« PředchozíPokračovat »
decide at what depths the rock is reached from the surface in the different parts of the City of Washington. Suffice it to say, that the results obtained from such excavations as have been made by digging wells and cutting down hills show a great variety of mineral matter and of successions of deposit.
We will speak first of the deposits as geological, then of the mineral matter, referring to the sources of it. At the lowest point penetrated, say 40 to 50 feet, we find successive beds of clays, sands, peaty earth, exogenous woods, in fragments, and silicified; others not silicified, but in a lignitous state ; others containing pyrites of iron; but these are confined to certain localities covered with water. In drier parts, where pyrites have been formed, the py* rites have undergone a decomposition, and the iron has become peroxydized, and shapen in various forms, according to that of the original nucleus. Thus we frequently obtain, in excavating, balls of iron, sand, or clay, like cannon-balls in form, but very light; and on breaking them open we find within the remains of a pine-knot or other vegetable matter, around which the sulphate of alumina or of iron had originally formed, on the carbonaceous matter. Subsequently the sulphur of the compound has been removed, and left the iron predominating in the state of iron-sand cemented together.
In 1856, or thereabout, in excavating I Street, at the junction of New Jersey Avenue, a log of silicified wood was removed at a depth of about 22 feet below the original surface. It was fully silicified throughout, with the strong marks of the grain of exogenous wood, of structure closely resembling the oak. It had crystals of quartz on its surface in great abundance.
The various strata of these deposits, especially the lowest, had evidently been deposited in quiet waters, as at the bottoms of lagoons or ponds, or stagnant pools where were first sand or gravel, then clays, then peat matters charged with iron; then, perhaps, some of these deposits repeated, and finally covered with ten or fifteen feet of drift. In all cases, the drift is on the top, and is very irregular in its character, generally consisting of masses or clumps of broken-up clay and loam, and pebbles, irregularly thrown together; and these constitute the mass of earth as found in the excavations of the streets in Washington and its suburbs. Clay, however, is the predominating earth throughout almost the whole District where the rock is covered with tertiary matters. The iron, being quite abundant and soluble in the carbonic acid of the air, is absorbed in the falling rain and surface water into the ground, and gives an iron deposit in nearly all water drawn from pumps and wells, conferring a degree of hardness which renders it objectionable for domestic uses.
The debris of the broken-down gneiss rock gives a clear, micaceous loam, that does not abound in clay ; hence, the hills about the District are more like other primary lands. The distinctive character of the mineral matter found in the tertiary of the District has been the result, not of one uniform action, but of several successive and different actions, and with long intervals between, in which peat and other vegetable growth has accumulated these followed by sudden inundation of sands or gravels, &c., and these at last by vast accumulations of drift. In all cases, the deposit was from an older formation;
and we find amongst the drift limestone and sandstone, in pebbles of various sizes, in rolled masses. In
the latter, the Delthyris arenosa, the peculiar fossil of the Oriskany -sandstone, in the New York system, has been repeatedly identified. But from what locality this fossil has come, is unknown. A sandstone has been also found amongst this drift, perfectly resembling, in granular structure, mineral matter, and shade of color, the Seneca sandstone, of which the Smithsonian building is constructed. These resemblances, although sufficient to indicate probability of source, are not sufficient to identify it. The mineral contents, beside those already named, are mostly siliceous, and such as would result from the debris of the gneiss of the substratum of the neighborhood
The principal water-course in the District of Columbia is the Potomac River, which, taking its rise in the Alleghany Mountains, receives the waters of several important streams, and, after a winding course of about four hundred miles, discharges into Chesapeake Bay. The principal tributaries of the Potomac are the Shenandoah, the Monocacy, the Conococheague, and the Anacostia, or eastern branch of the Potomac.
The tide-water of the Potomac ceases at the Little Falls, a romantic succession of cascades, three miles above Georgetown. At the Washington Navy Yard the average tide rises three feet, the Spring tide three and a half feet, and the neap two and a half feet. The southerly winds have a marked effect upon the height and continuance of the tides, and periodical freshets swell the volume of water. The Potomac is navigable as far as Greenleaf's . Point, for the largest class of vessels, as is evidenced by
the approach of the British squadron when the Capital was captured, and by the fact that the American frigate Minnesota, which was built at the Washington Navy Yard, was safely launched and successfully navigated down the river to Chesapeake Bay. Between the Navy Yard and Georgetown the channel has been filled up with denudations from the upper valley of the Potomac, but it has been recently dredged by the corporations of Washington and Georgetown, at a heavy cost, and is now navigated by the large steamers which ply between Washington and New York.
Within the District, the principal tributaries of the Potomac are Rock Creek, which separates Washington, on the west, from Georgetown, and the Anacostia, or east branch of the Potomac. The latter is a tidal stream, once capable of bearing large ships, and, until within a few years, navigated by a smaller class of vessels as far as Bladensburgh.
The scientific surveys of the Potomac have not yet been sufficiently accurate to determine the velocity of the current created by the tidal wave, and other important data require the researches of the United States Coast Survey. From the Potomac the following marketable fishes are obtained, amongst which the shad and herring, because of their abundance, are, in an economical point of view, the most important: cat-fish, chub, eel, gar, herring, perch (white and yellow), pike, rock-fish, shad, sturgeon, suckers, sun-fish, and various other small species. Of sturgeon, specimens have been caught weighing over three hundred pounds.
Mammalia.- Whatever may
have been the number of species of mammals inhabiting the District of Columbia in former times, the greater portion of them yet maintain a more or less permanent footing. Of those which formerly roamed over its surface, the wild cat (Lynx rufus), the panther, the American wolf, the black bear, the beaver, and perhaps the elk (Cervus Canadensis), are the only ones not found here at the present time, and it is even quite probable that the first-mentioned species still exists as a straggler. It is not probable that the buffalo ever lived in this region; the deer is not rare in the old ten-mile square; the otter even now is occasionally met with along the Potomac River, while foxes, rabbits, field mice, muskrats, and other species are more abundant than ever.
As far as accurate data are at our command, the following appear to be the characteristic features of the mammalian fauna of the District :
Of the Cheiroptera, or bats, about six species have hitherto been found. Of the Insectivora, there are three species of shrew mice, one of them a rare and little-known one, Sorex personatus. The common mole, Scalops aquaticus, and the star-nosed mole, Condylura cristata, also occur,—the latter here finding its southern limit.
Of the Carnivora, two species of fox, the red and the gray, are abundant. The ermine weasel, Putorius Noveboracensis, although not rare, is not often taken. It is too farsouth here to assume its white, winter dress, remaining brown the whole year. The mink (Putorius vison) was also common until the rise in the value of its fur caused