408 
ELEMENTS OE GEOLOGY. 
clay, and casts of the same, has before been explained, and 
referred to the drying and shrinking of mud, and the subse¬ 
quent pouring of sand into open crevices. It will baseen 
that some of the cracks, as at 5, c, traverse the foot-i’ints, 
and produce distortion in them, as might have been expect- 
Fig. 438. 
Slab of sandstone from the coal-measures of Pennsylvania, with foot-prints of air- 
breathing reptile and casts of cracks. Scale one-sixth the original. 
ed, for the mud must have been soft when the animal walked 
over it and left the impressions; whereas, when it afterwards 
dried up and shrank, it would be too hard to receive such in¬ 
dentations. 
We may assume that the reptile which left these prints 
