GEOLOGY OF THE SAN JOSE DISTRICT 257 



appears across the country in a line of huge rounded bowlders 

 which lie four or five feet above the andesite. A second dike, 

 two miles long, cuts the first almost at a right angle. Numer- 

 ous other dikes similar to these in field habit but smaller are 

 given on the map. It is not known whether they are older 

 than the basic series of dikes or not, since the members of the 

 one are not found intersecting those of the other. 



The basic dikes present considerable variety. They lie in 

 the nephelite syenite and in the andesite and diorite. They are 

 not found cutting across each other. None were met crossing 

 the flow of basalt. The camptonite dikes are as common as 

 those of diabase. The vogesite dikes are rare. The most in- 

 teresting of these is found along Bretana Creek half a mile 

 southeast of Mt. Parreno. Its appearance in the field is like 

 that of coarse diabase, but the feldspar proves to be orthoclase 

 and the dark silicate appears under the microscope as green 

 fibrous hornblende. 



The order of succession among the igneous rocks of the San 

 Jose region is, therefore, so far as known, beginning with the 

 oldest, as follows : 



1. Nephelite syenite and diorite. 



2. Andesite. 



3. Basalt, tinguaite, camptonite, diabase and vogesite. 



VI. PETROGRAPHY 



A. The Granitoid Types 



The granitoid rocks of the San Jose district are nephelite 

 syenite, exposed throughout the San Carlos Mountain range, 

 and diorite. The former may at times be free from nephelite 

 and then appears locally developed as true syenite. The dior- 

 ite is a medium-grained rock which is only found as isolated 

 patches underlying the andesite where the laccolithic mass has 

 been deeply eroded. Dikes and offshoots of the andesite cut 

 through the diorite, which antedates the formation of the lacco- 

 lith. The diorite has diabasic affinities. 



Nephelite syenite, the product of long-continued, deep-seated 



