6io SCIENCE PROGRESS. 



Geologically, the rocks which occur in Mexico, may be 

 divided into three distinct groups. First, we have the 

 ancient crystalline and schistose rocks, which are presumed 

 to be of Archaean age ; secondly, the sedimentary deposits, 

 which extend from the close of the Palaeozoic era to the 

 present ; and lastly, a great series of volcanic rocks, which 

 are mostly of Tertiary and Quaternary age. 



The crystalline rocks constitute a great mass, which occu- 

 pies a large part of the country at the southern angle of the 

 V formed by the great mountain chains. They extend also 

 for some distance along the western coast, and form a 

 considerable part of the hills of the Sierra Madre del 

 Pacifico in the state of Sinaloa. 



The sedimentary deposits are much the most extensive 

 of the three groups. They cover the northern, central, and 

 eastern parts of the country, and also the southern ex- 

 tremity. None have been proved with certainty of earlier 

 date than the Carboniferous, and neither these nor the 

 Trias nor Jurassic are found except in isolated patches.- 

 By far the greatest space is occupied by Cretaceous rocks, 

 which form nearly the whole of the Sierra Madre Oriental, 

 and also a large part of the central plateau, where, how- 

 ever, they are often concealed by Quaternary deposits. 



The volcanic rocks form nearly the whole of the Sierra 

 Madre del Pacifico, and in the latitude of Mexico and Puebla 

 they stretch almost across the width of the country. 



ANCIENT CRYSTALLINE AND SCHISTOSE ROCKS. 



The crystalline rocks, as already explained, form a large 

 nucleus which occupies the southern part of the state of 

 Puebla, some portions of the Sierra Madre in the state of 

 Chiapas, and extensive areas in Oaxaca and Guerrero. 

 Small scattered patches are also met with in the central 

 plateau and elsewhere, while there is a large mass in Sinaloa. 

 Crystalline rocks also form the axis of the mountain chain 

 of the Californian Peninsula. 



The rocks consist of porphyroid gneiss, gneissic and 

 argillaceous schists, penetrated by veins of granite, granu- 



