206 University of California Publications in Geology [Vol. 13 



Form and type of intrusion. — The observed localities at which the 

 actual contact of basic rock against the schist or the quartz diorite 

 is laid bare are three in number. At these places the contact plane 

 conforms essentially to the structure of the invaded rock. The map- 

 ping suggests that for perhaps a fifth of the periphery of the basic 

 intrusive mass the contact conforms approximately to the foliation 

 of the gneissoid quartz diorite and schist. However, along the greater 

 portion of the border of the mass the basic rock cuts across the pre- 

 existent structures. In most cases where there is any degree of paral- 

 lelism between the strike of the schist or gneiss and the direction of 

 the contact of the main Cuyamaca intrusive, the dip of the invaded 

 rocks is toward the intrusive at high angles, varying from sixty or 

 seventy to ninety degrees. 



There is no evidence of any metamorphism exercised by the basic 

 magma on the schist or quartz diorite. This is to be expected, as the 

 older rocks had a high degree of stability before the invasion of the 

 basic rock. 



Several small masses of schist occur within the basic intrusive area 

 and two fairly large granite masses were found in the saddle between 

 Cuyamaca and Middle peaks. These occurrences suggest that the 

 basic magma made way for itself, at least in part, by the stoping of 

 wall rock. 



From evidence presented in a previous section there can be no 

 doubt but that the basic magma enlarged its chamber, to a slight ex- 

 tent at least, by the assimilation of wall rock. It is doubtful if this 

 action was on a considerable scale for the reason that such basic rocks 

 as olivine gabbro and olivine norite are not only found in the central 

 portions of the mass but are rather common along certain stretches of 

 its periphery. 



A complex intermingling of different rock types was found not 

 only in the main mass of the basic rock but also in several of the 

 smaller outlying masses. No definite contact planes were found be- 

 tween the different types of rock. It seems probable then that the 

 heterogeneity of the Cuyamaca Basic Intrusive is due largely to dif- 

 ferentiation in place. 



The relatively fine grain of the rocks and the lack of contact action 

 on the wall rocks suggest that the magma solidified at moderate depth. 



It is believed that the structures of the wall rocks had much their 

 present attitude before the intrusion of the basic masses. The form 

 of the main intrusive body must then be an irregular mass elongated 



