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POPULAR SCIENCE MONTHLY. 



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 





Entrance to the Excavation, Looking South. 



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 extend- 

 ing nearly horizontally inward, there is a stratified layer of finer 

 material. At places this stratum is pinched out and scarcely distin- 

 guishable, 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 stratifica- 

 tion are indistinct or wanting. In some places there are whitish hori- 

 zontal streaks of limited extent, and some observers believe that dis- 

 tinct indications of stratification are shown in the disposition of the 



