508 



■FOOTPRINTS OF 



[Ch. XXV. 



which is represented in the foregoing drawing (fig. 559). It dis- 

 plays, 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 b, 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 



Fie. 560. 



Series of reptilian footprints in the coal-strata of Westmoreland County, Pennsylvania. 

 a. Mark of nail ? 



