3G0 ON THE ANTIQUITY OF THE CAVERNS 



factory way are wanting, ]jut there ai'e certain inferences which may bring us very near 

 the truth, or at least enable us to fix a superior limit for their duratiou. 



The computations which have been made as to the amount of wear over the surface of 

 the Mississippi valley, have shown that about one foot in each six thousand j^ears passes 

 away from the surface of the whole valley; as this quantity is determined by the amount of 

 solid matter passing into the Gulf of Mexico during a given time, it is of course averaged 

 over the whole basin. But it is a well known flict that the Ohio valley furnishes by far 

 the most considerable amount of sediment in proportion to its area of any of the large 

 rivers tributary to the main stream. Its vohmie of water is actually rather larger than 

 that furnished by the Missouri, thougli the area whence it comes is not quite half as 

 large, so that if the amount of sediment be proportionally as large, the amount of 

 erosion should be about douljl}' as great. This estimate of one foot of erosion in every 

 three thousand years is doubtless not far from the truth at the present time ; but as 

 the waste during the more ancient forest-clad state was somewhat less, we will suppose 

 the slower rate of one foot in six thousand years. It must be noticed that the stream 

 beds of any country must cut down more rapidly than the general surface of the country 

 they drain, and the difference in these rates of cutting is approximately measured by the 

 difference in the level of the river beds and the upland country. Now as the stream beds 

 occupy not over the twentieth part of the area in this country, they must cut down 

 much more rapidly than the table land about them. My own impression is that their cut- 

 ting rate is at present three or four times as fast in this particular section, but we will not 

 make any allowance for this difference, letting it go as a means of making the assurance of 

 the average rate of one foot in six thousand 3-ears the safer as a basis for calculation. 

 On this basis we have next to consider the thickness cut through since the streams struck 

 the limestone. At the Mammoth Cave we may estimate this at two hundred feet. As we 

 go to the east the level of the limestone rises rather more rapidly than the beds of the 

 streams, so that we find that aliout three hundred feet is the maximum height of the lime- 

 stone above the stream beds. At the rate of one foot in six thousand years we get 

 about t^vo million years as the maximum time during which these caverns could have ex- 

 isted. If we apply the correction for the difference in the rate of cutting of stream bed 

 and upland, we shall have to shorten this time to less than a million years, possibly to seven 

 handred and fifty thousand years. 



Although not exact, a defect common to all geological time ratios, these calcidations 

 have for us great value, as they serve to show that the caverns of this district are of com- 

 paratively modern origin. It is not reasonable to suppose that a million of years can carry 

 us much farther into the past than the early part of the pli(jcene period, the latest of the 

 complete divisions of the tertiary period in our northern section. If, then, these caverns 

 have not inherited their life from preexistent caves which have since been entirely destroyed, 

 then we may suppose that they will serve as measures of the rate of change in organic life 

 in a very satisfactor}- way. It is impossible to discuss this question in detail at the present 

 time, but even with our imj^erfect knowledge some hints may be had towards its solution. 

 The sandstone series al)ove tlie limestone furnishes no caverns, and as it has a thickness of 

 at least three thousand feet, and wears rather slowly, there could have been no succession 

 there. The only possil/dity would have been through the original extension of the carbon- 

 iferous limestone over the region now underlaid b}' the devonian series, including the Wa- 



