248 PROCEEDINGS OF THE CALIFORNIA 



argentiferous veins at Zaragoza, in Sinaloa, and east of Topisco, in Sonora. 

 These are traversed by numerous intersecting dykes of diorite, feldspar, and 

 quartziferous and feldspathic porphyries, especially well seen near Hermosillo, 

 and the Cajon de los Carrisos, east of Sau Antonio de la Huerta. There are 

 no metalliferous veins where the granite is thus intersected by dykes. 



Metalliferous Porphyries. 

 These may be divided into two groups. The first consists of a rock occurring 

 in large irregular masses or beds, and having a dark colored argillaceous base, 

 through which are disseminated small crystals of whitish feldspar. This variety, 

 which is probably older than the granites, includes some of the richest mines of 

 the Sierra Madre ; as those of Candelaria, Bolanos, Ciuco Senores. etc., near 

 San Dimas, in Durango ; and which have yielded over $20,000,000. There 

 are also rich veins in this kind of rock at Candelero, fifty-two leagues northeast 

 of Mazatlan, in Sinaloa. All these veins run northeast and southwest, and are 

 cut at right angles by dykes. The second variety of porphyry is a gray feld- 

 spathic rock, apparently made up of labradorite and magnetic iron ore ; this 

 overlies the greenstones, and is covered by the Triassic beds at Los Bronces and 

 San Javier, where there are three systems of argentiferous veins. The Nahuila 

 mine, one of the best in Sonora, is in this rock. 



Metamorphic Rocks. 



Heavy masses of metamorphic rocks maybe seen at various localities in Sina- 

 loa and Durango (Tecomate, Tenchoquelite, Arroyo del Ciruelo, Arroyo de San 

 Vincente) resting either on the granites or the metalliferous porphyries. These 

 rocks occur in masses or beds, sometimes distinctly stratified, and sometimes with- 

 out any traces of the original bedding. They are always much altered and broken 

 up. Their lithological characters are not well marked, although the series is 

 easily recognized. The rocks referred to in this division, are usually fine-grained, 

 of a greenish or bluish color, when not too much decomposed, and somewhat 

 argillaceous in composition. At the base they pass into porphyries. The ar- 

 gentiferous veins cut both the metamorphic and the porphyritic rocks at Tecom- 

 ate, on the Rio de San Ignacio, where the dip of the formation is to the northeast, 

 at an angle of 70°. Between La Pucrta and El Pilar, Arroyo de San Dimas, 

 they occur in jaspery layers, ribboned with green and brown hues, and resemble 

 some of the metamorphic Triassic rocks of Sonora. Near Candelero, the meta- 

 morphic rocks are associated with whitish, semi-crystalline limestone. The for- 

 mation in question may be observed in many other localities in Sinaloa, always 

 resting on granite and passing into porphyry ; it is also sometimes associated 

 with metamorphic slates. 



Greenstones. 



These rocks occur in heavy masses or in beds, and are made up of a fine- 

 grained, compact mixture of hornblende and feldspar, often containing mica, 

 and having a greenish color. The greenstone underlies the Triassic rocks, and 

 in many places it protrudes through the granite. This rock is highly metallif- 

 creus at Copala, Sinaloa, and also at Los Bronces and San Javier, in Sonora. 



