386 H. FOSTER BAIN 



Granite proper, cutting the mica schists and the phyllites and 

 shown at most of the Archaean exposures as well in the north- 

 west as the southern portion of the country. (3) Granulite, 

 cutting all the rocks of the Archaean. (4) Hornblende-granite 

 in frequent dikes and occasional stocks, cutting all the Archaean 

 rocks. (5) Pegmatite, passing into graphic granite and occur- 

 ring as dikes cutting the gneissic and true granites. There seem 

 to have been two distinct periods of eruption of the pegmatite. 

 The older, seen in Sonora, cuts only the gneissic granites, the 

 mica and the amphibole schists. The later and more common 

 type cuts all the Archaean rocks. (6) Greisen, associated with 

 the granites, forming segregation veins. (7) Diorite dikes, 

 later than the preceding rocks but earlier than the end of the 

 Palaeozoic. These are very abundant in the southern portion of 

 Puebla and the northern part of Oxaca and Guerrero. 



The Archaean forms considerable areas, and in addition to the 

 points mentioned is found in Zacatecas near Fresnilo, in Guana- 

 juato in the vicinity of the capital, in Sinaloa, near the crest of 

 the Sierra Madre, and near Vera Cruz. The rocks also form the 

 axis of the peninsula of Lower California. 



The Palaeozoic has few representatives in Mexico. The rocks 

 of this system whose age is definitely fixed belong to the Car- 

 boniferous. Although considerable areas have been referred 

 heretofore to the Silurian, and fossils characteristic of this terrane 

 and said to be from Mexico, may be found in collections, we 

 know of no exposures proven to be of that age. In the collec- 

 tion of the Institution is a piece of limestone holding beautiful 

 specimens of Orthis testudinaria Dalman, and sent to Professor 

 Castillo from the Cuesta de Santa Teresa near Cachuamilpa in 

 Guerrero. Careful search in the locality by Professor Castillo 

 failed, however, to reveal the source of the specimen. Many 

 geologists have assigned to the Silurian the slates found at Guana- 

 juato, Catorce and Zacatecas. Those at Catorce are Jurassic, 

 and while at the other localities there are beds older than the 

 Jurassic, there is no good reason for assigning to them so great 

 an age as the Silurian. With regard to the Devonian Professor 



