M. T. Hill — Cretaceous Formations of Mexico. 307 



Art. XXXYII. — The Cretaceous Formations of Mexico and 

 their Relations to North American Geographic Develop- 

 ment ; by Kobert T. Hill. 



Mexican geology has been little studied in the light of the 

 broader problems of continental development. In this paper 

 it is proposed to present a few facts in its Mesozoic history 

 which bear upon this relation and are based upon fragmentary 

 literature and upon the observations of the author. 



Mexico is a southern geographic continuation of the Cordil- 

 leran region of the United States. It consists of an interior 

 plateau or basin enclosed upon the coastal sides by groups of 

 marginal Sierras which are comparable to, although not geneti- 

 cally identical with, the Rocky Mountains and Sierra Nevada 

 systems of our own country. These are known as the Sierra 

 Madre of the east and the Sierra Madre of the west respec- 

 tively ; they unite into a common mass and terminate near the 

 longitude of the City of Mexico. The profile of the southern 

 union of the Cordilleras was the origin of the popular con- 

 ception that the interior of Mexico is a great plateau ; as seve- 

 ral writers maintain, it is, really, an intra-mountain basin 

 region between the crests of the Sierras, analogous to and 

 continuous with the Great Basin of our own southern border. 

 The present coastal plains of Mexico are but slightly devel- 

 oped and will not be discussed here. 



The geological formations of Mexico may be classified in 

 four general groups: 1. The Pre- Cretaceous rocks which were 

 more or less completely buried beneath the earlier Cretaceous 

 sedimentation and except possibly in Sonora are now only 

 slightly exposed by the erosion of the later formations. 2, 

 The two Cretaceous marine formations which constitute the 

 prevailing sedimentary rocks of all the mountain structure. 



3. The Post-Cretaceous eruptives which occur in associations 

 with the Cretaceous formations in the mountain structure. 



4. The detrital fresh water deposits which cover the intra- 

 mountain plains and valleys. Of these the events connected 

 with the Cretaceous and early Eocene history of Mexico are 

 the most important. 



Two contributions have lately been published upon the 

 occurrence and relations of the Cretaceous formation in Mexico, 

 both of which were founded upon personal observations by 

 the writers in the southern half of the Republic. They differ 

 widely in their conclusions, however, and show that their 

 authors were not entirely familiar with the sequence of the 



. Am. Jour. Sci. — Third Series, Tol. XLV, No. 268.— April, lfiJ)3. 

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