ON THE ORIGIN OF COAL. 169 



proofs of a very quiet flow of water that 

 deposited the red clay — the recession of such 

 water — the drying and cracking of the clay by a 

 hot sun or air, and the return of a sharp current 

 of water, bearing along with it the sand that 

 formed the casts of the moulds, — circumstances 

 of great interest, to those who speculate on the 

 physical condition of the globe at that remote 

 period. 



Numerous such thin beds of clay are to be met 

 with in the Coal measures, alternating with beds 

 of sandstone, formed of grains of different sizes, 

 still no trace of desiccation is to be found like 

 those in the new red sandstone last described. 

 Such may have existed, yet all evidence of them 

 in England has been lost ; but Mr. Lyell, in vol. 

 II., No. 4, p. 25, of the second series of the 

 American Journal of Science, states, that he has 

 discovered footmarks of an animal, resembling 

 the Cheirotherium, in the middle of the Coal-field 

 in Unity township, five miles from Greensburg, 

 in Westmoreland county, Pennsylvania. The 

 markings occur on slabs of stone, a few inches thick, 

 between which are thin partings of fine unctuous 

 clay, where casts of the animals' feet in sand are 

 z 



