200 University of California Publications in Geology [Vol. 13 



of the porphyry to the normal norite could not be made out ; but, as 

 the locality is situated over three hundred yards from the margin of 

 the basic intrusive, it is probable that this rock also is from a dike. 



The gabbro on the west flank of Cuyamaca Peak is distinctly 

 porphyritic for a distance of one-quarter mile from its west margin. 

 In no case, however, does the ground mass become as fine in grain as 

 are many of the even-grained rocks from the central portions of the 

 basic intrusive mass. The very width of the zone of porphyritic 

 texture makes it improbable that it is caused by marginal chilling. 



It is concluded that, for those portions of the intrusive mass mapped 

 as norites and gabbros, there are no marginal chilling effects. The 

 following observations of the average grain of rocks, other than dike 

 rocks, of the norite, gabbro, and closely related types bear out this 

 conclusion : In 6 rocks occurring within 50 yards of the margin, 2 

 of them immediately at the contact, the average grain was 0.6 mm. ; 

 in 8 rocks from Friday Mine it was 0.6 mm., and in 17 rocks, more 

 than one-quarter mile from the margin, including those from Friday 

 Mine, it was 0.56 mm. At two points on the contact of the diorite 

 east of Cuyamaca Peak with the gneissoid quartz diorite, the diorite 

 at the contact is finer grained than that a few feet distant. This de- 

 crease of grain without doubt is due to the cooling effect of the wall 

 rock. 



Assimilation of wall rock. — Five specimens of quartz norites and 

 quartz gabbros were collected, representing four or five small areas of 

 these rocks. Three of the specimens were collected from points within 

 25 yards of the contacts of gabbro or norite against quartz-mica schist. 

 The two other samples were found at distances of 100 and 200 yards 

 from the nearest schist. There can be little doubt that these rocks 

 are ordinary norites and gabbros that have acquired their quartz and 

 biotite from the schist. At the contact of basic diorite against gneissoid 

 quartz diorite, two miles southeast of Cuyamaca Peak, there is con- 

 clusive evidence of the assimilation of wall rock by the basic intrusive. 

 The contact, which is well exposed in the bed of a stream, strikes north 

 75° west and dips 40° to 60° north. Viewed as a whole, the contact 

 is an even surface, sharply separating the basic rock, which lies above, 

 from the quartz diorite below. The strike of the contact corresponds 

 to that of the gneissoid structure in the quartz diorite, but the latter 

 dips at a steeper angle, 70°, though in the same direction. 



When viewed in detail, the contact is in places straight and sharp, 

 but in other places the light and dark rocks penetrate one another in 

 complicated fashion. Starting at the contact, the basic rock becomes 



