56 
and crossing the Louisville rocks, one approaches the dry 
lands of the “ subcarboniferous fo rmations.”* 
Here the rainfall is carried off by “ sink-holes,” and except 
where important streams flow in open gorges 300 to 350 
feet below the plain, the drainage is all under ground. Sink- 
holes are sometimes isolated funnels in the middle of 
depressions which might still hold lakes unless so drained, 
and probably did so once, and sometimes are the outlets at 
the bottom of dry ravines which have no mouths. The 
water entering these sink-holes passes away through 
cracks and caverns hollowed out of the weaker places in 
the rocks. When such a cavern channel is formed near 
the surface, the roof falls in at places, and a more or less 
open ravine appears. At times the sink-holes, after using 
an upper series of caverns for ages, have drilled down- 
wards by means of the gravel and rock churning in the 
bottoms of potholes till the water has found lower outlets, 
and when it has eaten out on lower levels waterways 
sufficient for the rainfall of the district, the upper 
caverns are deserted, the muds washed in harden on the 
* All this region wants the small valleys which we are accustomed to 
see in any country ; but in their place the surface is covered by broad, 
shallow, cup-like depressions or sink-holes, in the centre of which is a 
tube leading down to the caverns below. All this region is completely 
honeycombed by caverns, one level below the other, from the surface to 
the plane of the stream below. In one sense this set of underground 
passages may be regarded as a continuous cavern as extensive as the 
ordinary branches of a stream when it, flows upon the surface. The 
sink-holes answer to the smallest extremities of the branches. Some 
idea of the magnitude of these underground ways may be formed from 
the fact that the Mammoth Cave affords over 200 miles of chambers large 
enough for the passage of man, while the county in which it occurs has 
over 500 openings leading far into the earth, none being counted where 
it is not possible to penetrate beyond the light of day. 
