Evening star. [volume] (Washington, D.C.) 1854-1972, 1885-07-11

braNch-nOtaDIy
southeast Of Uniontown and east of the Insane
Asylum. The pebbles are sometimes cemented
by carbonate or oxides of iron into firm con-
giomerate or puddingstone, which Is used loI
purposes of construction, just as the sands Of
the subjacent and superjacent divisions of the
formation are sometimes similarly cemented
into firm sandstone. The puddiugstones and
sandstones of this character are used in founda.
lions for buildings and abutments for bridges
the wall inclosing the grounds of the Insane
Asylum is constructed Of such ferruginOus pud.
dlngstones: Where not cemented, the graves
ern margin OF the formation the pebbles in this
medal division become coarser and less per
fectly rounded; and the division increases in ab
solute and relative thickness Its graveis gene.
rally cap the eminences of the western part Ol
the District; old Fort Reno, north of Tenaily.
structed Of these gravels and their associated
clays; the Rockville road, from the heights north
OF OF. Georgetown to Tenaliytown, exposes the
graveis In many of its cuttings; the LOughborC
road, extending from Tennallytown southwest
waTa us Is largely upon the graveis OF this division
tomac, from Chain bridge to near Fort Myer, is
crested with similar grayels.
The peculiar relations of the gravels Gaping
waterways Is one of the most puZzling features
IN the geology OF the District. In many cases
the ,grayels upon the hilltops have evidently re-
manned undisturbed since their original depo
sition. for they contain intercalated strata clay
and sand. But on the slopes Of the hills the
formation sometimes becomes modified, and
passes into talus made up mainly of the gray
els of this formation, but undergoing progress
I've modification and increase in volume from
finally merges into the Quaternary or recent
alluvial gravels and sands.
The uppermost or notified clay division OF
the Potomac formation is well exhibited in the
railway cutting between Washington and Bal
tmore, and in nearly every natural or artificial
excavation on the east side of the Eastern
branch. Like the lower division of the forlna,
ton, it is destitute OF definite structure-its
lines of bedding are seldom continuous through.
ont Of
the intercalated beds of gravel and sand are
lenticular in form, irregular in distribution and
quite local. Its material is mainly fine clay,
low, gray, brown, red, buff, or orange, by oxides
of iron. Like the lower division 1t contains oc-
casional bands of coarse sand or gravel not
readily distinguishable from those Of the me-
dial division and scattered boulders OF consid-
cradle dimensions.
some miles west of its present margin. The ma
terials in these outliers consist mainly of peb-
bles and boulders, much coarser than those
ton, associated with highly colOred clays. Such
outliers reach maximum attitudes of some
thing over 600 feet. The pebbles and clays in
evidently the shore deposits of the sea in which
the finer materials of the Potomac formation
were laid down at greater distances off shore,
but others are destitute of definite structure;
and since the easternmOst Outliers OF the Trias-
sic red sandstone contain similar pebbles, it is
possible that these are resdua left after the de-
composition of the matrix of red sandstone in
which they were originally embedded. A def
ntely bedded CUTLER unquestionably referable
to the Potomac formation occurs at Peach
Grove. five miles south of Great Falls a gravel
capped eminence, in which the gravels appear
t. be triassic residua. occurs about Bealsville,
Md; and a gravelly plateau, in which the rela-
tions of the materials have not been deter
mined. occurs.sx miles southwest OF Seneca, on
the Leesburg and Alexandria turnpike.
Though of different degrees of fineness, the
materials of the different divisions of the entire
formation are maniiestly of common origin.
The ,quartzitic sands, gravels and occasional
boulders OF the upper and lower divisions and
the pebbles OF the medal division are mani
testify derived in large part las already init
mated) from the quartz veins intersecting the
gneissic zone to the westward; but a consider
able portion of the pebbles are not of quartz but
ofquartzite lithoiogicaly identical with that
sugar Loaf Mountain, thirty to fifty miles to
the westward and are unquestionably derived
there!rom. \ few of the pebbles too, are of
limestone, sandstone, slate and other plastic
rocks, sometimes containing paleozoic fossils
evidently derived from the paleozoic forma-
tions OF the Appalachian mountains extending
from the Blue Ridge westward. The finer ma
terials of the formation are re-depOsited pro-
ducts of the decomposition of the Washington
gneiss, of the Triassic red sandstone (which lies
immediately west OF the gneissic zone, and OF
the paleozoic beds Of the Appalachians, to-
gether with finely triturated debris from all of
The structure of the Potomac formation indf-
Gates that it is shallow water deposit, and was
laid down near the oscillating shore in which
tude of the deposits proves that the surface Of
this sea rose 600 feet above present tide level.
With the exception of silicified and lignitized
wood, such as occurs In the new reservoir, the
Potomac formation is without fossils in the VII
entity OF Washington, and its place in the
geologic ,column mas accordingly not been
accurately ascertained. A few miles to the
southwark, however, abundant impressions
OF leaves have beer found in 1t by Prot. Fon-
species OF the plants upon which they grew
have been determined; and equally we'll pre-
served leaves have been found IN the vicinity
OF Baltimore. In so far as the testimony OF a
single class of fossils can be relied upon in the
establishment of geologic equivalence, these
plants indicate that the formation is lower
the European NeOcoman. The position of the
formation in the geologic column is more re-
liably but less closely determined stratigraphi-
cally: to the eastward it passes unconformably
beneath a1undantly fossilferous eocene marls
and green sands, and it is therefore PREFER
tiary. Also its undisturbed condition proves
CURED in this region since 1t was deposited
while the older Mes0zoic deposits TO the west
and it must accordingly be much younger than
the older MeSOZOic.
HOW THE CONTINENT LOOKED THEN.
Although so well preserved, the fossil leaves
found by Prof. Fontaine at Fredericksburg and
Richmond and by Prof Uhler at Baltimore are
unlike those of the plants that prevail in the re-
gion to-day. They represent palm-like cycads
coDiferOus trees which find their living represen.
u OIL
ferns which apparently fiourshed with tropical
luxuriance over the low swampy plains form
ing the shores OF the sea in which the Potomac
formation was deposited and other plants of
archaic or foreign aspect, associated with the
earliest known dicotyledOns-an order Of
plants including the major portion OF the pres
ent flora of these regions. 1t Is partly by means
of these faint leaf impressions that we are a BILE
to determine how great a period has elapsed
since the formation was laid dOwn-hOw slowly
and gently the sometimes violently active geo
logic agencies have wrought upon the eastern
side OF our continent: when the Potomac sands
and gravelS were washed about by wave and
tide the general coniiguration of the eastern
from and only one or two hundred miles in
land of the present Atlantic coast; but the CONT
nent had not reached half of its present area A
great bay extended from the southern
now northeastern Alabama northwestward
through the Mississippi valley, in western ILL
nois and Wisconsin and eastern Iowa and Min
nesota, far into British America-perhaps to the
palms, ferns and strange tropical plants OF the
Potomac formation grew upon the shores of the
gradually contracting Atlantic and were swept
into the sea by tropical storms, most Of the
region now forming the Rocky mountains was
q quiet sea bottom. When the prototype of the
Potomac river wendell its way from the Appa-
lachian mountains to the sea in very nearly its
present upper course, II was equal in size and
length to the infant Mississippi, the grand
Colorado, now flowing in IL channel mile in
Arkansas and other great rivers Of the south
west had no existence. No better illustration
of the variable activity of certain geologic
agencies Is known to biologists than is offered
the valley of the Potomac with the many times
more rapid development Of the great mesa
country drained by the Colorado. A like vari;-
aud ta1o fi<,y, of Dot