AVESTERN COAL-FIELDS OF THE OHIO VALLEY. 11 



tail! that these carboniferous strata are now wearing down at the rate of 

 one foot in about three thousand yeai's, or about twice as fast as the 

 average erosion of the valley in which they lie. On the supposition that 

 only ten million years have elapsed since the erosion of this country 

 began, there must have been something like three thousand feet of erosion 

 upon the carboniferous section. 



When we add to these considerations the fact that the present erosion 

 rate is probably much less than it was during the greater rainfall of the 

 glacial period j and further, that the time that has elapsed since the car- 

 boniferous period is in all probability twice as long as that we have 

 estimated, — we see how great is the probability that the coal-measures 

 once covered all the surface of the continent, fi'om the western plains 

 to the Atlantic, and north to the position of the great lakes. 



There are several important conclusions which follow from this evi- 

 dence of the former wider extension of the coal-measures. The most 

 important of these is that the uplifting and down-sinking of the sea, or 

 of the continent, which brought about the rapid changes in the nature of 

 the deposits of the coal time, must have aifected, not portions of the 

 continent, but nearly the whole of its area. This much increases the 

 difficulty of the problems brought us by the conditions of the carbon- 

 iferous period. It is not possible to discuss them here; they Avill, how- 

 ever, be treated in the final report on the geology of Kentucky, Avhich 

 is now in preparation. 



