308 FOSSIL FOOT-PRINTS [CHAP. XXXVII. 



but as they cut their way into the bank, the mass ot 

 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 faith 

 ful 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 undersides 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 , of various sizes, which must have ex 

 isted in the clay. Such casts are produced by the 

 drying and shrinking of mud, and they are usually 

 detected 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 unfre- 

 quently 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 

 sions, whereas, when it afterwards dried up and shrank, 

 it would become too hard to receive such indenta 

 tions. I have alluded, in my former &quot;Travels,&quot;* to the 

 recent foot-prints of birds called sand-pipers ( Tringa 

 mi?iuta\ which I saw running, in 1842, over the red 



* Vol. ii. p. 168 



