6i2 SCIENCE PROGRESS. 



Other rocks which have been referred to the Carboni- 

 ferous, have since been found in some cases to be of 

 Cretaceous, and in others of Triassic, age. 



MESOZOIC. 



Trias. — The Trias appears at one time to have covered 

 a considerable extent of country ; but it is now visible only 

 in a few scattered patches, of which the best known is 

 perhaps that of Los Brancos in Sonora, The formation 

 is also met with in Puebla and other states. 



The beds consist of quartzose sandstones and argil- 

 laceous deposits. They are very variable in character and 

 change not only vertically but also horizontally. 



Ceratitcs has been recorded from the Sierra de la 

 Encantada, in the state of Chihuahua ; but this range is 

 stated by Aguilera to consist of Cretaceous deposits. 



The fossils which have been found are chiefly plants, 

 and among these are species of Gangamopteris and Ma-, 

 croptcEuiopteris, genera which are characteristic of the 

 Glossopteris flora of India and the southern hemisphere. 

 Jiirassic. — The Jurassic rocks also are met with only in 

 scattered patches, the largest of which occurs on the borders 

 of Zacatecas and Coahuila, They consist of argillaceous 

 marls and marly sandstones, covered by calcareous marls, 

 and finally by limestones, which pass up quite conformably 

 into the Cretaceous. 



Fossils are abundant, but have not yet been closely 

 examined. They include a considerable number of Cephalo- 

 poda. Most of the species are held to be distinct from 

 those of Europe, but the most abundant are of Upper 

 Jurassic facies. 



Cretaceous. — The Cretaceous System, unlike the pre- 

 ceding, occupies a very considerable area. It forms the 

 greater part of the Sierra Madre Oriental and covers most 

 of the great central plateau, excepting where it is concealed 

 by Quaternary deposits. It also occurs in large patches in 

 the southern part of the Republic, south of the great 

 plateau. 



