Page:Popular Science Monthly Volume 62.djvu/472

466 length, a thick, nearly horizontal stratum of carboniferous limestone, but the material excavated is believed, in most part at least, to have been deposited by either the river or the recurrent freshets in the tributary ravine. The excavation has a width of about ten feet, and a height of seven or eight, the roof being slightly arched, which, with the walls, have retained their shape without support. Upon the floor, for a thickness of one or two, or in some places perhaps more, feet, there are many broken pieces of limestone, and shales, for the most part worn and disintegrated, that were evidently the talus from the adjacent hillside of carboniferous rocks. With these, however, and sometimes at higher altitudes there are not a few larger masses of

sharp-angled limestone masses, lying horizontally. In this talus material a number of fragments of water shells were found. About three feet above the floor, on the west side of the tunnel, and extending nearly horizontally inward, there is a stratified layer of finer material. At places this stratum is pinched out and scarcely distinguishable, and later excavations show that it does not extend further than the end of the tunnel as first excavated.

Its material is not unlike that of the walls of the tunnel elsewhere, though less coarse. Above this stratum, evidences of water stratification are indistinct or wanting. In some places there are whitish horizontal streaks of limited extent, and some observers believe that distinct indications of stratification are shown in the disposition of the