WESTEBN COAL FIELDS "I I 111. OHIO VALLK1 11 



tain that these carboniferous Btrata are now wearing down at the rate ol 

 en.- foot in about three thousand years, or about twice as fast ns 1 1 1 * - 

 osion of the valley in which they lit- On the supposition that 

 only ten million years have elapsed since the erosion of thi> country 

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

 upon the carboniferous section. 



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



- probably much less than it was during the greater rainfall of the 

 glacial period; and further, that the time that has elapsed Bince 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 Burface <>f the continent, froin the western plains 

 td the Atlantic an I north to the position of the great lakes. 



There are Beveral important conclusions which follow from this evi- 

 of the former wider extension <>!' the coal-measures. The most 

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

 <»t' the continent, which brought about the rapid changes in the nature ol 

 tin- deposits "i the coal time, must have affected, not portions of the 

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

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



- period. It is not possible to discuss them here: they will, how- 



l in the final report on the geologj ol Kentucky, which 

 i- now in preparation. 



