FOOTPRINTS OF 



Fif?. 512. 



[Ch. XXV. 



Scale one-sizih the original. 



Slab of sandstone from the coal-measures of Pennsylvania, with footprints of 



air-breathing reptile and casts of cracks. 



m North America. These occur in the coal strata of Greensburg, in 

 Westmoreland county, Pennsylvania; and I had an opportunity of ex- 

 amining them in 1846. I was at once convinced of their genuineness, 

 and declared my conviction on that point, on which doubts had been 

 entertained both in Europe and the United States. The footmarks were 

 first observed standing out in relief from the lower surface of slabs of 

 sandstone, resting on thin layers of fine unctuous clay. I brought away 

 one of these masses, which is represented in the accompanying drawing 

 (fig. 512). It displays, together with footprints, the casts of cracks (a, a') 

 of various sizes. The origin of such cracks in clay, and casts of the 

 same, has before been explained, and referred to the drying and shrinking 

 of mud, and the subsequent pouring of sand into open crevices. It will 

 be seen that some of the cracks, as at 5, c, traverse the footprints, and 

 produce distortion in them, as might have been expected, for the mud 

 must have been soft when the animal walked over it and left the impres- 



