CHAP. XXXVII.] FOSSIL FOOT-PRINTS IN COAL STRATA. 229 



organization than fishes, were created till after the carboniferous 

 strata had been elaborated. 



During my stay in Westmoreland County, I was indebted to Dr. 

 King for the most active assistance in the prosecution of my inqui 

 ries. He kindly devoted several days to this object, and we first 

 visited together a stone quarry in Union township, six miles 

 southeast of Greensburg, on a farm belonging to Mr. Gallagher, 

 where the foot-marks had been first observed, standing out in 

 relief from the lower surface of slabs of sandstone, resting on thin 

 layers of fine clay. These slabs were extracted for paving-stones, 

 and the excavation was begun in the bank of a small stream, 

 where there was at first a slight thickness only of shale overlying 

 the harder beds ; but as they cut their way into the bank, the 

 mass of shale became so dense as to oblige them to desist from 

 the work. Between the slabs of stone, each a few inches thick, 

 were thin parting layers of a fine unctuous clay, well fitted to 

 receive and retain faithful impressions of the feet of animals. On 

 the upper surface of each layer, Dr. King saw the foot-steps im 

 pressed more or less distinctly ; but, as the clay was left exposed 

 to the weather, it had crumbled to pieces before I examined it, 

 and I had only an opportunity of seeing the casts of the same 

 projecting in relief from the under sides of slabs of argillaceous 

 sandstone. I brought away one of these masses, of which the 

 annexed figure (fig. 12) is a faithful representation ; and it will 

 be observed that it displays not only the marks of the foot-prints 

 of an animal, but also casts of cracks, a, a , of various sizes, 

 which must have existed in the clay. Such casts are produced 

 by the drying and shrinking of mud, and they are usually detect 

 ed in sandstones of all ages in which foot-marks appear. It will 

 be seen that some of these cracks, as at b, c, traverse the foot 

 prints, and they not unfrequently 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 impressions, where 

 as, when it afterward dried up and shrank, it would become too 

 hard to receive such indentations. I have alluded, in my former 

 &quot;Travels,&quot;* to the recent foot-prints of birds called sand-pipers 

 * Vol. ii. p. 168. 



