A SKETCH OF THE GEOLOGY OF MEXICO Z^7 



Aguilera has succeeded no better than in searching for the Silu- 

 rian. A careful study of localities said to have furnished 

 Devonian fossils has only resulted in showing the presence of 

 certain prc-Jurassic rocks whose age, because of the absence 

 of fossils, and the metamorphism which the rocks have suffered, 

 cannot be definitely fixed. 



Carboniferous rocks of undoubted authenticity occur along 

 the Guatemala border directly below the Cretaceous. The 

 rocks are compact ash-gray limestones, containing Productus 

 semireticulatus. Large areas of rocks in the central and northern 

 portion of the country, assigned by Frazer, Hall and others to 

 Silurian, Devonian and Carboniferous, are now known to be either 

 Cretaceous or of unknown age. 



In the absence of rocks belonging to the first two periods of 

 the Paljeozoic, it seems probable that Mexico, which during the 

 Archaean was reduced to a group of islands, or perhaps to a single 

 narrow peninsula stretching from California to Tehuantepec and 

 Chiapas, suffered during the Silurian and Devonian a continu- 

 ation of the ascendant movement which began at the end of the 

 Huronian. The complete absence of stratigraphic and palaeon- 

 tologic data relative to the first subdivision of the Permo-Car- 

 boniferous authorizes the belief that during this time the eleva- 

 tion continued, and makes acceptable the hypothesis that it was 

 during this time that the various islands became united and 

 formed the skeleton of the country. 



The Mesozoic is represented in Mexico by beds of the Upper 

 Triassic and Jurassic, as well as the whole of the Cretaceous. 

 The Triassic beds indicate a period of depression. The sedi- 

 ments accumulated in marshes and estuaries along the western 

 coast to a thickness probably of lOOO meters; sediments 600 

 meters in thickness being now found in Sonora. The deposition 

 was interrupted by minor elevations as is shown by lithological 

 variations and the presence of basal conglomerates. The depos- 

 its cover a considerable area and are composed mainly of gray, 

 red and yellow sandstones and gray to black slates. In general 

 the rocks outcrop on the crests of low hills and ridges, being 



