THE GEOLOGY OF MEXICO. 615 



Pliocene and reached their maximum in the Quaternary- 

 period. 



RhyoHtes and trachytes, however, also occur. 



The fossils of the Tertiary are very abundant, and in- 

 clude both marine and terrestrial forms. Among the latter 

 may be mentioned Hippothei'ium and Protohippus. 



QUATERNARY. 



Quaternary deposits occupy a wide area in Mexico. 

 They cover a large part of the central table-land, and, in a 

 greater or less degree, the slopes of the mountains and the 

 floors of the valleys. But, excepting the great lava-flows 

 of the period, which are basaltic in character, they do not 

 offer any special points of interest. 



Philip Lake. 



