DEPARTMENT OF THE INTERIOR 

FRANKLIN K. LANE, Secretary 

UNITED STATES GEOLOGICAL SURVEY 

GEORGE OTIS SMITH, Director 

Professional Paper 95 

SHORTER CONTRIBUTIONS TO 

GENERAL GEOLOGY 

1915 

DAVID WHITE, CHIEF GEOLOGIST 

WASHINGTON 

GOVERNMENT PRJ;NTING OFFlCE 

1916 
NOTES ON THE GEOLOGY OF GRAVINA ISLAND, ALASKA. 

By PHILIP s. SMITH. 

INTRODUCTION. 

Gravina Island is in the extreme southern p art of Alaska and is about 20 miles long and 10 

miles wide.. It l

1

ies about 30 miles north of the international boundary and is separated by a 

narrow channel froni the town of 

Ketchikan, the southernmost port 

of call in Alaska. Its general position with regard to ot~er parts 

of southeastern Alaska is shown 

on figure 34. An opportunity. of 

studying the coast exposures of 

'this island was afforded the writer 

in the summer of 1913. This 

work, amplified by the earlier investigations of Brooks 1 rnd the 

Wrights/ showed the presence of sar---:~~~~·---'--~---4~~~~~~~~;;:n-:z:-;(-T 

Triassic rocks, whicr pr~viously C'),.... 

had been reported definitely at ~ 

<:':. 

only one other locality in southeastern Alaska. Inasmuch as the 

structure and general relations of 

Gravina Island furnish · an, insight 

into the geology of this region, 

the following notes are presented. 

Little of the island away from the 

seacoast has been studied, and 

therefore the geology of the interior is practically unknown. 

r----- -4--~-------~~~~~~~~---

Both igneous and sedimentary 

rocks are represented in the region. 

1> 

Oc..._._-'---'-_._::5.::...0 J...-L.-L-J._ji0:...:_0 ____ __,200 MILES 

The sedimentary rocks range in ~~~==~~'~ ~~~~~~~~='~34=·~~~~~~~~~ 

character from thoroughly recrys- FIGURE 34.-Map of southeastern Alaska showing position of Gravina Island (indicated 

tallized schists to unmetamor- byshading). 

phosed conglomerates, sandstones, and limestone . The igneous rocks include both deep-seated 

intrusives and surface flows. The general distribution of these rocks is shown on the geologic 

map (fig. 35). 

SEDIMENTARY ROCKS. 

GNEISS AND A:MPHmOLITE. 

The oldest rocks recognized on Gravina Isla d are exposed at the extreme southern point, 

Dall Head. . At this place j they consist of highly metamorphic gneisses and amphibolites which 

1 Brooks, A. H., Preliminary report on the Ketchikan mining district, Alaska: U. S. Geol. Survey Prof. Paper 1, 120 pp., 1902. 

2 Wright, F. E. and C.W., The Ketchikan and Wrangell mining districts, Alaska: U.S. Geol. Survey Bull. 347, pp. 210,1908. 

97 
98 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1915. 

have been intruded by igneous rocks. Undoubtedly the older rocks have been metamorphosed 

locally by the younger intrusives, but apparently they were considerably metamorphosed 

dynamically before they were intruded. No definite determination of the age of the gneisses 

and amphibolites has been made. Judged by their physical character they seem to be much 

older than any of the known late Paleozoic rocks. Although their aspect may be due in large · 

measure to the contact effects produced in proximity to the large intrusive masses, the impression gained in the field, and not dispelled by later investigations in the office, is that those rocks 

are probably at least as old as the middle or earlier part of the Paleozoic era: 

0 2 4 6 8 10 MILES 

LEGEND 

SEDIMENTARY ROCKS 

[[2] 

Sandstones, shaiEls, and slates 0 

~ ~ ~ ~ Limestone 

Conglomerate ~ 

Slates 

-

and shales 

I~<~>~ ~~ 1 

Limestone 

E2l ·Conglomerate 

Green 

-

schists 

. 

~ Schists 

0 

Gneiss and 

amphibolite 

IGNEOUS ROCKS 

a: 

1-

z 

~ 

z 

0 

> w 

0 

·~ ~~ 1-

~3 Lo.JO 

o:::w Granite uu 

FIGURE 35.-Geologic map of Gravina Island, Alaska . 

. The relation of the old gneisses and amphibolites to the other sedimentary rocks of the 

region is mdefinite, because they occupy only a small area at the extreme end of the island and 

are surrounded on all sides by sea or by a large area of younger igneous rock. No other rocks o~ 

·Gravina Island are correlated with these'J'ocks near Dall Head. 

ROCKS NEAR VALLENAR BAY. 

Near the north end of Gravina Island, in the vicinity of Vallenar Bay, sedimentary rocks 

are exposed which lithologically simulate other rocks on the island, but which paleontologically 

are apparently quite distinct. These rocks are considerably deformed and their relations are 

not everywhere evident. At the base, apparently, they consist of a small amount of conglomerate 
NOTES ON GEOLOGY OF G'RA VIN A ISLAND:, ALASKA. 99 

composed of pebbles few of which are over an inch in diameter. Thepebbles are dominantly 

of schist or of quartz. The strike of the conglomerate ranges from northwest to northeast and 

the dip from northeast to southeast. Apparently the structure is · that of a steep eastwardpitching anticline. 

Overlying the conglomerate is a thin limestone, and this is succeeded by black slates and 

sandy beds. The rocks are not much metamorphosed and in the field were correlated with the 

Triassic rocks on the southwest coast. Thi.'3 correlation, however, is not confirmed by fossils 

found at a locality about a mile east of South Vallenar Point. According to the Wrights 1-

Fossils were first found at this locality by Brooks in 1901 and were determined by Charles Schuchert to be 

Devonian. In 1905 a collection was made at this place by E. M. Kindle, and in 1906 more material was gathered 

by the writers. Kindle reports as follows: 

"One mile west of Vallenar Bay, Gravina Island.- The material from this locality, while generally insufficient 

for specific determination, is much better than that obtained last year (1905), and leaves no doubt as to the Devonian 

age of the beds west of Vallenar Bay. Several specimens of Atrypa reticularis are present. This fixes the horizon as 

not later than Carboniferous, while the association of Chonet es cf. manitobensis, Spirifer sp., Proetus sp. , and Cyclonema 

sp. indicate a horizon of Devonian age, probably middle Devonian." 

The finding of Triassic fossils in rocks of similar lithologic character in the central part of 

Gravina Island suggested the need of a review of this early determination, and Mr. Kirk reexamined the collections from this locality. In an informal communication he states that the rocks 

from the vicinity of V allenar Bay are clearly of Paleozoic age and that he sees no reason to 

question their assignment to the Devonian, as was originally suggested by Schuchert. 

There seems to be no reasonable room for doubt that Paleozoic rocks, in part of Devonian 

age, occur in the vicinity of Vallenar Bay. The relation of these rocks, however, to the other 

sedimentary rocks is unfortunately obscured by the presence of later effusive rocks which unconformably overlie them and mask their contacts with the other rocks. 

SCHISTS ON NORTHEAST COAST. 

East of the eastern entrance to Blank Inlet and extending in a belt about a mile wide 

nearly to Gravina Point are metamorphic rocks consisting of schists and schistose limestones. 

The rocks of this belt trend dominantly northwest and dip apparently eastward at steep angles. 

As will be shown later in more detail, the major $tructure of Gravina Island ,is believed to be that 

of an overturned syncline. Therefore, although the dominant dip of the schists appears to be 

eastward, the geologic sequence is believed to be reversed, so that the top of the schist member is 

toward the west and the bottom toward the east. 

The schists are considerably faulted, and the sequence of the individual beds is by no means 

constant. The contacts of the schist with the fossiliferous beds on Gravina Island are not 

such as to show definitely the relations of these rocks, and consequently interpretation of their 

age is based more on general features than on specific details. Apparently the schists are much 

more metamorphosed than the Devonian rock already described. This difference in amount 

of metamorphism 4oes not seem to be due to the proximity of the schists to the great intrusive 

masses of the Coast Range batholith, for apparently the rocks both at Vallenar Bay and on the 

northeastern coast of Gravina Island are abou~ equally distant from the known large areas of 

intrusive rock. The difference in the amouni of metamorphism therefore seems to be best 

explained by assuming that the rocks on the northeastern coast of Gravina Island have been 

affected by dynamic metamorphism to whicb the other rocks were not subjected. If this 

interpretation is correct it seems probable thati the schists are older than the Devonian rocks. 

Inasmuch, however, as this suggestion is not confirmed by other facts it must be regarded as 

only a tentative correlation, and further data must be sought. Although definite correlation 

· is not now made, little doubt is felt th~t these schists are of Paleozoic age, and they have 

consequently been represented on the accompanying map as belonging to the group called, for 

brevity, the undifferentiated Paleozoic rocks. 

1 Wright, F. E. and C. W., op cit., p. 50. 
100 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1915. 

GREEN SCHISTS NEAR KETCHIKAN. 

East of the belt of schists just described is another schist which forms the coast line for 

about 7 miles northwestward from Gravina Point. This is a light-green, thinly cleaving rock 

that ~eems distinct from the other schists. It has been variously interpreted as a schistose tuff, 

a sheared intrusive or extrusive igneous rock, or a sedimentary rock. No conclusive proof of 

its origin has yet been adduced. Its general appearance under the microscope suggests that 

it may have been a tuff, but in the specimens examined metamorphism had obliterated the 

diagnostic features. 

This rock occurs not only on Gravina Island but also on the east side of Tongass Narrows, 

in the vicinity of Ketchikan, and it is the rock on which Brooks based the formation entitled 

on the map accompanying his report 1 "greenstone schist, Paleozoic." If this rock underlies 

the water-covered area which separates Gravina Island from Ketchikan it must occupy a belt 

nearly 2 miles wide. Apparently it has considerable longitudinal extent, for it has been recognized to the southeast at Foggy Bay and near Cape Fox, the latter place distant nearly 50 miles 

in an air line, and to the northwest on Cleveland Peninsula, a distance of at least 20 miles. 

The continuity .of the belt of green schists, the preservation of a semblance of tuffaceous 

composition, and the physical appearance of the rocks suggest that the green schists are younger 

than the schists to the west. This conclusion is further supported by the general eastward 

dip of the rocks, but, as has already been pointed out, this dip is not believed to be conclusive, 

for part of the structure is believed to be overturned. Probably an undetected fault or an 

unconformity separates the green schists from the schists to the west. 

TRIASSIC ROCKS. 

WEST COAST OF GRAVINA ISLAND. 

On the west coast of Gravina Island, about lt miles north of Dall Head, a conglomerate is 

exposed. South of this place the coast line is formed of granitic rocks. The relation of the 

southernmost conglomerate exposure to the granitic rock was not observed. None of the granite 

was recognized in the pebbles of the conglomerate, and the presumable age of the granite sug- , 

gests that the contact relation is either that of intrusion or of faulting. At this place the conglomerate trends nearly east and stands nearly vertical. Thin sandstone beds alternate with 

beds of pebbles and cobbles, all thoroughly indurated so that the rocks fracture across the grains. 

Little or no cleavage has been developed, and the jointing is rather widely spaced. 

A short distance to the north a small bay indents the coast, and on th~ ialands in the bay 

a gray, reddish-weathering gritty limestone crops out. The strike of this rock swings abruptly 

from a general easterly trend on the southern islands to a northerly trend on the northern 

islands. The structure is that of an appressed fold pitching east. On the northern headland 

of this bay the conglomerate again appears, having apparently passed seaward of the limestone 

islands. This interpretation is further suggested by the presence of a submerged rocky shoal 

in the position that projection of the structure of the limestone would indicate should be occupied 

by the conglomerate. On the northern point of the bay the conglomerate and limestone dip 

30° E. and strike about N. 25° W. Plate VII, A, shows a characteristic view of the conglomerate 

at this place. The conglomerate apparently is conformably overlain by the limestone. 

A collection of fossils from this bay was made in 1906, but the precise locations of the outcrops from which the collections were obtained were not recorded. The lithology of the matrix 

containing the fossils, however, leaves nq room to doubt that the collections came from this 

rather narrow band of limestone. The fossils were not diagnostic, but· they were provisionally 

referred by the Wrights to the lower Carboniferous, though they stated that they might represent 

a Triassic horizon. G. H. Girty, 2 who examined the collections, reported: 

1 Brooks, A. H., op. cit., pl. 2. 2 Cited by Wright, F. E. and C. W., op. cit., pp. 5~53. 
NOTES ON GEOLOGY OF GRAVINA ISLAND, ALASKA. 101 

Zaphrentis sp. Pteria? sp. 

Lithostrotion sp. Tetinka? cf. bellula Barrande. 

Martinia? sp. Loxonema? sp. 

Dielasma? aff. bovidens Tschern. non Martin. Euomphalus? sp. 

Dielasma? aff. millepunctatum. Pleurotomaria sp. 

A viculipecten? sp. Naticopsis sp. 

Halobia? sp. Several undetermined forms. 

By the presence of Halobia (?),which is very similar if not identical with that which occurs in lot 17 on the 

Yukon [now considered Triassic], as well as by certain other particulars, a correlati•m of the two horizons is suggested, and perhaps for the present that is the best disposition that can be made of this very ambiguous collection. 

At the same time the two faunas are rather widely different. 

East of the limestone and separated by a short interval in which no rocks are exposed 

are outcrops of shales, slates, and thin sandstones. These beds apparently trend parallel to the 

conglomerate and limestone. Thus at the northern part of the bay they strike nearly north and 

dip at moderate angles to the east; ·in the eentral part they form a closely appressed synclinal 

fold pitching east, cut by small faults; still :farther south they trend nearly east and dip steeply 

north. Cleavage rather steeply inclined to t he bedding has been induced in the weaker members 

to such an e?Ctent that the rock fractures into small angular slaty bits. Owing to this structure, 

bedding that would be easily recognizable in outcrop is extremely obscure in the small cleavage 

flakes into which the rock breaks. This precludes hasty search and doubtless accounts for the 

lack of success earlier geologists have had in obtaining fossils. Where good bedding planes are 

found fossils are abundant. All the forms seen consisted of one genus, but doubtless further 

search would reveal others. These forms are so umerous that the imprint of their shells forms 

an almost unbroken surface. No shell material remains. The fossils are simply imprints of the 

shells in the shale, and their presence usually is not determinable in sections of the rock other 

than those parallel to the bedding. 

The collections of fossils obtained north of the center of the small bay on the northern limb of 

the appressed pitching syncline (8704), and also from the cove 6 miles north of Dall Head (8705) 

were submitted toT. W. Stanton, who reported upon them· as follows: 

The only fossil species recognized in these two lots is a Halobia, which is closely related to if not identical with 

H . superba Mojsisovics, an Upper Triassic species. The rocks from which these collections came are therefore referred 

to the Triassic. 

The exposures on this bay were examined in 1914 by G. C. Martin, who obtained several 

collections of Triassic and one of Devonian :fossils. It is Mr. Martin's opinion 1 thatthe Triassic conglomerate rests unconformably upon Devonian rocks, like those of Vallenar Bay, at the north end· of 

Gravina Island, and the Devonian fossils which have been collected near the Triassic fossil localities on the ''cove 

3 miles north of Dall Head," were obtained either from Devan.ian bowlders in the Triassic conglomerate or from unrecognized Devonian rocks occurring in complex structural association with the· Triassic beds. 

The fossils collected by Mr. Martin were determined by T. W. Stanton, as follows: 

8829 (G. C. M. No.1). Gravina Island, north arm of cove 3 miles north of Dall Head. Float on outcrop of shale and 

thin limestone beds of localities 8834 and 8835. 

Undetermined corals. Possibly Triassic. 

8830 (G. C. M. No.2). Gravina Island, south arm of same cove as 8829. Massive limestone outcrop near anchorage 

behind wooded island. 

Undetermined corals of Mesozoic type-two or three genera represented. 

Ostrea? sp. · · 

Pseudomelania? sp., internal cast. 

Arcestes?? sp., fragment; may not even be an ammoni te. Probably Triassic. 

8831 (G. C. M. No. 3). Gravina Island near 8830. Large limestone nodule in conglomerate. 

The whole nodule is a nautiloid with deeply lobed suture, possibly referable to Cosmonautilus. On the back 

is a Rhynchonella (?); 

Probably Tri.assi~. 

8832 (G. C. M. No. 4). Gravina Island, near south foreland on arm of cove 3 miles north of Dall Head, near contact 

with conglomerate. 

Undetermined coral fragment. 

Pentacrinus sp., segment of column. 

Probably Triassic. 

f Alaska (in preparation). 
102 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1915. 

8833 (G. C. M. No.5). Gravina Island, south arm of cove 3 miles north of Dall Head. Near zone of nodular masses 

of limestone in conglomerate. 

This lot appears to be Devonian. [See Kirk's report, below.] 

8834 (G. C. M. No. 6). Gravina island, north arm of cove 3 miles north of Dall Head. Massive limestone in reef west 

of cabin. Probably about 100 feet below 8704: 

Corals, probably several genera. 

Cassianella sp. 

Myophoria?? sp. 

Natica sp. 

Murchisonia? sp. 

Triassic. 

8835 (G. C. M. No. 7). Gravina Island, near 8834, from thin-bedded limestone interbedded with shale about 20 or 30 

feet below 8834. · 

Corals--several genera represented. 

Spiriferina? sp. 

Myophoria? sp. · 

Natica sp. 

Turri tella? sp. 

Pseudomelania? sp. 

Trachyceras? sp., small fragment. 

Triassic. 

The fossils from locality 8833 have been examined als by Edwin Kirk, who submitted 

the following report on them: 

Devonian (Middle). 

No. 5. South arm of "cove 3 miles north of Dall Head." Near zone of no ular masses of limestone in conglomer~te. 

Cladopora sp. 

Diaphorostoma sp. 

These fossils are clearly of Devonian age. P. S. Smith collected cor Is identical with those in this lot from 

Kasaan Bay, Long Island, in 1913 at locality 13AS49. 

The conglomerate is exposed continuously along the coast for 3 1niles north of this bay 

and throughout this distance strikes nearly parallel to t.he c ast and dips at low angles eastward. Deformation then causes the strike to swing Inore to the norLhwest, or seaward, and 

the conglomerate disappears from the coast, probably being I covered by water. Overlying it, 

however, appears the sn1alllimestone band, and this in turn is overlain by shales and sandstones 

similar to those farther south. Halobia is found in the shales at this place (8705) also. 

The shales and sandstones were traced northward almost contmuously for about 5 miles. 

Throughout this distance the rocks are approximately parallel to the coast, dipping eastward, 

but here and there showing a small amount of deformation such as that illustrated by Plate 

VII, B. Although folding of the sort shown by the picture complicates measuring the thickness 

of the formation and introduces minor irregularities, it is evidently of a relatively simple type 

and on a broad scale. 

EAST COAST OF GRAVINA ISLAND. 

On the east side of Gravina Island the structure is somewhat more involved and the several 

lines of evidence lead to somewhat antithetical conclusions. The conglomerate on the west 

coast at the southernmost locality trends nearly east. lts direct continuation on this trend 

is apparently interrupted by granite and other igneous rocks which form the country rock on 

the east coast as far north as Bostwick Inlet. · 

On the headland northeast of the entrance of Bostwick Inlet a conglomerate trending 

nearly north and dipping 70° E. is exposed. It is somewhat finer grained than the conglomerate on the west coast, but lithologically resembles and seems to be equivalent to the western 

conglomerate. If this correlation is valid the structure of the island as a whole is that of a 

somewhat overturned synclinorium whose western limb dips about 25° E. and the eastern 

limb about 70° E. This fold is not simple, for its general synclinal structure is complicated 

by subordinate anticlinal flexures. This is well shown by the numerous folds observed along 

Blank Inlet, where small anticlines separated by minor synclines give the for!p.~tion a much 

greater areal extent than the steep dip and normal thickness would otherwise admit. Fault-
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 95 PLATE VII 

.A. TRIASSIC CONGLOMERATE 3 MILES .NORTH OF DALL HEAD, GRAVINA ISLAND, 

ALASKA. 

B. TRIASSIC SANDSTONES AND SHAL E~ ON WEST COAST OF GRAVINA ISLAND, 

ALASKA. 

0. ANDESITIC AGGLOMERATE ON WEST COAST OF GRAVINA ISLAND, ALASKA~ 

NOTES ON GEOLOGY OF GRAVINA ISL,AND, ALASKA. 103 

:ing on a small scale has also produced some ·duplication of strata and some apparent anomalies 

·of distribution. 

To the west of the conglomerate on Bostwick Inlet are black sandstones and thin impure 

limestones, in general striking north and dipping east at high angles. Fossils were collected in 

1913 by the writer 1 from these rocks on the west shore of Bostwick Inlet and submitted to Mr. 

Girty for exainination. The following is his report to Mr. Stanton: 

Three lots from Bostwick Inlet, Gravina Island, contain round crinoid stems (13AS170) and a single compressed 

pelecypod (13AS169), suggesting the genus Posidonomya. The third lot (13AS171) contains a ptlecypod fauna interesting and varied but entirely new to me. As thes~ fossils at btst show only the shapes and some of them the sculpture, 

and as pelecypods of similar external appearance may belong to widely different· genera, the identifications made here 

.are offered with doubt.1 

Though similar doubt tacitly smTounds many pelecypod identifications in faunal lists and 

.elsewhere, I have expressed it in this case by the use of question marks beeause the age of the whole fauna is involved 

in such uncertainty. In lot 171 five types are 'represented more or less abundantly and by specimens more or less 

good. These are Glossites? cf. G. lingualis, Schizodus? cf, S . appressus, Paracyclas? cf. P. ellipticus, Crenipecten? cf. 

C. crenulatus, Elymella? cf. E. nuculoides. Besides these1 however, there are a good many indeterminate forms, some 

'()f which suggest the ge-p.era Leda, Chonetes??, Pseudomonotis??, A viculipecten, and N ucula. The age of the fauna is 

.quite uncertain. You declined for the time being to admit it into the Triassic, and nothing resembling it has thus far 

been brought in from the Alaskan Carboniferous. The generic and specific resemblances suggested above might indieate a Devonian fauna, but neither has any Devonian fauna related to this been obtained from Alaska. The genus 

{Jhonetes, if definitely present, would at least limit the geologic age to the upper Paleozoic, but the specimen is so 

:imperfect that even the identification as a brachiopod is doubtful. 

Thus the age of this, the best fauna in the collectio~1, must for the present remain undesignated. It would be 

highly rash to attempt any definite age determination or conclusion for the other lots, containing as they do, for the 

most part, only crinoid stems (even that being doubtful i some cases), though they may tentatively be placed in the 

Paleozoic. 

As Mr. Girty points out, this fauna is enth·ely unlike any other from Alaska. It therefore 

.affords little aid in solving the' stratigraphic position of the be:ds on Gravina Island. Certain 

significant deductions, however, suggest themselves. In general in southeastern Alaska the 

known Carboniferous rocks are limestones and not lithologically similar to the rocks of Gravina 

Island. The rocks here discussed are much less dynamically metamorphosed and are less 

intensely deformed than the known Devonian or Carboniferous rocks of the region. Their relation to the andesitic agglomerates and flows fits· in better with an assumed Mesozoic age than 

with a Paleozoic age. The almost uninterrupted succession of beds from a basal ( ?) conglom-

-erate to shales containing unquestioned Triassic fossils on the southwestern coast suggests that 

the doubtfully de ermined genera from that locality, provisionally assigned to the Carboniferous, 

justify the suggestion made by Mr. Girty at that time that they may mark a Triassic horizon. No 

Lower Triassic is known in Alaska. Is it not possible that this fauna from Bostwick Inlet, 

which is unknown elsewhere in Alaska, fits into this as yet unfilled gap? 

The fossils represented in the following list were obtained by G. C. Martin at or near this 

same locality. These fossils were determined by T. W. Stanton as follows: 

.8836 (G. C. M. No.8). Gravina Island, Bostwick Inlet, w4;1st shore near entrance. From angular nodules in a brecciated(?) nodular limestone. 

Terebratula sp. Myophoria sp. 

Spiriferina? sp. Myophoria or Trigonia sp. 

Pecten sp. Nucula sp. 

Plicatula? sp. Astarte? sp. 

Cassianella sp. Arcestes? sp. 

Triassic. This assemblage suggests the fauna of the lower part of the Modin formation in California, which 

was tentatively assigned to the Jurassic. 

The eastward-<lipping structure which is dominant on Gravina Island indicates that the 

beds of Bostwick Inlet are equivalent to or higher rather than lower than the definitely determined Triassic beds on the west side of the isla,nd. An interpretation of the structure as that 

<>f an overturned anticline, in which the b~ds in the central part of the island are the oldest, 

is possible, but is opposed by the observed dips and by the broader correlations already pointed 

<>ut. The interpretation that these conglomerates, thin limestones, shales, and sandstones 

may be of early Mesozoic age seerns to be a reasonable working ·hypothesis. Further strati-
104 SHORTER CONT~IBUTIONS TO GENERAL GEOLOGY, 1915. 

graphic and paleontologic evidence should be sought and the field relations carefully scanned 

to test this interpretation, but until work of this ehmacter presents data more in accord with 

another view the rocks on the west coast of Gravina Island and on the east coast as far 

northeast as Blank Inlet should be regarded as of the same age. 

In this conneCtion it may be interesting to note that Brooks long ago grouped these rocks 

together and regarded them as of Mesozoic age. He included these rocks in his Gravina series, 

which he described as follows: 1 

This is a series of massive conglomerates overlain by black shales or slates and closely infolded with the rocks of 

the Vallenar series. * * * It is thus probable that the placing of the Gravina series in the Cretace::>us is correct. 

Subsequently,2 however, he said: 

The Gravina was correlated with Dawson's Queen Charlotte group (Cretaceous), but on reviewing the evidence 

its identity with the Vancouver series (Triassic) seems equally probable. 

About 2~ miles southeast of V allenar Point, immediately beyond the belt of undifferentiated 

Paleozoic schists, is a conglomeratic rock. This differs from the conglomerate noted on Bostwick and Blank inlets in that it is smnewhat more schistose. Like the conglo1nerate at those 

places it is rather fine grained, few of the pebbles being more than an inch or two in dia1neter, 

and it thus appears quite different from the very coarse phase seen on the wesb coast of the 

island. The relations of this northernmost conglomerate are obscured, as the beds have been 

considerably disturbed by faulting subsequent to the deposition of the next overlying formation. Inasmuch, however, as this conglomerate is essentially in line with the conglomerate 

on Blank Inlet, has a corresponding strike and dip, and is not more than 7 miles distant, 

and as the general relation at the two places are essentially the same, the two are correlated. 

IGNEOUS ROCKS. 

ANDESITIC EFFUSIVE ROCKS. 

Immediately north of the Triassic shales on the west coast is a volcanic breccia of andesitic 

composition. Large bowlders of this formation had been recognized along the beach farther 

south and had apparently fallen from the hills that rise steeply from the coast. Plate VII, 0, 

shows some of these large bowlders at a point 1 mile north of the Triassic fossil locality that 

was noted as 6 miles north of Dall Head. The detrital character of the rock and the distribution of the float indicate that it has a distinct stratigraphic position above the Triassic rocks. 

Just what the relation is was not determined, as the contacts were not well exposed and the 

depositional structure was not evident, but n1arked discordance was apparent. The andesitic 

breccia or agglomerate was recognized also on the east coast of Vallenar Bay and at Seal Bay, 

on the east coast of the island. 

Stratigraphically overlying the andesite agglomerate or breccia is a considerable thickness 

of andesitic flows, with here and there a relatively small amount of interbedded sedimentary 

material. These rocks are si!rikingly similar t.o those in the region north of Juneau, described 

by Knopf as augite melaphyres and referred by him to the Upper Jurassic or Lower Cretaceous. 

According to Knopf ,S these melaphyres were earlier than the great period of Coast Range 

intrusion. This age determination is based on the fact that though the melaphyres are nowhere 

cut by the Coast Range igneous rocks they belong to the sequence of formations that have 

been metamorphosed by contact with the igneous masses which were injected at that period. 

On the west coast of the island the andesites form the shore line for about 6 miles, occupying that part between the Triassic and Devonian exposures. On the north end of the island, 

extending along the coast for 2 to 3 miles, both south and east of Vallenar Point, the andesites 

. form the country rock. In 1nost exposures they are rather massive and heavily jointed, but at 

several places thinly laminated phases occur. The rock is dark green, and some of it is 

amygdaloidal. 

1 Brooks, A. H., op. cit., p. 45. 

2 Brooks, A. H., The geography ana geology of Alaska: U.S. Geol. Survey Prof. Paper 45, p. 226, 1906. 

a Knopf, Adolph, The Eagle River region, southeastern Alaska: U.S. Geol. Survey Bull. 502, pp. 18-20, 1912. 
NOTES ON GEOLOGY OF GRAVINA ISLAND, ALASKA. 105 

Andesites correlated with t.he andesites a the north end . of the island were seen also on 

Bostwick Inlet. Presumably the andesites at Bostwick Inlet and at V allenar Point are continuous inland between the two places. The andesite exposed on the west coast probably 

also extends southward for a considerable distance. The and~site inland from the coast was 

not examined by the wTiter but is indicated on the map (fig. 35), on the basis of the Wrights' 

map, on which the igneous rock on the north side of Bostwick Inlet and east of the head of 

Vallenar Bay is shown as continuous in the hills) loca~ly known as the California Range, between 

the two places. The Wrights, however, considered the igneous rocks on Bostwick Inlet as 

belonging to the group of Coast Range intrusi;ves d the rocks of V allena1 Point as belonging 

to the group described in the legend accompanying their map as ''greenstone lava flows interstratified with volcanic tuffs and black slates." 

Although, as has already been pointed out, the andesite agglomerate and andesite nowhere 

appear to be unconformable with the underlying rodks, the fact that they are in juxtaposition 

with rocks of different ages and kinds clearly indicates that the relation is either one of faulting or an unconformity. From the facts at pre ent known the interpretation that the andesites 

unconformably overlie the sedimentary rocks previously described seems to be most probable. 

COAST RANGE INTR USIVE ROCKS. 

Few new facts were obtained bearing on t:Ue age and broader relations of the Coast Range 

intrusive rocks. Some new details were obtained as to the distribution of these rocks, but as 

these facts are most clearly displayed on the accompanying map specific description will be 

.omitted. The following excerpts from the Wrights' report 1 give the more important facts 

regarding the lithology of the Coast Range intrusive rocks: 

The Coast Range massif, as it has been defined by Dawson, is not of the same composition throughout, but is composed of different kinds of igneous rocks ranging from granite to diorite and even gabbro, quartz hornblende diorite or 

-tonalite being the dominating type. The most noteworthy feature of the entire Coast Range mass of intrusives is 

their general uniformity in texture and their continuity. The variations across the range are apparently not so gradual 

~ those along its trend. The Coast Range massif consist of many separate interlocking batholiths, or batholiths 

·within batholiths, intruded at successive epochs but during the same general period of irruption. 

Knopf 2 has shown that in the region north of 1uneauthe diorite [part of the Coast Range massif] invades the rocks of th~ Berners forn;tation and is consequently post-Jurassic 

in age. The upper limit is indicated by the fact that on Admiralty Island Eocene conglomerates are found to contain 

pebbles of the diorite, showing that the intrusion took place in Cretaceous time. The general history of southeastern 

.Alaska suggests that the· diorite is of early rather than late Cretaceous age. 

RHYOLITIC LAVAS. 

The high peak west of Seal Cove is formed of a light-colored igneous rock which has been 

determined to be a rhyolite. It has been son1ewhat faulted, but appears to overlie unconiormably all the other rocks in the vicinity. This rhyolite was correlated by the Wrights 

with the Tertiary, ~nd there seems to be no reu,son to dou~t this correlation. The area occu-

·pied by this rock has been delineated mainly according to the map by the Wrights, as the 

-exposures in the interior of the island were not visited by the writer. Some changes, however, 

nave been made as to its eastern limits on the coast to conform with the new information obtained 

jn 1913. 

Rhyolites were also recognized by Brooks in 1901, for he states: 3 

Rhyolites are not ~ncommon in the Ketchikan region. They were found as dikes and small flows in all parts of 

the district. * * * These rhyolites probably belong to the same general period of extr~sion as the Kasaan greenstone. 

The age of the Kasaan greenstone was thu stated by Brooks: 3 

The Kasaan greenstone was extruded after the deformation of the Wales and Ketchikan series. It seems probable that it is of later origin than the Coast Range granite, ior it shows evidence of having suffered during the crustal 

disturbances which accompanied this intrusion. 

1 Wright, F. E. and C. W., op. cit., pp. 61-63. 

2 Knopf, Adolph, op. cit., p. 27. 

a Brooks, A. H., Preliminary report on the Ketchikan mining district, Alaska: U.S. Geol. Survey Prof. Paper 1, p. 50, 1902.