56 



THE AGE OF MAMMALS 



Fig. 12. — The imbedding of skelituu.s in ituiiim, ur wind-drift depo!>it». Alju\e : Recent 

 times. Partially imbedded skeleton of an ox on the plains of South Dakota. Below : Miocene 

 times. - Partially exhumed skeleton of the fossil forest horse, Hypohippus, on the plains of 

 Colorado. 



The chief kinds of mammal-bearing rocks, that is, sorted as to mineral 

 composition, are as follows: 



1. Conglomerates {"Conglojnerats," " Konglomerate") . Composed of weather- 

 worn pebbles, gravels, and sands. They are evidence either of advancing or re- 

 treating shore lines of the sea or of river channels, or mountain streams where they 

 spread upon the plains. 'Mud-ball' conglomerates are rather common in the 

 western tertiaries. On the old sea borders of Eocene France we find the Gravier 



