ACADEMY OF NATURAL SCIENCES. 247 



The lithological character of the eruptive materials is extremely varied, and 

 there seem to have been several periods of igneous action preceded by as many 

 disturbances of the strata, all of which took place after the close of the Creta- 

 ceous epoch. Three different series of volcanic rocks may be observed in Sina- 

 loa and Sonora, unconformable with each other ; and these may again be sub- 

 divided into groups, after a thorough examination has been made of the extensive 

 suite of specimens which has been collected. The lower, or oldest series, affords 

 several hundred varieties of porphyries, characterized by crystals of feldspar or 

 augite. There are also very peculiar trachytic rocks, resembling granite in 

 appearance. These volcanic materials occur in beds or in masses, and are fre- 

 quently cut by dykes ; but they are quite destitute of veins containing gold or 

 silver, the only metalliferous ores they contain being those of copper (?) and 

 iron, and these in small quantity. Various volcanic ridges in Sonora belong to 

 this class. The second series consists of extensive beds of micaceous, trachytic 

 tufas, and breccias, all more or less uplifted since their deposition, and covering 

 the different igneous and sedimentary formations as well as the older volcanic 

 porphyries. These attain a great thickness, between San Dimas and San Igna- 

 cio, in Durango and Sinaloa. 



Above these formations occur ancient alluvial deposits, with bones of extinct 

 animals (elephants) at two localities ; near La Noria, northeast of Mazatlan, 

 and in the Arroya de la Palma, two leagues east of La Casita, in Sonora. 



Sheets of basaltic lavas, somewhat similar to those of California, and probably 

 of the same age, forming with tufas the upper volcanic series, overlie the other 

 formations, occupying a nearly horizontal position. 



The most recent formation is that of the terrace deposits of sand and gravel, 

 which occur in Sonora. 



Having thus given a general sketch of the principal groups of rocks devel- 

 oped in the region in question, I pass to a more detailed description of the differ- 

 ent formations. 



Granites. 



Underlying all the rocks in Durango and Sinaloa, and probably posterior to 

 the Carboniferous limestones, which they have in places extensively metamor- 

 phosed, are masses of granite. These may be seen in many places between the 

 coast and San Dimas, either occupying the bottoms of the valleys, or forming 

 independent hills. There are two well-marked varieties : Of these the first are 

 syenites, more or less fine-grained, and consisting of a mixture of i'eldspar, va- 

 riously colored,, quartz, black or green hornblende, and black or brown mica, the 

 latter usually in hexagonal plates. Localities of this variety are : Haval, Las 

 Higueras, San Ignacio, Santa Apolonia, Candelero, La Noria, Zaragoza, etc., 

 in Sinaloa; San Marcial and Tecoripa valleys, Hermosillo, in Sonora. The 

 other variety is either very coarse-grained, consisting of white feldspar, gray 

 quartz, and plates of silvery mica, or else finer grained, and chiefly made up of 

 feldspar ; these occur, forming mountains and ridges in Sonora, in the Sierra 

 del Amole, near La Magdalena, Sierra del Espinaso Prieto, near Hermosillo, 

 and the Sierra de Mazatau, south of Ures. The fine-grained granites contain 



