1922] Hudson: Geology of the Cuyamaca Region of California 203 



The rocks of the Cuyamaca mass are of finer grain than any of 

 the type rocks considered above. On the other hand, an inspection 

 of the micro-drawings of typical norites given by Harker 10 gives one 

 the impression that the average grain of his specimens varies from 

 0.7 to 1.2 mm. This is about the same as many of the Cuyamaca rocks. 



Despite their relatively fine grain the Cuyamaca rocks rarely ex- 

 hibit porphyritic textures. While in all the specimens the largest 

 grains are considerably larger than those of average size, there is a 

 graded series of sizes, from slightly below the average to the maximum. 



Diabasic texture was found nowhere in these rocks. A slight 

 tendency to automorphic outline of the feldspars, seen in several of 

 the specimens, probably corresponds to hyperitic texture. Large 

 luster mottled crystals of pyroxene or brown hornblende are of fre- 

 quent occurrence. The included mineral grains have rounded outline 

 and the boundaries of the host mineral against the surrounding min- 

 erals are very ragged. The typical ophitic texture is rarely seen. 



In general the rocks have the texture typical of gabbros, in that 

 the minerals all have but slight tendency to euhedral outline. 



PetrograpJiic summary. — The rocks of the basic intrusive mass are 

 fine to medium-grained rocks, belonging in general to the gabbro, 

 norite, and diorite families. Their textures are those typical of gab- 

 bros, there being little or no tendency to diabasic structure. Ex- 

 tremely fine-grained and porphyritic textures are only locally devel- 

 oped along the margins of the mass. .There is good evidence that the 

 basic intrusive rocks made way for themselves in places by the assimi- 

 lation of wall rock. 



Of the silicate minerals, olivine was clearly the first to crystallize. 

 Perhaps all the olivine crystallized before any of the other silicates. 

 The period of crystallization of the pyroxenes runs parallel to that 

 of the plagioclase and overlaps at either end, so that some of the 

 pyroxene separated both before and after the plagioclase. The brown 

 hornblende is believed to be a primary mineral of the rocks. Its period 

 of formation was later than that of much of the pyroxene ; yet appar- 

 ently a portion of it separated before part of the plagioclase. 



The order of crystallization is : 

 Olivine 



Pyroxenes 



Plagioclase 



Brown Hornblende 



10 Harker, A., Petrology for students, 1908, p. 87. 



