

may be described as a quartz-muscovite schist 

or a quartz-chlorite schist, in which the mineral constituents are so minute that on the 

cleavage planes no one mineral is distinguishable. The fresh rock is light silvery gray 

and very schistose in character and is com~ 

posed of quartz and mica or chlorite. The 

quartz, which is visible only on the edge transverse to the cleavage plane, occurs in lenses 

completely wrapped around by the micaceous 

layers; on cleavage planes mica alone is visible 

in microscopic flakes that are indistinguishable 

in the hand specimen. These micaceous constituents form a surface with a smooth "feel," 

which has been described as "soapy." The 

color varies from gray in the fresh rock to 

green or yellow in the weathered rock. 

In the thin section the schistose character 

of the rock is very apparent. Small grains of 

quartz, showing hornstone texture, form continuous bands throughout the rock or occur 

in lenses enveloped by twisted fibrous muscovite or muscovite and chlorite; muscovite may 

be present in fine blades and chlorite may be 

almost entirely absent, or muscovite and 

chloritized biotite may occur together, associated with secondary quartz, hematite, and 

magnetite. The accessory minerals are pyrite, 

magnetite, perofskite, titanite, rutile, and 

apatite. 

The sedimentary origin of the schist is shown 

both in its field relations and in its composition. It is the upper member of a conformable 

series of sandstone, limestone, and shale, and 

has the restricted mineral content and hornstone texture of a sedimentary deposit that 

has undergone metamorphism. 

Analysis.-The composition of the schist is 

shown by the following analysis: 

Analysis of Octoraro schist. 

Si02 •••••••••••••••• 

Al20 3 ••••••••••••••• 

Fe20 3 ••••••••••••••• 

FeO .............. . 

MgO (deficient) .... . 

N a20 (deficient) ... . 

K 20 .............. . 

39. 35 Ignition. . . . . . . . . . . 6. 05 

31. 92 Ti02 ••••••••••••• • • • 1. 20 

2. 19 P20 5 •••••••••••••• ,. • 49 

9. 00 NiO (trace of cobalt) . 06 

3.08 

1.98 100.58 

5.26 

Mica schist, 1,222 feet from Bird in Hand Tavern, on 

road from Gulf Mills to Bryn Mawr, Montgomery County, 

Pa. Pennsylvania Second Geol. Survey Rept. 06, p. 133, 

1881. 

The small content of the alkalies and alkaline 

earths relative to the silica and alumina, the 

absence of lime, and the preponderance of potash over soda in the analysis point to a sedimentary origin of the formation in which the 

more readily soluble substances, such as lime, 

soda, and the alkalies, have been removed in 

solution. 

Thickness.-In the syncline of the South 

Valley Hills · the thickness . of the Octoraro 

schist seems to be about 2,000 feet. 

Age and correlation.-The Octoraro schist 

conformably overlies the Shenandoah limestone, grading down into the limestone through 

a calcareous schist, and is therefore of Ordovician age. It has been correlated with the 

Berkshire and Hoosac schists of New England. 

Name.-The name Octoraro has been given 

to the schist because it is particularly well 

exposed along the banks of Octoraro Creek, 

between Lancaster and Chester counties. 

IGNEOUS ROCKS. 

EARLY CAMBRIAN OR OLDER. 

METAGABBRO. 

Distribution.-About a mile northwest of 

Coatesville the Baltimore gneiss is penetrated 

by two intrusive gabbro masses. Only the 

southern extremity of one intrusion is visible 
WISSAHICKON MICA GNEISS OF DOE RUN AND AVONDALE REGION, PA. 21 

in the region under discussion; the other mass I gabbro. It is at present a metagabbro or, beextends eastward from the western boundary cause of its secondary banding, a hornblende 

of the region mapped (Pl. I) to Valley station, gneiss. 

on Rock Run. A small dike occurs on the Analysis.-The following analysis 1 repreeast side of Rock Run. sents a dike of similar metagabbro which ocIn the southern part of the Doe Run district curs a mile north of Bryn Mawr station: 

and throughout the Avondale district metagabbro occurs in narrow dikes in the Baltimore and Wissahickon gneisses and the Chickies formation. The outcrops of the dikes north 

of Avondale range in width from a few feet to 

half a mile. About Wickerton, New Garden, 

and ~andenberg the dikes are of considerable 

size. South of Landenberg a series of narrow 

dikes, roughly parallel in direction, may be 

traced for 5 or 6 miles southwestward to the 

southern edge of the Coatesville quadrangle. 

Field relations.-The dikes of hornblende 

gneiss cut the country rock with varying strike, 

and are in places weath~red into a soft darkgreen rock in which hornblende may still be 

distinguished. 

Lithologic character.-The rock of the two 

areas northwest of Coatesville is a holocrystalline, granular, or only slightly gneissic medium 

Analysis of metagabbro. 

Si02 •••••••.••••••• 

Al20 3 •••••••••••••• 

Fe20 3 •••••••••••••• 

FeO .......... ... .. . 

MgO .............. . 

CaO .............. . 

Na20 ......... . ... . 

K20 ............... . 

H2 0+ ............. . 

H 20-............. . 

C02 ••••••• __ •• __ • _. 

Ti02 ••••••••••••••• 

48. 68 Zr02. . . . . . . Not estimated. 

14.39 P20 5 ••••••••••••••• 0.29 

4. 00 CL . . . . . . . . . Not estimated. 

10. 09 S ...............•... Trace. 

6.32 Cr20 3 ••••••••••••••• None. 

9. 23 NiO ............... Trace. 

2. 31 MnO..... . . . . . . . . . . . 22 

. 47 BaO .......... Faint trace. 

2. 03 SrO .......... ------ None. 

.46 Li20 ........ . ...... Trace. 

None. 

1.69 100. 18 

The analysis gives the following norm: 

Norm of metagabbro. 

Quartz............. 2. 82 Magnetite .......... . 

Orthoclase.... . . . . . . 2. 78 Ilmenite .......... . 

Albite............. 19. 39 Apatite ............ . 

Anorthite ........... 27.52 Water ............. . 

5.80 

3. 19 

. 67 

2.49 

to fine grained gabbro. The only constituents Diopside ........... · 13.67 

· 'bl th k d 1 · 1 d k Hypersthene....... . 21. 78 

100. 11 

v1s1 e to e na e eye are p agwc ase, a ar -

green pyroxene, and brown hornblende in slender crystals. The metagabbro is a fine-grained 

dark-colored rock, for the most part green to 

greenish black, with leucocratic constituents 

mottling it. Lustrous hornblende, the most 

abundant and conspicuous constituent, has a 

parallel arrangement and thus gives the rock a 

banded appearance. The feldspar occurs in 

minute grains of a dull chalky aspect. In thin 

section it is found that this aspect of the feldspar is due to saussuritization, which obscures 

the original character of the feldspar. The 

character of the extinction angle indicates that 

the feldspar is labradorite-bytownite. Hornblende has been formed by uralitizationthat is, by the alteration of the original augite 

into hornblende. A core of augite is pres@nt 

in the interior of some of the hornblende crystals, where its pale-green to colorless character, 

its high index of refraction, and its occasional 

basal sections with rectangular cleavage separate it sharply and conspicuously from the surrounding hornblende. The gneiss, with its 

basic feldspar, uralitized augite, and accessory 

The rock falls in Class III, order 5, rang 4, 

and subrang 3 and is a hornblende granoauvergnose; it is perfelic, docalcic, and presodic. The compound prefix is gl.yen because 

hornblende is the only abnormative mineral 

and because the texture of the rook is hypautomorphic granular. 

Origin.-There can be no question of the 

igneous origin of the metagabbro, for the field 

relations, mineral constituents, and chemical 

composition concur in pointing to such an 

ong1n. Since the intrusion and consolidation of the gabbro pressure has converted the 

augite into hornblende and has produced a 

rock cleavage parallel to the prevailing southeastward dip of the cleavage of the region. 

Age.-The gabbro has nowhere been found 

intruded into recognized Paleozoic sediments, 

but large n1asses of gabbro have been found in 

pre-Cambrian rocks throughout the eastern 

Piedmont belt, and it can therefore be regarded as belonging to the period of preCambrian igneous activity. The presence of a 

1 Geol. Soc. America Bull., vol.16, p. 319,1905. Analyst, W. F. Rillezircon, has the constitution of a quartz-augite brand, u.s. Geol. survey. 
22 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916. 

dike of hornblende gabbro in the Chickies formation makes it seem likely that a portion 

of the original magma which furnished the 

material for the pre-Cambrian intrusions may 

have been injected into fissures either during 

the period of Lower Cambrian deposition or 

at some later time. 

PRE-SILURIAN OR YOUNGER. 

SERPENTINE. 

Distribution and field relations.-The occurrence of serpentine in the Doe Run district is 

restricted to three small lenticular areas, 

which range in length from a . quarter of a 

mile to half a mile, and are about one-eighth 

of a mile in width. Two of these areas are 

situated near Laurel, on the West Branch of 

Brandywine Creek, and the third is about a 

mile southeast of Goosetown. The outcrops 

are nowhere prominent, being confined to 

cuttings in the hillsides near Laurel and to a 

few scattered bowlders in the fields near 

Goosetown. Owing to the small extent of the 

areas it is impossible to recognize the presence 

of serpentine by its characteristic thin soil and 

stunted vegetation. The lenticular shape of 

the areas and the character of the rock itself 

are evidences of its igneous origin. The 

Laurel outcrops are within the confines of the 

Wissahickon mica gneiss; the Goose town outcrop is partly surrounded by the Ordovician 

schist. In the A von dale district there is a 

small outcrop of serpentine on the Toughkenamon and London Grove road 1! miles north 

of Toughkenamon. The rock, which is exposed 

in a road cut, is only a few feet in width and 

does not reappear along the strike. The 

exposure is surrounded by Cambrian gneiss 

belonging to the Chickies formation. 

Lithologic character.-The serpentine in the 

region of Laurel is a soft green, extremely 

talcose schist, almost approaching a soapstone; in the neighborhood of Goosetown it 

is a massive green serpentine. The thin section of the Goosetown rock shows that it is 

composed mainly of serpentine; other alteration products are talc, calcite, and iron oxides. 

The rock has been so completely altered to 

serpentine that the original character is difficult to determine, but a few small grains of 

residual olivine remain, showing that the 

rock was derived from a peridotite. The rock 

north of Toughkenamon is almost a tremolite schist; it is green in color, fibrous, and 

soft. 

Age.-Throughout the eastern Piedmont 

district of Pennsylvania there are many serpentine dikes whose occurrence, except in the 

isolated areas near Goosetown and Toughkenamon, is restricted to pre-Cambrian rocks. 

In many places in the immediate vicinity of 

Philadelphia and around Chester and West 

Chester serpentine is intruded into the preCambrian gneisses, but it nowhere occurs in a 

formation later than the Wissahickon gneiss. 

Owing to the common occurrence of serpentine in pre-Cambrian material and to its 

almost total absence from Paleozoic material, 

it seems possible that the serpentine near 

Goosetown, which is a metamorphosed peridotite, belongs to the pre-Cambrian period of 

igneous activity and. that it was carried . in 

a small patch of igneous material along with 

the overthrust Wissahickon gneiss over the 

surface of the schist and left by subsequent 

erosion. The area appears to be partly 

surrounded by the Wissahickon mica gneiss, 

but the cultivation of the farm land in which 

the serpentine occurs makes it difficult to 

map the border accurately: The irregular 

character of the boundary between the Wissahickon gneiss, serpentine, and Octoraro schist 

is such as would be expected to result from 

the partial removal by erosion of the overlying rock. It is possible that the serpentine 

of the area north of Toughkenamon is a 

remnant of overthrust pre-Cambrian material 

left by erosion, or that it is a land area which 

remained an island during the whole or part 

of Cambrian and Ordovician time. 

In Syracuse, N. Y., serpentine occurs in 

the Salina formation, of Silurian age, formerly 

called "Onondaga salt group," and Williams 1 

has called attention to the fact that this 

serpentine bears a strong resemblance to the 

Ca~boniferous peridotites of Kentucky described by Diller.2 The occurrence of postCarboniferous serpentines in the Appalachian 

province suggests that the serpentine here 

discussed may possibly be of later age than 

1 Williams, G. H., On the serpentine (peridotite) occurring in the 

Onondaga salt group at Syracuse, N.Y.: Am. Jotlf. Sci., 3d ser., vol. 34~ 

p. 137, 1887. 

2 Diller, J. S., Peridotite of Elliott County, Ky.: U.S. Gool. Survey 

Bull. 38, 1887. 
WISSAHICKON MICA GNEISS OF DOE RUN AND AVONDALE REGION, PA. 23 

Ordovician and therefore intrusive in the 

Octoraro schist of the Doe Run district and 

in the Cambrian gneiss of the Avondale district. In view, however, of the absence of 

conclusive evidence showing the Paleozoic 

or post-Paleozoic age of the serpentine in the 

Goosetown and Toughkenamon areas, and 

in vi_ew of the preponderance of igneous 

material in the pre-Cambrian rocks, it seems 

impossible to determine the age of the serpentine without further discovery of its occurrence in Paleozoic material. 

PEGMATITE. 

Dist'!ibution and field relations.-P egma ti te 

occurs at eight places in the Doe Run district. 

Four dikes extend across the country in the 

neighborhood of Coatesville, striking northeast. 

Three of these dikes occur northeast of the 

town, showing themselves in bowlders sea ttered 

through the fields; the fourth, which lies directly east of Coatesville, is a seam of kaolin 

within the confines of the limestone. Of the 

four pegmatite localities in the southern part 

of the Doe Run district, three lie in a continuous line northeast of Upland. There are only 

two direct outcrops of the rock in the Doe Run 

district. One is an old spar pit on the Sharpless farm at Upland; the other, in direct line of 

strike with the first, is a dike of quartz-feldspar 

rock in Logan's limestone quarry. 

The Avondale district contains many pegmatite dikes. With the exception of an isolated outcrop north of Chatham, the areas of 

pegmatite north of Avondale may be grouped 

into five dikes, the largest of which is an eighth 

of a mile wide. The northernmost dike appears 

a mile south of London Grove, whence it may 

be traced to Baker; the other four lie to the 

southeast, each separated from the next by 

about half a mile and the southernmost cropping out just north of Avondale. Between 

Avondale and Landenberg the pegmatite dikes 

are scattered and small, but south of Landenberg, in the neighborhood of the limestone, 

four dikes cut the metagabbro, the Baltimore 

gneiss, and the Shenandoah limestone. Many 

other dikes too small to be mapped cut the 

country rock of that region. 

Lithologic character.-The pegmatite is a 

coarse-grained white to gray rock composed of 

quartz, light-gray or pink feldspar, and abundant muscovite in large plates. The feldspar 

when fresh has lustrous surfaces but when 

weathered is abundantly coated wi11oh a yellow 

iron oxide stain. The feldspar may be orthoclase with Carlsbad twinning or, rrLOre rarely, 

with twinning according to the Baveno law; or 

microcline with the gridiron texture; or oligoclase; or mieroperthite. The most prominent 

of the long list of accessory minerals associated 

with the pegmatite are iron pyrites, tourmaline, and garnet. 

The absence of banding, the eoame crystallization, and the diversity in texture and mineral 

composition are all evidences that the pegmatite is not a normal plutonic rock which has 

undergone the pressure to which the other 

rocks of the region have been subjected. On 

the other hand, the trend of the dikes, discordant to the strike of the country rock, is 

unlike that of the veins of the region. 

Origin.-The pegmatite dikes of this region 

represent the most highly differentilated phase 

or end product of an igneous magma. They 

show more igneous than aqueous ch~:LracteristiGs 

and may be explained as the result of slow 

crystallization of an igneous magma saturated 

with water. They are the youngest pre-Triassic 

igneous rocks of the region and represent the 

expiration of the igneous activity which began 

in pre-Cambrian time and which is :represented 

in the Coatesville region by the 1netagabbro 

and in the Philadelphia region by the porphyritic granite, gabbro, and ultrabasic peridotite and pyroxenite. 

The points of difference between pegmatites 

and plutonic rocks have already been mentioned. The pegmatite is coarse and irregular 

in texture and has not the uniform eonstitution 

of a granite. It is like a granite in its order of 

crystallization and like a dike in its occurrence 

in the field. 

Age.-The pegmatite has not been found in 

rocks younger than the Shenandoah limestone. 

Its intrusion was subsequent to the pressure 

that produced the thrust fault of the Wissahickon mica gneiss, and its age :may be preSilurian or younger. 

TRIASSIC. 

DIABASE. 

Distribution and field relation8.-Diabase 

occurs in five dikes in the Doe Run and Avondale region. One line runs through the Doe 
24 SHOR.TER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916. 

Run district in a southwesterly direction from 

a locality about 1! miles southeast of Goosetown to the neighborhood of Green LaWn, and 

the great trap dike of Downingtown, northeast 

of Coatesville, may be regarded as a continuation of it. West of this long dike a shorter 

one extends from Springdell to Green Lawn. 

On each side of a line drawn from Woodville 

to Chatham a dike of diabase may be traced 

southwestward for about 4 miles. A fifth dike 

near Woodville extends for about 1! miles. 

Except for a few ledges, the existence of the 

diabase is indicated only by a trail of bowlders 

across the country underlain by the rock. It 

strikes N. 45° E., and the width of the dikes 

where exposures exist is about 20 feet. 

Lithologic character.-The Downingtown dike 

is a rather coarse grained diabase, composed of 

crystals of plagioclase and green pyroxene. 

The rock of the dikes near Wood ville is typical 

fine-grained diabase, in which the only mineral 

constituent recognizable with the naked eye is 

pale-green feldspar sparkling in a dark greenish-gray groundmass. Microscopic study 

shows that the feldspar is labradorite-bytownite and occurs in automorphic lath-shaped 

crystals so arranged as to produce the ophitic 

texture characteristic of diabase. The microscope also shows pyroxene, which is pale-green 

augite and unusually fresh and which occurs 

as xenomorphic individuals within the network of feldspar. The accessory minerals are 

magnetite, epidote, quartz, biotite, apatite, 

and chlorite, with secondary ilmenite. The 

rock has a conchoidal fracture and when exposed to the atmosphere for some time weathers into spheroidal bowlders with a yellow 

exterior and a greenish-gray interior. 

Analysis.-The following analysis represents a specimen of the Conshohocken dike in 

West Conshohocken, Montgomery County, Pa.: 1 

Analysis of diabase from Conshohocken dike. 

Si02 ••••••••••••••• 

Al20 3 •••••••••••••• 

Fe20 3 •••••••••••••• 

FeO ............... . 

MgO ........ -...... . 

CaO .............. . 

N~O ............. . 

51. 36 K20... . . . . . . . . . . . . . 1. 46 

17.38 Loss................ 2.15 

6. 57 T.i02 ............... 1. 63 

3. 85 P04 ••••••••••••••• • 13 

3. 42 Li20 ............... Trace. 

10.19 

2. 19 100. 53 

1 Pennsylvania Second Geol. Survey Rept. 03; p. 275, 1880; Rept. 06, 

p. 134, 1881. Analysis by F. A. Genth, jr. 

This analysis gives the following norm: 

Norm of diabase from Conshohocken dike. 

Quartz ............ . 

Orthoclase ......... . 

Albite ............ . 

Anorthite .......... . 

Enstatite .......... . 

Wollastonite ....... . 

Apatite ............ . 

10. 14 Ilmenite........... 3. 04 

8. 90 Magnetite......... 7. 66 

18.34 Hematite........... 1. 28 

33. 08 Water........ . . . . . . 2. 15 

8.60 

6. 96 100.49 

-34 

This diabase is a dosalane which is docalcic 

and presodic-in other words, the salic constituents are dominant over the fernie and the 

lime over the alkalies. Of the alkalies soda 

preponderates. The rock is an augite ophitibandose. 

Age.-In its freshness, mineral constitution, 

and microtexture the rock found in the Doe 

Run and Avondale region resembles the Triassic diabase. It is in line of strike with the 

Downingtown dike, which has been traced 

northeast into Triassic formations and which 

extends intermittently southwest as far as 

Maryland, crossing the . Phoenixville and West 

Chester quadrangles and appearing in the 

Coatesville quadrangle as here described. 

STRUCTURAL GEOLOGY. 

STRUCTURE OF THE PIEDMONT PLATEAU. 

The crystalline rocks of the Piedmont region 

comprise closely folded and faulted Paleozoic 

and pre-Paleozoic rocks, partly concealed by a 

cover of gently folded, unmetamorphosed Mesozoic shales and sandstones, which dip gently 

northwest and are broken by a series of normal 

and reverse faults with a southeast hade. In 

strong contrast to the simple structure of the 

shales and sandstones, the belt of crystalline 

rocks that lies southeast of the Mesozoic cover 

presents a succession of close unsymmetrical 

folds, sharply overturned· toward the northwest 

and dislocated by thrust faults with a prevalent southeast hade. The folding appears to 

be isoclinal, because the limbs of the folds are 

compressed closely and the folds are usually 

overturned. They strike in a general northeasterly direction, but the axis of the major 

folds is usually inclined more or less gently, and 

this pitching of the axis shows that there has 

been gentle folding at right angles to the major 

folding. The period of stress that began in 
WISSAHICKON MICA GNEISS OF DOE RUN AND AVONDALE REGION, PA. 25 

Archean and Paleozoic time was marked by the 

gradual accumulation in the earth's crust of 

compressive forces which sought relief in two 

directions, thereby producing two systems of 

folding-one, compressed and overturned major 

folds; the other, gentle and open minor folds. 

This difference in the intensity of folding may 

be due to the fact that the pressure originating 

along the minor axis was less than that along 

the major axis; or it may be due to the greater 

resistance of the strata along the minor axis, or 

to the combination of these two causes. The 

compressed folds lie along a northeast axis, . 

producing a succession of overthrusts that have 

been gradually developed as the result of a 

major force working from a southeast direction. 

A thick limestone formation was the controlling factor in the folding of the Paleozoic sediments, and the more deeply the layer was 

buried the more readily it yielded to pressure. 

In the sharp overthrusts of the major folds, 

warping of the earth's surface produced a weakness which, by concentration along the axis 

of overthrust, has caused a repeated sliding of 

the earth's crust. In the zone of combined 

fracture and flowage, by means of lateral pressure, combined with slight overloading, the 

elasticity of the material is exceeded so that the 

rock is fractured along the limbs of the folds, 

with movement along both sides of the rupture. 

Such faults have a gentle hade in a southeasterly 

direction and along them the deeply buried 

Archean rocks were pushed up until they overlay the younger Paleozoic sediments. The 

faults strike in a general northeast direction, 

and their linear extent is extremely variable. 

The faults in the Triassic beds are normal, with 

a hade of 75° SE. to 90° and with slight displacement. 

Cleavage, fissility, and jointing are also characteristic features of the crystalline schists of 

the Piedmont Plateau. Jointing is caused by 

the yielding of the rock along shearing planes 

and is usually found in three directions nearly 

normal to one another; it is supplemented by 

the plane of cleavage and fissility, which is in 

many places almost normal to the joint planes. 

Fi.ssility is a structure resulting from heat, 

pressure, and metamorphism, by virtue of which 

rocks are separated into parallel lamin::e. In 

the gneisses and schists rearrangement and 

flattening of the old minerals and a production 

of new minerals have caused an orientation of 

the mineral particles with their longer axes 

parallel. The parallel lamin::e in crystalline 

schists are usually coincident with the bedding 

planes, differing from them only where the 

sediments are much plicated and the plane of 

fissility does not follow the close crumpling of 

the beds. 

The yielding of the rock along shearing planes 

which produces jointing, cleavage, and fissility is, 

like folding, caused by lateral compression, owing 

to a tangential thrustfrom the direction of the 

Atlantic Ocean segment which continued during a long period of time and which acted most 

strongly in a direction at right angles to the 

strike of tl:..e major folds. At the same time 

vertical movements, bringing the beds to their 

present position, probably aided the work of 

horizontal compression, which is operative 

only at great depths in the zone of flowage and 

fracture. The two components o:f the force 

are lateral and vertical thrust. 1When open 

upright folds have passed by means of lateral 

thrust into closed and compressed folds, the vertical thrust tends increasingly to overturn them. 

The uniformity of the southeast dip of the 

formations shows that the major force was 

from that direction; the minor folding, normal 

to the major, resulted from suboceanic spread 

from the direction of the Gulf of Mexico. Tangential thrust began in pre-Paleozoic time and 

continued throughout the Paleozoic era, culminating at the close of the Ca,rboniferous 

period. The belt of crystalline rocks which 

lies on the northwestern border of the Triassic 

cover presents the structural features described 

above, modified to some extent by its greater 

distance from the position of initial stress. In 

the southeastern belt an anticlinal arch brings to 

the surface the pre-Cambrian gneiss as the core of 

a great major fold. This arch, passing under the 

Triassic shales, is to some extent obscured by 

them but becomes plainly apparent along the 

northwestern boundary of the eastern belt of 

metamorphosed crystalline rocks. Farther 

southeast it passes into a synclinal trough of 

Ordovician material, of which the northwest 

li~b forms Chester Valley and the southeast 

limb underlies a faulted cover of Wissahickon 

mica gneiss. A great anticline of pre-Cambrian 

gneiss follows the syncline and is succeeded by 

several folds which finally disappear under the 
26 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916. 

Cretaceous sediments of the Coastal Plain, on 

the southeast border of the Piedmont Plateau. 

The structure of this pre-Cambrian anticline 

is much obscured by many intrusions of 

igneous material, and in Maryland the anticline 

itself is broken up by igneous material. 

STRUCTURE OF THE DOE RUN AND AVONDALE 

REGION. 

FOLDS. 

In the major folds of the Doe Run and Avondale region are represented the three great anticlines and the synclines described above. In 

the northwest corner of the region the Baltimore gneiss forms the core of the North Chester 

Valley anticline, which is flanked on the southeast limb by Cambrian quartzite, succeeded by 

the Cambrian and Ordovician Shenandoah 

limestone. Farther southeast, on the South 

Valley Hills, the Ordovician Octoraro schist 

comes to the surface in a wide, shallow syncline 

termin~ted by a long thrust fault, which has 

brought the Wissahickon mica gneiss into direct 

contact with the Ordovician schist. The peculiar course of this fault plane is treated in detail 

on page 27. 

The rocks underlying the overthrust gneiss 

form a shallow synclinorium, here called the 

Modena-Chester Valley synclinorium. On the 

southeast limb of the synclinorium the underlying limestone comes to the surface in the 

crests of secondary anticlines, where stream 

erosion cutting through the overthrust cover 

of mica gneiss has established anticlinal valleys 

in the limestone. 

An overturned syncline of quartzite and 

limestone separates the Wissahickon gneiss 

from the southern dome of the Baltimore 

gneiss, in which the rock forms a steep anticline 

overturned to the northwest. Southeast of this 

anticline, here known as the West Marlboro 

anticline, a synclinal wedge of limestone has been 

faulted upon the Baltimore gneiss along a 

fault plane having a very gentle southeast hade; 

the fault plane is split and the overthrust Wissahickon gneiss almost completely covers the 

syncline, leaving only a small remnant of limestone exposed. These structural features are 

shown in Plate III, section A-A', which is 

drawn along a line normal to the average strike 

of the formations. The secondary and tertiary 

anticlines and synclines that compose the great 

folds are the controlling factors in the areal 

distribution of the formations. The Shenandoah limestone occupies, for the most part, 

synclines and it represents the troughs of folds 

remaining after the erosion of the rest of the 

formation, a relation of the limestone to the 

adjoining formations which is illustrated in 

Plate III. 

South of Avondale the Baltimore gneiss is 

closely folded into small anticlines and synclines whose character is indicated by the prevailing southeasterly dips. South of Landenberg is an area of limestone which exhibits similar structure, the southeastern limb of a syncline cut off by an overthrust from the southeast. 

This structure, which is illustrated by the 

sections, conforms to that prevailing in the 

southeastern part of the Piedmont region. 

The only striking divergence occurs in the limestone which extends from Baker southwestward 

and lies north of the W estgrove area of quartzite, where there is a normal upright syncline. 

The primary folds may be traced along the 

strike to the northeast and southwest. The 

Cabin John and Buck Ridge anticline extends 

from Maryland to New Jersey and throughout 

its length is bounded by synclines; the northern syncline passes in to the anticline of the 

North Valley Hills, and the southern syncline 

extends southwestward until it is overlain by 

younger deposits of the Coastal Plain or 

obscured by pre-Cambrian igneous rocks. 

The major folds, however, do not explain ~he 

pinching out of the Baltimore gneiss, the 

Chickies formation, and the Shenandoah limestone parallel to the strike, while to the northeast and southwest there is a uniform exposure 

of the Wissahickon gneiss. The explanation of 

the lack of formational continuity must be 

sought in the minor folding, which is rather 

pronounced in character. The anticline of preCambrian gneiss represents a dome whose 

northeast and southwest flanks slope away in 

gentle minor synclines. A little farther northwest the same minor folding is apparent, but 

the axis is shorter. The northern finger-like 

limestone areas, from Springdell to Doe Run 

station and from Doe Run post office north to 

Buck Run, represent the crests of small minor 

anticlines. These anticlines. pinch out toward 

the north, and the small minor syncline of Wissahickon mica gneiss which separates the lime-
28 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916. 

in both these systems is almost vertical, and rium, the widening waters of the Appalachian 

the joints of the two systems, intersecting Straits slowly transgressed upon the shore 

nearly at right angles, combined with the bed- lines of Appalachia in a period of submergence 

ding plane, cut the rock into rhombohedral which lasted until the end of Ordovician time. 

blocks. The third joint system, striking about Disintegrating feldspathic gneisses and igneous 

N. 45° E., includes strike joints which are rocks furnished large pebbles of quartz and 

utilized as the back faces of quarries, as in the feldspar which were carried down by the streams 

quartzite quarry on the north side of the road and dropped along a receding shore line, forming 

leading southwestward from London Grove, the basal conglomerat~ that marks the close of 

where a triple system of joints shows very dis- the period of erosion that resulted in the pretinctly. The fourth joint system, which strikes Cambrian unconformity and the· beginning of a 

N. 45° W., is found in the Baltimore gneiss and long period of quiet sedimentation. North of 

Wissahickon mica gneiss. Philadelphia, traces of this conglomerate, exHISTORICAL GEOLOGY. posed by the upward pitch of a syncline, underlie a fine-grained white quartzite which reprePIEDMONT PLATEAU. sents the Lower Cambrian and which extends 

In order t_o understand the history of the in varying thickness and amount throughout 

Piedmont Plateau it is necessary to discuss that the Piedmont region. At the beginning of 

of the Appalachian region. A land area which Cambrian time a slow subsidence of the upland 

has been called Appalachia existed along the · was flooding the stream valleys at their mouths, 

present eastern border of the United States forming estuaries in which was deposited shore 

from pre-Paleozoic time, and its streams material of coarse sand, often impure, because 

drained westward into the Appalachian Straits, wave action was not sufficient to sort the matean interior sea whose eastern shore was some- rial. In the northern part of the Piedmont 

where in the Piedmont Plateau and whose region this sand by consolidation and metawestern shore was east of Mississippi River. morphism has become quartzite, or mica schist, 

The highly feldspathic shore material carried or mica gneiss; in the south, with little or no 

down by the Appalachian streams subse- metamorphism, it has become sandstone and 

quently formed in the northern part of the shale. Its thickness is variant owing to changes 

Piedmont region the great expanses of Balti- in level of the strand line during deposition. 

more and Wissahickon gneiss, and in the south- These local oscillations next gave rise to a 

· ern part the thick deposits of Carolina gneiss. deposit in which micaceous mud alternates 

After the lapse of a period of time whose dura- with sand. Subsequent continuous erosion of 

tion can be inferred only from the great thick- the Appalachian land brought to the sea finer 

ness of pre-Cambrian sediments, a change in and finer sediments, and over the Lower Camconditions set in. A gradual emergence of the brian quartzite calcareous deposits were laid 

land and a westward migration of the shore line down in a transgressing sea, so that gradual 

took place, accompanied by igneous activity, denudation of the land, accompanied by an 

during which large masses of granite, diorite, expanding and deepening of the sea, resulted 

and pyroxenite were intruded into the pre- in a thick accumulation of late Cambrian and 

Cambrian gneisses, and a process of mountain Ordovician limestone, the Shenandoah limebuilding, similar to that which produced the stone. Conditions remaining unchanged, the 

Sierra Nevada and the Coast Range, was begun Cambrian period passed into the Ordovician 

by the uplift of the deep-seated pre-Paleozoic without stratigraphic break, so that it is only 

rocks. The ancient Appalachian streams, re- by study of the changes in marine life that the 

juvenated by uplift, began to cut down through periods can be separated. In later Ordovician 

the newly formed land and once more found time there was a negative displacement of the 

their way by lengthened courses to a sea which strand line and the positive element, Appalachia, 

had migrated toward the west and was reduced once more increased in extent, while in the west 

to little more than a strait. ' a Cambrian island wa.<; elevated above the · inAs the conditions of heat and pressure, which terior sea and formed ,the Cincinnati arch. The 

had led to igneous action, resumed an equilib- streams, quickened by rejuvenation, carried 
WISSAHICKON MICA GNEISS OF DOE RUN AND AVONDALE REGION, PA. 29 

down from the late Cambrian and Ordovician 

lowlands large quantities of a fine clayey mud 

and deposited it conformably upon the limestone, forming the Ordovician shale, which 

produced the Martinsburg shale of the southern 

Piedmont area, and probably by later lnetamorphism the Octoraro schist of the. northern 

Piedmont. 

The emergence that marked the close of the 

great period of limestone deposition continued 

in this region until the close of the Paleozoic 

era, while elsewhere a varying succession of 

sand and shale was laid down. At first shale 

succeeded limestone in all parts of the region, 

but when the uplift was accomplished the shale 

was confined to deeper-water sedimentation. 

Uplift continued until the Appalachian Sea 

was closed at the north and became a gulf. 

In the southeastern Piedmont belt of Pennsylvania no sediments younger than Ordovician are found. Silurian and Devonian sediments may have been laid down and later 

removed by erosion, but it is likely that this 

area, during the deposition of the coal measures 

in western lagoons, was a land mass which 

received no further sediments until the end of 

the Triassic period. The Appalachian Gulf was 

being slowly filled, and the shore line was 

moving west, until, at the end of Carboniferous 

time marine sedimentation in the Appalachian ' ' Straits was brought to an end. · 

During the post-Carboniferous period of 

erosion the uplifted continent was worn down 

by the degradation of upper Paleozoic sediments, so that if any post-Ordovician Paleozoic 

formations were ever present in the Piedmont 

Plateau they were completely removed. A 

slow transgression upon the eastern shore of the 

Appalachian continent followed the emergence 

of the western coast from the interior Paleozoic 

sea. Exactly where the Atlantic coast line 

stood in Mesozoic time can hardly be determined, but the evidence of the deposits 

of the Newark group would indicate that 

toward the end of the Triassic period a series 

of lagoons and tidal estuaries extended along 

the eastern border of the continent, then worn 

down to the level of Ordovician sediments. In 

the shallow, brackish water of these lagoons 

rivers were depositing the mud and sand from 

the eroded Paleozoic rocks which subsequently 

formed the ripple-marked red shales and sandstones of theN ewark group. Volcanic activity 

accompanied the sedimentation, resulting in 

flows of basic lava and in the intrusion of sills 

and dikes of diabase which extend with various 

interruptions from the great Palisades of New 

York to isolated dikes in Maryland. With the 

emergence of the Newark sediments the last 

sedimentation distinctive of the Piedmont 

Plateau came to a close. The history of all 

succeeding deposits may be read in the adjoining Coastal Plain province. 

DOE RUN AND AVONDALE REGION. 

The formations of the Doe Run and Avondale region present an epitome of the geologic 

history of the Piedmont Plateau from early 

pre-Cambrian through Ordovician time. In 

order to understand the somewhat irregular 

sedimentation of this area the conditions of 

deposition must be clearly borne in mind .. It 

will be remembered that the Appalachian 

continent, worn by erosion to a region of low 

relief, lay to the east of the present Pie~mont 

district, and the interior sea, which had widened 

in the south probably as far east as the present 

Atlantic shore and had migrated in the west 

across the great Appalachian Valley to Missouri and Wisconsin, was rapidly narrowing 

in its northern part to little more than a strait. 

The sediments which form the Wissahickon 

mica gneiss were being laid down in a thick 

deposit along the shore line of Appalachia. 

That the southern portion of the Doe Run and 

Avondale region remained, in part at least, a 

land mass until the beginning of Cambrian 

time is a conclusion suggested by the absence 

of the Wissahickon mica gneiss between the 

Baltimore gneiss and the Cambrian quartzite 

in the Woodville-Chatham anticline, and by 

the presence on the flanks of. the igneous core 

of a sedimentary transition facies of the 

Baltimore gneiss. Pumpelly 1 has described 

a gneiss from Hoosac Mountain, Vt., which 

presents the same anomalous feature-that of 

a transitional form apparently in direct continuity with two formations otherwise separated by a great time break and an erosional 

unconformity. He explains the gneiss of 

Hoosac Mountain as a local consolidation of 

1 Pumpelly, Raphael, The relation of secular rock disintegration to 

certain transitional crystalline schists: Geol. Soc. America Bull., vol. 2, 

pp. 209-224, 1890. ' 
30 SHORTER CONT'RIBUTIONS TO GENERAL GEOLOGY, 1916. 

decayed rock material; according to him the 

consolidations favorable for deep rock disintegration are "a land surface exposed during 

a long period to the influence of a moist climate 

and protected by vegetation." 

Toward the end of pre-Cambrian time the 

land surface of the Avondale district had 

reached an advanced stage of disintegration 

similar to that described by Pumpelly as consisting of three zones. On the surface was a 

layer of kaqlinized material, forming a clay; 

between the semikaolinized material and the 

fresh rock was a zone in which the process of 

disintegration had advanced only far enough 

to weaken mineral cohesion by alteration of 

the micaceous or hornblendic layers. A residual soil produced under such conditions would 

remain in place, forming a deep zone that would 

grade insensibly into a layer in which disintegration had not been accompanied by decay. 

That compressive stress was · already acting 

upon the Baltimore gneiss of the Pennsylvania. 

Piedmont is shown by the intrusions of gabbroitic magma now found in the pre-Cambrian 

gneiss; at the same time folding and metamorphism were taking place, thereby giving a 

gneissic character and simulated conformity 

to both igneous and sedimentary formations. 

During this period of stress the semidisintegrated material in the zone of weakened cohesion, overlying the true gneiss, yielded readily 

to pressure and was consolidated, assuming the 

gneissic structure of ·the parent rock. 

The transgression which ushered in the 

Cambrian period was contemporaneous with 

the development of the thrust force which 

metamorphosed pre-Cambrian rocks into 

gneisses. The residual clay soil, washed away 

by the rapidly advancing waves, was deposited 

in deep water far from the shore; the coarse 

semikaolinized material, an easy prey to 

erosion, was carried to the sea and deposited 

near the shore, subsequently forming the conglomerate that marks the base of the Cambrian; 

the removal of these two upper zones exposed 

the lower beds of secularly disintegrated rock 

that had been consolidated in place; and by 

the erosion of this transitional gneiss material 

was furnished for the formation that was later 

consolidated and metamorphosed into the 

Lower Cam brian mica gneiss of the Chickies 

formation. 

During later Cambrian and Ordovician time 

the sedimentation of the Doe Run and Avondale region was quiet and continuous, uninterrupted by any marked disturbance. 

Streams working in the pre-Cambrian gneisses 

of the northern and southern areas were cutting 

out a fine quartzose sand which was deposited 

at river mouths in deepening estuaries. 

Submergence continued without local uplift, 

so far as the evidence shows, and deepening 

water in Upper Cambrian time received the 

wide limestone deposit which continued into 

the Ordovician period and probably extended 

over the whole surface of the Doe Run and 

Avondale region. A slight negative movement of the strand line ensued, and a load of 

fine mud was poured into the clear waters of 

streams that had been carrying lime in solution. The first result of the change in stream 

content was the deposition of a calcareous shale 

upon the surface of the limestone. On further 

uplift ero.3ion washed down from the lowlands 

large quantities of finely divided clayey material, which was subsequently consolidated to a 

shale and metamorphosed to the Octoraro 

schist. Deposition of Silurian sediments upon 

the Ordovician .shale seems probable, but all 

positive evidence of such sedimentation has 

been removed by subsequent erosion. In the 

southern part of Pennsylvania adjacent to the 

Piedmont Plateau, as estimated by Stose/ the 

total thickness of the Paleozoic sediments is 

20,000 feet, and about 4,000 feet of this thickness represents post-Martinsburg sedimentation; it is therefore probable that in the Doe 

Run and ,Avondale region a light load of sediments inay have covered the Ordovician shales. 

In the pre-Cambrian sea southeast of the 

Doe Run and Avondale region the Wissahickon 

mica gneiss had been deposited in the forn1 of 

an arkosic argillite, which during the late preCambrian period of diastrophism was consolidated and metamorphosed. Before the beginning of the Paleozoic era the Wissahickon 

gneiss suffered erosion in the pre-Cambrian 

mountains of Appalachia, and when the western 

part of Appalachia was buried beneath Paleozoic sediments the gneiss was brought into the 

zone of anamorphism and was subjected to the 

1 Stose, G. W., The sedimentary rocks of South Mountain, Pa.: Jour. 

Geology, vol. 14, pp. 201-220, 1906: U. S. Geol. Survey Geol. Atlas, Mercersburg-Chambersburg folio (No. 170), 1909. · 
WISSAHICKON MICA GNEISS OF DOE RUN AND AVONDALE REGION, PA. 31 

compressive forces that operated throughout 

the Paleozoic era. Through the whole of early 

Paleozoic time a slowly accumulating earth 

stress was making itself felt in deeply buried 

sediments. The resistant power of the Shenandoah limestone, combined with its extent, 

made it a competent stratum in the process of 

folding. According to C. W. Hayes 1 the requisites for great overthrust ~ovements are a 

light load upon a competent stratum. The 

Shenandoah limestone at first offered the requisite resistance to the strong thrust force, and 

later, after it had been folded into gentle open 

folds, it gave way to the accumulating stress 

which thrust the Wissahickon gneiss, originally 

deposited on the western shore of Appalachia, 

far over the surface of the folded limestone. 

From a study of the structure in the Doe Run 

and Avondale region it seems probable that 

there were two periods of folding during Paleozoic 

time. ItisanacceptedfactthattheAppalachian 

Mountains are a pre-Permian range which is connected, by way of the mountains of Newfoundland and Nova Scotia, with the pre-Permian 

mountains of Europe-the Armorican-Variscan 

ranges. In the Piedmont Plateau the effects 

of the pre-Permian diastrophism which formed 

these mountains are seen in the folding and 

faulting of the Paleozoic formations. In Scandinavia, Scotland, and Ireland the Armorican 

~fountains follow the line of an older rangethe pre-Devonian Caledonian Mountains. The 

pre-Cambrian platform in the Caledonides, 

according to Peach and Horne, 2 ''has been 

driven forward until the Archean rocks have 

been carried over the truncated edges of the 

Silurian strata." In the Scottish Highlands the 

presence of Lower Devonian strata, unaffected 

by this period of thrusting and faulting, makes 

evident the age of the movement. In the 

Piedmont Plateau there is a similar series of 

pre-Cambrian and Cambrian and Ordovician 

rocks. Here also an overthrust has brought 

pre-Cambrian gneiss over Paleozoic sediments, 

but in the Doe Run and Avondale region erosion 

has removed the Silurian and post-Silurian 

rocks, so that proof of the age of the thrusting 

can not be found in the presence of undisturbed 

material. 

I Hayes, C. W., The overthrust faults of the southern Appalachians: 

Geol. Soc. America Bull., vol. 2, pp. 141-154, 1891. 

z Peach, B. N., and Horne, J., Nature, vol. 31, p. 33, November, 1884; 

see also London Geol. Soc. Quart. Jour., vol, 44, p. 378, 1888. 

In Brazil and in northwestern Europe there 

are pre-Devonian mountains whose formation 

was due to the same cause-tangential compression derived from suboceanic spread. It 

is probable that this compression affected 

Appalachia, the great positive element of the 

eastern Atlantic region, as well as the northern 

and southern Atlantic regions, though the 

resulting diastrophism may not have been 

exactly contemporaneous. Schuchert considers 

the Taconic revolution (the pre-Silurian period 

of folding and thrusting in eastern America) 

the most marked disturbance of the era except 

the Appalachian revolution; it may be that 

the time of thrusting in Appalachia was preSilurian, instead of pre-Devonian, corresponding to the age of the Caledonides in northern 

Africa. It is possible that the thrusting was a 

gradual movement which kept pace with the 

deposition that probably occurred in eastern 

Pennsylvania during Devonian time. By the 

end of the Carboniferous period a reaccumulated earth stress once more found relief in a. 

period of folding by which the already overthrust gneiss was folded along with the underlying rocks. 

To these Paleozoic earth stresses, therefore, 

the rocks of the Doe Run and Avondale region 

owe their present complicated and overturned 

structure, which was already established before 

erosion had carried its work so far as to expose 

to view the Ordovician schist. Upon the 

degraded surface of this schist later Tertiary 

and Quaternary deposits have been laid down 

and again completely removed by erosion. 

The fact that these deposits once extended 

over the whole region is proved by the character of the underlying drainage. The courses 

of the larger streams show them to be out of adjustment to the underlying formations, and 

such a discordance can be explained only as a. 

superimposition of drainage inherited from a 

cover of material that was sufficiently deep to 

establish the streams firmly in their present 

courses. 

CONCLUSION. 

Nowhere in the Piedmont Plateau does the 

Wissahickon mica gneiss have easily interpreted relations to other formations. Near 

Philadelphia it is in contact with the Baltimore gneiss, an ancient pre-Cambrian formation which served as a floor for sediments of 
32 SHORTER CO~T'RIB1!TIONS TO GENERAL GEOLOGY, 1916. 

later age, or it is adjacent to younger sedimentary material from which it is separated by 

faulting. Elsewhere in this paper it has been 

. stated that the Wissahickon mica gneiss and 

the Octoraro schist are in contact in the Doe 

Run region and the immediately adjoining 

area. The question of their age and of their 

structural relations has been a moot point, 

not only because of the difficulty in determining the age of the Wissahickon · gneiss but 

also on account of the ambiguous structural 

relations existing between the mica gneiss and 

the Ordovician limestone. In the Coatesville 

quadrangle small outcrops of limestone occurring within the mica gneiss exhibit a relation 

which has a significant bearing upon the 

problem of the age of the mica gneiss. 

The age of the limestone must be first determined, but absence of fossils, metamorphism, 

and complicated structural relations make this 

determination difficult. The limestone may 

belong to the pre-Cambrian and be the equivalent of the Franklin limes tone toward the 

east, or it may be the representative of the 

Cambrian and Ordovician Shenandoah limestone. The limestone of the Doe Run and 

Avondale region is lithologically dissimilar to 

the Franklin limes.tone, and it is not associated 

with graphitic gneiss, which usually accompanies the Franklin limestone. On the other 

hand, the limestone of the Doe Run· and Avondale region bears a complete lithologic resemblance to the Shenandoah limestone of Chester 

Valley. It occurs in a conformable series 

with a quartzite which has been correlated 

with the Chickies quartzite of the North Valley 

Hills. Moreover, the southern limestone 

areas of the Doe Run and A von dale region are 

in the direct line of strike with the Shenandoah limestone of Cream Valley, west of Conshohocken. For these reasons the limestone 

of the region under discussion is correlated 

with the Shenandoah. 

The readily determined relation between the 

Ordovician mica schist and the great belt of 

Shenandoah limestone, which has been proved 

by fossil content to be of Cambrian and Ordovician age, establishes· the age of the mica schist 

a.s Ordovician.1 The conformable contact of 

the schist upon the limestone is shown by the 

1 Merrill, F. J. H., U. S. Geol. Survey Geol. Atlas, New York folio 

(No. 83), p. 4, 1902. Bascom, Florence, idem, Philadelphia folio (No. 

162), p. 5, 1909. 

lithologic gradation of the limestone through a 

calcareous mica schist into the overlying formation, as seen in Chester Valley near Coatesville, and also by the constant presence of a 

single bed of geodiferous, siliceous material 

along the line of contact/ thereby proving that 

the contact can not be a fault line. 

The fact that no fossils have been found in 

the Wissahickon mica gneiss makes it necessary 

in determining its age to rely altogether upon 

its stratigraphic relations to other formations. 

In dealing with the relations of the mica gneiss 

to the Ordovician Octoraro schist it must be 

decided whether the gneiss is of the same age 

as the schist and grades into it, or whether the 

gneiss is of different age from the schist and is 

separated from it by a thrust fault. The 

evidence in the Doe Run and Avondale region · 

seems to indicate a difference in age notwithstanding a marked resemblance between the 

two formations. A certain resemblance between the gneiss and the schist, both in the 

hand specimen and in the field, has caused 

considerable difficulty in distinguishing them 

and has given rise to the supposition that the 

mica gneiss is probably of Ordovician age. 

On close examination, however, it becomes 

evident that the apparent gradation of gneiss 

into schist across the line of strike is not 

actual. A detailed study of the region has 

established both lithologic and structural 

differences, shown by the following facts which 

have been used as a lithologic basis of separation between the two rocks: 

1. The mica gneiss always contains feldspar, 

usually in large amounts; the mica schist is 

almost free from feldspar. ~ 

2. The gneiss shows close crumpling and 

many fine plications; the schist is straight 

bedded. 

3. The gneiss is garnetiferous; the schist is 

not. 

4. The gneiss shows evidence, in coarser 

crystallinity, of having undergone greater 

metamorphism than the schist; and the change in 

metamorphism is too abrupt and well marked 

to be explained as the result of gradual eastward increase of the metamorphic action that 

'prevailed throughout the Piedmont Plateau. 

5. The chemical analysis of the gneiss shows 

the composition of an arkose which has under2 Bascom, Florence, idem. 
WISSAHICKON MICA GNEISS OF DOE RUN AND AVONDALE REGION, PA. 33 

gone injection and impregnation, while the 

schist has the composition of an argillite. 

6. The dip of the gneiss is gentle, about 

30°; that of the schist averages about 60°. 

7. There are four well-marked joint systems 

in the gneiss, whereas the schist possesses only 

three joint systems. 

The coarser crystallinity of the gneiss and 

the development of deep-seated metamorphic 

minerals not found in the schist, together with 

the presence in the gneiss of one joint systern 

and many plications that are absent in the 

Ordovician material, are evidences that the 

mica gneiss has been subjected to the influence of one more period of diastrophism than 

the schist and is therefore much older than 

Ordovician. 

As the Wissahickon gneiss Inust be older 

than Ordovician, let us examine the stratigraphic column in the Doe Run and Avondale 

region in order to decide at what horizon the 

gneiss is most likely to belong. It can not be 

considered younger than the limestone--

that is, underlying the schist and overlying 

the limestone--because of the conformity 

which has been established between the limestone and the mica schist. That the mica 

gneiss can not be included within the Shenandoah limestone is shown by the prevalent calcareous nature of the Shenandoah deposits seen 

in the Great Valley and in the Piednwnt region 

of eastern Pennsylvania. The generally conceded conformity between the Lower Cainbrian deposits and the Shenandoah limestone 

exeludes the gneiss from a position between the 

limestone and the quartzite and proves that 

the Wissahickon is as old as or older than the 

Chickies forn1ation. 

It therefore remains to be considered whether 

there is sufficient evidence to warrant the inclusion of the Wissahiekon gneiss in the Paleozoic succession by placing it within the Lower 

Cambrian. The character of the Wissahickon 

and the absence of sin1ilar rock within the 

Lower Cambrian of other regions Inake it seem 

unlikely that a great thickness of arkosic nlaterial may be included within Lower Ca1nbrian 

rocks, which elsewhere in the Appalachian 

province indicate a persistently arenaceous 

sedimentation. 

It has been shown that the ntica gneiss can 

not be younger than the Shenandoah limestone, 

that is, of Ordovician age-and also that it 

can not be of Ca1nbrian age; hence by exclusion fron1 the Paleozoic succession the mica 

gneiss becontes pre-Cambrian. Positive evidence of this age is found in the relation which 

the igneous intrusives bear to the Wissahickon 

formation. The Inica gneiss is penetrated by 

granite, gabbro, and ultrabasic intrusive rocks 

whose horizon is bounded by pre-Cambrian 

rocks. These intrw;;ive rocks are found in the 

Baltinwrc gneiss and the Wissahickon mica 

gneiss, but are not found to a significant extent 

in Paleozoic rocks of known or questionable 

age. The Palem~oic rocks are therefore separated from the vYissahiekon mica gneiss by an 

unconformity which determines the pre-Cambrian age of the 1nica gneiss. 

A further confirn1ation of the pre-Cambrian 

age of the Wissahickon gneiss is the fact that 

the formation can be traced westward fron1 the 

Doe Run and Avondale region through southeastern Pennsylvania into }.tfaryland.aud thence 

into the District of Columbia and Virginia. The 

formation in the neighborhood of Washington 

which is lithologically similar to the mica 

gneiss of Doe Run and A von dale has been described by Keith 1 as the Carolina gneiss and 

detern1ined as pre-Cambrian. 

The Shenandoah limestone on the south 

side of the Cabin John and Buck Ridge anticline has been folded into a syncline which is 

made up of many secondary folds. The Wissahickon mica gneiss occupies the center of 

many of these synclines, most of which are overturned toward the northwest, giving the strata 

isoclinal dips. In order to explain the presence 

of a pre-Cambrian gneiss in immediate contact 

with the Cambrian and Ordovician limestone 

and the Ordovician schist it is necessary to 

assume the existence of a deep overthrust 

fault, the origin and nature of which have been 

described and which has brought the older formation along a gentle hade to lie upon the 

younger Paleozoic rocks. The resulting discordance has been obscured by a subsequent 

diastrophic movement that has folded and 

faulted the thrust plane as if it were a plane of 

stratification. 

In the neighborhood of Philadelphia Miss 

Baseom 2 has established two presumably continuous fault lines along Huntingdon and Cream 

I Keith, Arthur, U. S. Geol. Survey Geol. Atlas, Washington folio 

(No. 70), p. 2, 1901. 

2 Bascom, Florence, U. S. Geol. Survey Geol. Atlas, Philadelphia 

folio (No. 163), p. 16, 1909. 
34 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916. 

valleys. The Cream Valley fault has brought 

up the Buck Ridge anticline of pre-Cambrian 

gneiss until it overlies the syncline in the Ordovician schist. The westward continuation of 

this fault lies along the contact of the Wissahickon gneiss and Octoraro schist and extends 

directly into the Doe Run region. The change 

in dip between the schist and the gneiss is 

structural confirmation of the existence of a 

fault which is in complete accord, not only 

with the prevalent type of Piedmont structure as determined in the eastern Atlantic 

States, but also with the structure of regions 

sinrilar to the Pied1nont of the lTnited Statesfor example, northern Scotland and Scandinavia, where overthrusting of like character 

and magnitude has been worked out. 

Therefore, as the existing evidence seems to 

indicate a pre-Cambrian age for the Wissahickon m.ica gneiss, and as the structure necessary to explain the relations of the gneiss to 

the Paleozoic sediments is in keeping not only 

with the intrinsic structural evidence of the 

formations but also with the structure of regions 

similar to the Piedmont of Penns:ylvania, it 

seems fair to conclude that the Wissahickon 

mica gneiss is separated from the Shenandoah 

limestone and the Octoraro schist by a thrust 

fault which has been obscured by post-Ordovician metamorphism.