308 University of California Publications. [Geology 



rocks, to be described later, which have an important relation 

 to copper ore bodies. It is to be remarked, however, that rocks 

 of the monzonite class are being more and more recognized in 

 regions where copper ore abounds. Possibly the rock contains 

 traces of copper which cannot be detected by the ordinary 

 methods of analysis, and this may be brought up from the deeper 

 portions of the irruptive mass, whence come the aqueous ema- 

 nations that induce the formation of garnet and other contact 

 metamorphic minerals, including chalcopyrite. 



MONZONITE PORPHYRY DYKES. 



At several localities within the district, but most abundantly 

 on the border of the area mapped on the north side of Lane 

 Valley, there occur dykes of a remarkably coarse porphyritic 

 rock, a petrographical study of which reveals its character as a 

 monzonite porphyiy. To the north of Lane City the dykes are 

 of large dimensions and at their contact with the limestone 

 there is commonly developed an abundance of garnet rock. 

 Specimens taken here may be described as having a greenish 

 gray ground mass in which are imbedded huge crystals of fresh 

 orthoelase with flesh tinted, lustrous cleavages, smaller feld- 

 spathic phenocrysts and black hornblendes. 



Petrography. — Under the microscope the ground mass ap- 

 pears as a microgranitic aggregate of non-striated feldspar and 

 quartz, the former predominating. In this ground mass are 

 imbedded idiomorphic phenocrysts of orthoelase, plagioclase, 

 green hornblende, green augite, abundant titanite, a few large 

 apatites, and an occasional much corroded crystal of quartz. 

 The plagioclase has the optical characters of oligoclase. It is, 

 however, commonly zoned. Combinations of Carlsbad and albite 

 twinning are of frequent occurrence. The hornblende is strongly 

 absorbent. The augite is of a clear chrome-green color, non- 

 pleochroic, octagonal in sections transverse to the prism, and 

 exhibits a strongly marked cleavage, against which the extinc- 

 tion in longitudinal sections reaches 40°. The titanite is a re- 

 markable feature of the slides, some large crystals showing fine 

 twinning. The apatite is in part inclosed in the hornblende. 

 There are also a few grains of opaque iron ore. The smaller 



