GEOLOGY OF THE SAN JOSE DISTRICT 255 



vey, on "The Laccoliths of the Black Hills." The laws gov- 

 erning drainage changes on eroded laccoliths are there devel- 

 oped in a most interesting manner. The drainage of the San 

 Jose region is similar to that of the Citadel Rock type, but at 

 San Jose two originally radial streams have taken possession of 

 the dome and cut into the exposed mass of andesite. The 

 stream which flows past Mt. Anacuas is working at a lower 

 level than that one which lies beside Mt. Laureles. The divide 

 between the two drainage systems is slightly to the northeast of 

 a line joining Mt. Armadillos and Mt. Baril. It is a low divide, 

 and with a few feet of forward cutting capture of the Laureles 

 drainage would result. 



V. FIELD RELATIONS OF THE IGNEOUS ROCKS 



The igneous rocks of the San Jose region are granitoid nephe- 

 lite syenite and diorite, andesite, effusive basalt, and dikes of 

 tinguaite, diabase, camptonite and vogesite. 



The nephelite syenite appearing over the southern portions of 

 the map is continued southward in the San Carlos Mountains 

 beyond the limits of the sheet for fifteen miles. The andesite 

 touches it in a fairly even east-and-west line along the northern 

 slopes of the Baril Range. At the contact the effects of quick 

 chill in the andesite are seen. The rock at this point is apha- 

 nitic, although elsewhere uniformly coarse-grained, as is com- 

 mon in laccolithic masses. The nephelite syenite is older than 

 the andesite. It is probable that the main mass of the San 

 Carlos mountains was faulted up through the sedimentary series. 

 The limestones and shales, in every part of the region which 

 was visited, are stripped back from the nephelite syenite, and the 

 actual contact between this rock and the sedimentary beds was 

 not found. 



There are many slight faults in the San Jose region. It is 

 probable that some of the lesser stream courses have been 

 directed by them. There is no break between the nephelite 

 syenite and the andesite by faulting. The latter rock would 

 seem to have come in on the edge of the nephelite syenite under 



