GEOLOGY OF THE SAN JOSE DISTRICT 253 



to the south. It receives the drainage from a much larger area 

 than the Arroyo Grande does, and makes its way to the plains 

 to the west through an extended canyon cut in limestone. The 

 interior valley at its head is similar to that about San Jose. 



III. STRATIGRAPHY. STRUCTURE 



It will be seen by an inspection of the map that the sedimen- 

 tary rocks surround the central mass of andesite, adjacent to 

 the town of San Jose, almost entirely. The Baril Range ap- 

 pearing on the south is nephelite syenite. The sedimentary 

 series about San Jose is composed of limestone and shale. The 

 blue limestone is the important member of the series. It is a 

 heavy bedded rock, in regular, gently dipping layers, capping 

 the andesite on mounts Parreno, Tinaja, Ladinas, Anacuas and 

 Armadillos. The limestone is siliceous, and has a dense even 

 grain. It scarcely ever shows traces of organic life. Two 

 poorly preserved fossils were found in it, a belemnite and an 

 exogyra. It is a part of the great mass of Cretaceous sedi- 

 ments in eastern Mexico. Rarely shaly layers, an inch or less 

 in thickness, are met in the limestone. They were carefully 

 examined for fossils but none were found. The limestone once 

 extended over the entire San Jose region. It has been stripped 

 away on all sides from the andesite beneath it. The dips, as 

 indicated on the map, are radially outward from San Jose at 

 every point. They average nearly 35°. The mass of andesite 

 is a laccolith. These relations are expressed in the sections 

 [Plate II] . A few small outliers of the limestone have been 

 left standing over the andesite near San Jose. A much larger 

 remnant is found a mile to the south above Bretana Creek. 

 Three or four small outliers, besides these, are found to the 

 south near the Baril Range. These are all identical with the main 

 body of limestone. Like it they are but slightly affected, near 

 the contacts, where they stand against the igneous rock. Rarely, 

 the limestone is changed to marble for a foot and a half from 

 the contact. Smaller limestone inclusions in the andesite have 

 been thoroughly metamorphosed and made over into garnet 



