A SKETCH OF THE GEOLOGY OF MEXICO 389 



The beds of the lower division are largely shales, clays, marls, 

 and greensands. The middle formation is mainly made up of 

 compact ash-gray limestones frequently magnesian, though not 

 in general constituting true dolomite. The limestones are rich in 

 fossils, though they have been much metamorphosed. The upper 

 member occurs only in the northeastern portion of the country 

 and is represented by fine to medium grained, gray to red and 

 yellow sandstones, alternating with clay shales of gray to black 

 colors. 



The advance of the sea begun in the Jurassic, continued until 

 the country was converted into an archipelago at the end of the 

 Middle Cretaceous. There was then a general elevation, carry- 

 ing all but the northeastern portion of the country along the Rio 

 Grande, above the sea. This elevation was accompanied by fold- 

 ing and mountain-making, continuing into the Upper Cretaceous. 

 It was at this time that the main masses of the Sierra Madre of 

 the east and west coasts were ridged up. 



The Upper Cretaceous was laid down by a retreating sea and 

 by the close of the Mesozoic, Mexico had its present general out- 

 line. There were, however, certain differences. Although the 

 country then as now formed a great triangle with the apex in 

 Central America, the width of the landmass was much less 

 than at present. The Pacific coast line was farther west and 

 Lower California was not yet separated from the mainland. 

 The Gulf of Mexico had a more irregular coast line and extended 

 to the west and southwest, probably uniting with the Pacific 

 south of Guatemala. Yucatan and Florida were as yet covered 

 by the ocean. 



During the Eocene there was a series of vertical oscillations, 

 but the total result was an increase of territory. In the Miocene 

 there was along the east coast an invasion of the sea, though not 

 so as to cover the whole of the Eocene area. At the same time 

 the Pacific advanced inland and the first peninsula of the Repub- 

 lic was cut off, forming Lower California. Toward the close of 

 the Miocene a new movement of elevation in the Atlantic region 

 caused the sea to abandon most of its former dominion, the ele- 



