GEOLOGY OF THE SAN JOSE DISTRICT 279 



more acid, tinguaites and offshoots from the andesite mass, and 

 the basic series. They tend to assume two general directions 

 in their distribution. Most of them strike nearly north and 

 south. The great tinguaite dike which extends from Mt. 

 Anacuas to the south of Mt. Parreno is an exception to this 

 rule. A second set of dikes follows the east- and- west direction 

 at right angles to the first series. The great tinguaite dike 

 which belongs with these, running from the foot of Mt. 

 Armadillos to the slopes of Mt. Parrefio, has been faulted aside 

 for 1 50 yards near the town of San Jose. 

 The tinguaite dikes fall into two divisions : 



1. The more acid types which in the field are easily recog- 

 nized by their abundant porphyritic crystals of orthoclase. 



2. Those which are aphanitic and are characterized micro- 

 scopically by the presence of analcite. 



The tinguaites cut the limestones and the andesites, but they 

 were not found in the nephelite syenite. The other dikes are 

 all of them presumably later, and they are beheved to represent 

 several periods of eruption. They do not cut across the tingua- 

 ites in the region examined. In only one instance was a dike 

 found intersecting another. The basic dikes are seen cutting 

 through the nephelite syenite as well as the other rocks of the 

 district. Good exposures occur along the roads leading from 

 San Jose to the Santa Helena, San Narciso and Vegonia mines. 

 These dike rocks are all of them aphanitic and black-looking, 

 somtimes slightly inclined to bluish, brownish or grayish. The 

 following basic types are included in the descriptions : 



Camptonite. 



Vogesite. 



Limburgite. 



The tinguaites show a uniform field habit, with the charac- 

 teristic green color due to microscopic prisms of aegirite. They 

 are more numerous than any other of the San Jose dikes. They 

 may be without phenocrysts, and again are notably porphyritic 

 by reason of the orthoclase crystals which they contain. They 

 are found not only in the andesite, and cutting the limestone, 

 but far to the northwest of the San Carlos Mountains as well, 

 Annals N. Y. Acad. Scl, XIV, March 22, 1904—19. 



