10 EXTINCT TYPE OF DOG. 



into a fine clay that is easily borne away by the streams that run at the 

 foot of the mountain. In this way the Cumberland escarpment has been 

 forced to make a very rapid retreat from the middle of the great anticlinal, 

 in which lies Powell's river, back to its present position. The total amount 

 of its retreat since the elevation of the Powell anticlinal, which probably 

 took place in the Triassic time, is not less than live, and may be as much as 

 seven miles. As the caves do not extend more than two or three hundred 

 feet back from the face of the escarpment, it is evident that they cannot 

 represent a very great lapse of geologic time. I do not believe that the 

 present series of caverns in this escarpment have endured mure than about 

 one hundredth part of the time occupied in the retreat of the escarpment 

 from the centre of the Powell anticlinal. Estimating the duration of time 

 since the Triassic period at from thirty to forty millions of years, the cav- 

 erns now lying in the bed of the Subcarboniferous limestone cannot have 

 been in existence for more than about three to four hundred thousand years. 

 Owing to the peculiar position of these escarpment caverns, there is little 

 chance that the waste arising from the destruction of one set of caverns 

 can be carried into another successive set of caves, as is the case in many 

 other regions. These considerations make it clear that a very great geo- 

 logical age cannot well be represented in these caves of the Subcarbon- 

 iferous series. 



The other series of caves in this region, the series to which the Ely 

 cave belongs, are very differently conditioned. They do not lie in vertical 

 escarpments, but upon the surface of a broad field of limestones, which 

 have a structure eminently adapted to favor the preservation of caves. 

 These limestones are of the Cambro-Silurian age, corresponding approxi- 

 mately in geological position to the series of rocks elsewhere called the 

 Cincinnati group. These beds represent a thickness of over a thousand 

 feet of limestones, which are, in the main, of the ordinary carbonate of 

 lime, but contain many thick beds of a dolomitic nature. These dolomitic 

 layers here, as everywhere, afford stoul barriers to erosion. Water works 

 its way through joints in their layers into the more erodable beds below, 

 and there excavates broad, flat caverns, which extend their arches until the 

 roof becomes too wide to support itself, when the cavern is opened to the 

 day. These layers of dolomitic limestone prevenl any rapid ablation of 

 the surface they cover, so that, while the face of the escarpment in which 

 tlie caverns of the Subcarboniferous limestone are found has retreated 



