If. B. Emery — Igneous Geology of Carrizo Mountain. 355 



tains the following minerals porpbyritically embedded : fine 

 crystals of translucent oligoclase, minute crystals of sanidite, fre- 

 quently associated with the oligoclase, small crystals of biotite 

 (rare), and a few small enclosures of quartz."* 



Cross later studied the very specimens collected by Holmes, of 

 which he says : 



" Three specimens from the Carrizo Mountains, collected by 

 Holmes, have been examined by the writer. They are all horn- 

 blende-porphyrites, almost identical in character with those of 

 the El Late group. There are abundant phenocrysts of black 

 hornblende and plagioclase, 1 to 5 millimeters in length, in an 

 even-grained groundmass, chiefly made up of quartz and ortho- 

 clase. Biotite is rare or wanting. Quartz phenocrysts were not 

 seen."f 



Both Holmes and Cross have brought out the similarity of 

 the rocks constituting the numerous laccolithic intrusions of 



Fig. 5. 





LEGEND 



SCALE OF MILES 



S E2 E3 



men+ary rocks Dionte porphyry Melanocratii 



Fig. 5. Structure section across geologic map on line AA. 



the Southwest. The writer wishes to further emphasize this 

 similarity. So closely does the description of the rock of the 

 Henry Mountains, given by Cross, correspond to the diorite 

 porphyry of Carrizo Mountain, that his description might be 

 employed here with only minor changes. 



Microscopic description. — In thin section the rock is seen to 

 consist of phenocrysts of plagioclase and hornblende set in a 

 groundmass of quartz and orthoclase. Phenocrysts of quartz 

 were observed in a number of slides, biotite in one. Iron ore 

 is present in two generations in some places ; in all slides it is 

 seen in the groundmass. Apatite is a constant accessory and 

 titanite is not uncommonly present. A few small crystals of 

 zircon were noticed. 



Hornblende is of the common, green, strongly pleochroic 

 type. In some places, however, it is rather light in color. 



* Holmes, op. cit., p. 275. 



t Cross, Whitman, The Laccolithic Mountain Groups of Colorado, Utah, 

 and Arizona, 14th Ann. Rept., U. S. Geol. Survey, Part II, 1895, pp. 210- 

 211. 



