Trans. N. V. Ac. Sci. 60 Dec. 12, 



fully 6° lower than has been commonly supposed, and may be taken as 

 a fair representation of that of the crust of the earth in the country 

 immediately surrounding it. 



Mr. Stevens exhibited a geological map of Kentucky, showing the 

 area of sub-carboniferous limestone in which the Mammoth Cave is 

 situated. This is overlaid with a thin stratum, mostly of sandstone, 

 that is pierced by thousands of sink-holes, through which the surface 

 drainage is carried down into limestone fissures and thus to the 

 general drainage level of the Green River. This stream passes at the 

 distance of less than a mile from the Cave Hotel, the floor of the latter 

 being 312 feet above the water and 118 feet above the mouth of the 

 cave. He briefly explained, with a diagram, the general mode of cave- 

 production in limestone strata, showing that subterranean tunnels must 

 be started by the solvent action of slightly acidulated rain-water, and 

 subsequently enlarged by erosion, along the fissures in the limestone. 

 These agencies are still at work in portions of the cave, and the whole 

 of this limestone country is thus honey-combed with caverns. No 

 tunnel can be thus formed at any point lower than the general drain- 

 age level, since there must be an exit for the saturated water. The 

 production of the fissures is referable to the general upheaval of this 

 area at the close of the coal period : but, that there has been subsidence 

 since the completion of much of the Mammoth Cave, is indicated by 

 the fact that at its lowest parts to-day the floor is covered with water to 

 the depth of thirty feet or more, having subterranean connection with 

 Green River. The fissures intersect at various angles, but many of 

 them are nearly or quite coincident with the dip of the strata, which 

 is very gentle. Water passing through these forms the tunnels, while 

 that passing through the vertical fissures scores out the pits which 

 pierce them. The same pit, starting from a sink-hole at the surface, 

 may have successively lower tunnels as exit passages. If the visitor 

 encounters it while walking through the higher, and therefore older, 

 tunnel, the upper part appears to him as a dome, the lower as a pit. 



The rate of erosion in the Mammoth Cave has been variable. The 

 older parts are perfectly dry, and entirely free from .stalagmitic deposits, 

 indicating rapid erosion, followed by elevation, so as to deviate the 

 water completely into other channels. In the newer parts the water is 

 still dripping from the surface above, and depositing stalactites and 

 stalagmites ; but as a whole the cave is by no means remarkable for 

 these formations, being much sui passed in this respect by the neigh- 

 boring White's Cave, of more recent origin. Those which do occur are 

 moreover deeply colored with iron, which exists in the soil in the form 

 ■of both oxide and sulphide. In the dry parts, the ceiling of the cave is 

 jTiore or less covered with efflorescent calcic, magnesic and sodic sul- 



