THE GEOLOGY OF MEXICO. 613 



Palaeontologically and lithologically it may be divided 

 into three great subdivisions. 



The Lower Cretaceous is composed of green argilla- 

 ceous and marly rocks and calcareous and marly sand- 

 stones, covered by variegated clays. Sometimes it rests 

 directly upon the crystalline rocks and sometimes upon the 

 Upper Jurassic. 



The Middle Cretaceous consists of grey, blue, or blackish 

 limestones, which occasionally emit a foetid odour when struck 

 with a hammer. This is much the most widely spread of the 

 three subdivisions. It predominates in the north and east 

 of the country and forms the greater part of the Sierra Madre 

 Oriental. Sometimes it rests upon the Lower Cretaceous, 

 sometimes upon the Upper Jurassic, and sometimes upon 

 the Trias ; and it has suffered much from folds and faults. 



The Upper Cretaceous consists of light-coloured sand- 

 stones alternating with grey and blackish argillaceous de- 

 posits, which locally become more or less marly or calcareous. 

 This division is not so much affected by folds and faults as 

 the preceding, and frequently lies horizontally. 



Towards the close of the Cretaceous period, or at the 

 beginning of the Tertiary, began the great volcanic eruptions 

 which form so striking a feature in the recent geological 

 history of the country. 



The fauna of the Cretaceous is remarkably rich and 

 includes a considerable number of European forms. Hippu- 

 rites, Radiolites and allied orenera are met with ; and Ammo- 

 nites are abundant. The fauna of the middle division 

 (which corresponds with the Cenomanian, Turonian, and a 

 part of the Senonian) is naturally the most abundant, since 

 this division is the most widely spread.^ 



TERTIARY. 



The Tertiary deposits are found along the sea-coasts 

 and also on the central table-land. In the former case they 

 are marine, and in the latter lacustrine. 



^ I have not referred hitherto to the wealth of minerals in the various 

 systems ; but it is worthy of note that the Cretaceous Limestones are in 

 places impregnated with cinnabar and other ores of mercury. 



