Evening star. [volume] (Washington, D.C.) 1854-1972, 1885-07-11
OUR PATCH OF EARTH.
THE GEOLOGY OF THE DISTRICT
Story Of the Rocks. Gravels and Ter.
races as Interpreted by y Savant--
Records Of the Time when the Sea
Rolled Against Our Hills-The For.
nations Found in the District, &c.

nations Found in the District, &c.
Mr.. w. j. MCGEE, of the geological survey, q
synopsis of whose interesting address on the
terraces of the Potomac valley was published In
recent issue of THE STAR, has prepared the
following paper upon the geology OF the Distric
of Columbia:
1t is remarkable that although Washington
| has been the home of leading American geolo
gists for a generation, less has been known o
| the geology of the District of Columbia, unit
within the last few months, than of any othel
equal and equally populous area in the United
States. There are various reasons for this deartt
of knowledge of the geology of the District
First the biologists located in Washington
have generally been occupied in celebrating the
material collected in investigations elsewhere
second the geology of the region of which the
District of Columbia forms the central part il
exceedingly complex, and presents many prob
lens SO profound as to discourage the amateur
and third, there Is not in existence a topo
graphic map of the District. and contiguous pOr
tions of Virginia and Maryland necessarily in
volved in a geologic investigation of Washing
ton and Its environs, sufficiently detailed anC
accurate 10 be employed in reconnaissance.
Al the instance 'Of the director of the United
States geological survey. Major j. w. Powell
the writer began to devote his leisure moment:
TO the study of Washington geology in the lat
ter part of 1888; although it will be impossible
to set forth intelligibly the entire results of the
curate topographic maps now in course of con
struction, a few of the leading observations and
some legitimate differences therefrOm, may bt
brought together.
THE FORMATIONS OF THE DISTRICT.
In geologic nomenclature the term "forma
ton is applied to rock masses (which NAV be
characterized by any degree of lithificatioI
rom 1ncoherent sand or clay to the most Obdu-
rate quartz or granite) produced by definite
agencies during q definite period The range in
applicability Of the term cannot be illustrated
better than by describing the rock masses OF
the District of Columbia to which the term has
been applied.
But three essentially distinct formations occur
Within the District of Columbia. These are: a.
the washington gneisS, which includes the
gnelsses, mica shists, amphibolites (nassive
green stones and related crystalline rocks SO
Well exposed along the Potomac above George
town; (2) the Potomac formation, comprising
the laminated clays, sands and gravels spread
over the highest lands of the District and 49,
the Columbia formation, made up of the brick
Gays, sands and gravelS prevailing throughout
the blufi-bOund amphitheater in which Wash
lngton Is located. The first of these formations
represents an early stage in geologic history;
the second represents one of the middle stages;
the third represents the latest of the geologic
epochs and the cons intervening between the
formation of the first and the second, and again
between the second and the third. are en-
tlrely unrepresented in the District by the de-
posts known to have been laid down elsewhere
during each period of their continuance.
THE WASHINGTON GNEIss
is exposed along the Potomac as far east as the
mouth of Rock creek. and still further to the
eastward appears occasionally in the natural
and artificial excavations on both sides of the
river. Its last appearance on the south side of
the river is near the angle in the Old George
town and Alexandria road in the eastern part
of the village of ROsslyn. On the north side OF
the river appears in the street cuttings tar
east as oath street, and was reached in the water
works shaft, just east of Howard university,
about 100 feet beneath the surface. 1t would
probably be found ut about that depth on Capt.
toi Hill, and at corresponding depths through
out the city generally. The various exposures
OF the formation about its eastern margin indi-
cate that 1t is an irregular surface sOpng gently
to the eastward, but, whether buried beneath
newer deposits or not. diversified by hills and
valleys of height and depth perhaps equal to
those Characteristic OF its present surface further
to the westward.
Every citizen of Washington is familiar with
the rocks derived from this formation. They
are quarried on both sides of the Potomac below
Chain bridge, on the W cooley road near Rock
Creek, and else where and are extensively used
for paving, for curbstones, for foundation
walls, and for other purposes. They have also
been removed in gre.; quantities from the new
water works tunnel extending from the dis
tributing reservoir to Howard university.
The gneisses, mica states, and green stones of
the formation are deeply decayed specifically;
and the depth of this decay illustrat 5 the vast
length of the periods with which the geoloeist
has to deal. Paving stones a few inches in
thickness resist atmospheric decay and the
wear OF street traffic for years: the curbstones
put in place when the streets of Washington
were first paved are Yet sometimes as perfect
as the day they were laid down: the stones in
over-ground foundation walls built half a cen
fury ago have but a film of partially decayed
rock on their exposed surfaces: but the decay of
the undiSturbed rock masses. which has nrO-
crossed not one whit faster than that of the
curbstOnes and rubbie of the foundation walls
has gone on through . period so great that it
extends to depths OF ten feet, twenty feet, fifty
feet, and even, In one known case more than q
hundred feet! And even one hundred feet does
not represent the total amount of secular de-
composition of the gneiss since its surface was
raised above the level of the seattle quartz
veins and the abundant quartz pebbles of the
Potomac and Columbia formations tell nf eX an
greater decay in the earlier geologic cons, other
wise unrepresented in the deposits of the Dis
trIct: The gneiSs Is intersected by many veins of
quartz, sometimes associated with mica, gar
nets, tOurmaline, and other minerals. This
vein-quartZ is one OF the most obdurate rocks
known. and has pretty etfectually resisted the
secular decay by which the gneiss is so pro-
foundly affected. SO the wider veins are rep
resented geographically by knobs and ridges
from which the quartz is extensively auarried
tor road-making, etc. But with the diSap-
pearance of the walls between which the veins
were originally confined, even the obdurate
quartz rock has yielded to its own internal ex-
pansiOn, To the action of frost, of beat
ing rain, ripping rll, and rushing tor
rent, to such an extent as to give origin
to great numbers of angular fragments which
now lie scattered over the surface. During the
geologic period represented by the Potomac for
nation such fragments were even more album
dainty scattered over the surface then existing
and were gathered up by the streams swept
into the sea. and washed about by wave and
tide in an ocean rising 500 feet above present
sea level, until they and the sand grains
formed ny t.ielr trIturation Were finally depOs-
ited in the sea bottom. Thus, by their elevation
above the contiguous gneiSS the quartz ridges
about TenallytOwn and Fort Marcy tell of
deeper decay OF the gneiSs than is indicated by
the water works shafts: for the gneiSS once rose
as high as the highest of the quart ridges. and
by their abundance in the Potomac formation
the quartz pebbles tell of still deeper decay dur
ing the geologic past: for by just as MANY feet
as the quartz veins We're worn down to furnish
these pebbles, by just So many feet was the in
teryening gneiss dissolved away. An accurate
amount
and Of the time during which it was accom-
piished is impossible without more definite
data: but the most conservative geOlogist
familiar with the facts would readily cOncede
that the atmospheric decay of the Washington
beating rain and corradin9 stream-is measured
by hundreds OF feetand the time by hundreds
oil thousands, if not millions of years
Thus far the Wtshin%ton gueiss has not been
certainly correlated with the other crystalline
formations of The United States. Like other
crystalline rock masses il is unfo,siIif ferrous.
and since II Is Oycrlain on the east by newer
NesCzDc and on the west by older Mesozoic
rocks, It Is Impossible 10 determine its strait
graz nc relations TO the fileoZoic formations nf
the Appalaciiaus. II is doubtless the continual
ton oi The belt OF gnelsS and mica shists reach
ing SO great extent In New England forming
the sub>traiun on which New York is built
sinking beneath the surface in Staten Island
Fearing again near Philadelphia. and extend
ing thence through southeastern PENNSYLVANIA
into Maryland bru while most of the practical
questions connected with this great oneisSic
agreed whether Ii should be referred To the
laturentlan or regarded as metamOrphic Paleo-
Zoics.
THE POTOMAC FORMATION.
The name Potomac formation" is applied tn
the notified clays, gravels and Illuminated sands