AXD CAVERN LIFE IX THE OHIO VALLEY. 357 



point of the existence of a single cave above the tide level. So far from caves being evi- 

 dences of marine action, it is a safe statement that their occurrence in any district may 

 generally be taken as evidence of long continued immunity from depression below the sea. 



Taking the geological limitation of caverns, we notice at once that they are practically 

 confined to limestone districts. When they occur in sandstone regions they are generally 

 limited to narrow rifts that practicall}' afford no sufficient field for cavern life. In a few 

 cases I have found these sandstone caves enlarged by the occupancy of wild animals. Bears 

 especially, and to a cei'tain extent other animals often serve to enlarge caverns by rubbing 

 against their sides, carrying away considerable quantities of dust upon their bodies, and 

 distributing it in their wandering as it is shaken from them by the bushes in their track. 

 Throughout Kentucky, in the regions of the carboniferous sandstone and the Waverly 

 sandstone, we have thousands of shallow depressions cominonlj^ called "rock houses," 

 caused by the more rapid recession of the base than the summit of a cliff; but, while the 

 area of these chambers is often considerable, I have never, in the hundreds I have visited, 

 seen a single case in which they were prolonged into true caverns, or had any part of 

 their area beyond the reach of dajdight. In fact, we cannot regard these shallow caves 

 as giving us either the darkness or the uniform temperature essential to the development 

 of the peculiar species of cavern animals. 



The whole of the true caverns of the Mississippi valley, so far as I have been alile to 

 ascertain, lie within the limestone regions of that valle3^ The limestones of the Cincinnati 

 group, especially the upper himdred feet, contain a few of these ; they are all very shallow, 

 and I have grave doubts whether, in a single case, they extend much beyond the light of 

 da}'. Their channels are so narrow that the water floods sweep them as with a besom, 

 driving out all the life that might attempt a lodgment there. When the streams cutting 

 deeper into the rock leave the cavern quite dry, their walls arc so feeble that they rapidly 

 become closed with rubbish. I have never yet found a cavern in this series of rocks with 

 a surface which did not bear the impression of being stream swept. It is only when 

 we get into the massive limestone which lies just below the coal that Ave find true cav- 

 ei'ns on an extended scale. Here we find a marvelous development of such structures. 

 Within the State of Kentucky there are not far from eight thousand square miles where the 

 carboniferous limestone lies in positions suitable for the joroduction of caves. In this region 

 the limestone varies in thickness from a few feet to three hundred feet or more ; it generally 

 exceeds one hundred feet, and may be safely averaged at one hundred and fifty feet. When- 

 ever wo have this rock between the surfixce and the drainas;e level all the small streams 

 at once desert the surface, and only the considerable rivers find the light of day. We ma}- 

 journey for fifty miles or more in the wet season without crossing running water ; when we 

 find it at all it is in the shape of a considerable river whose waters with nearly unvarying 

 temperature in all seasons find their way into its bed through arches at the level of the stream. 

 These arches lead us into caverns which rise stage on stage into the region on either side of 

 the river. On the surface we find hills and the similitude of stream channels arranged as 

 in ordinar}^ countries, the valleys branching as in those which are stream swept, but there 

 are no channels ; only in the bottom of the valleys, be they great or small, a great number 

 of more or less circular depressions, each usually with a pit leading down into the caverns 

 below. Over most of this country these pits will average about one hundred to the square 



MEMOIRS BOST. Son. NAT. lilST. VOL. II. 90 



