Trans. N. Y. Ac. Sct. 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 
sull 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 suipassed 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 
-ot both oxide and sulphide. In the dry parts, the ceiling of the cave is 
more or less covered with efflorescent calcic, magnesic and sodic sul- 
