306 University of California Publications. [Geology 



often show Carlsbad twinning. A contact facies of the mon- 

 zonite where it conies in contact with the Nevada Limestone on 

 the roadside just north of the Monarch Claim was studied in 

 specimens taken from a prospect pit on the contact. The rock 

 is a medium grained, gray, porphyritic rock made up apparently 

 chiefly of feldspar with a subordinate admixture of a dark min- 

 eral. In thin section the rock appears as an allotriomorphie 

 aggregate of orthoclase and plagioclase in which are imbedded 

 numerous phenocrysts of light green augite and a few green 

 hornblendes. The accessory minerals are apatite, titanite, and 

 zircon in rather numerous but small crystals and a few granules 

 of iron ore. Another specimen from the same pit is finer grained 

 and more decomposed. It effervesces feebly with dilute acid. 

 In thin section the rock is an aggregate of more or less idio- 

 morphic feldspars now wholly replaced by calcite and kaolin and 

 some yellow epidote. Nearly colorless chlorite occurs in nests, 

 representing, doubtless, the decomposition product of an original 

 ferromagnesian constituent. Large crystals of apatite are nota- 

 ble features of the slides, and considerable titanite is present. A 

 very little secondary quartz may also be detected. 



Chemical Composition. — The identification of the rock as a 

 monzonite on the basis of microscopic study is confirmed by a 

 chemical analysis which has been made by Mr. Herbert Ross 

 since the above was written. The sample selected for analysis 

 was taken from a prospect pit on Weary Flat to the south of 

 the road between Copper Flat and Pilot Knob. The analysis 

 of the rock from Hodritsch, Hungary, quoted from von Hauer 

 by Brogger* as that of a typical monzonite is cited for compari- 

 son. The two analyses are given in the following table : 



* Die Eruptionsf olge der triadischen Eruptivgesteine bei Predazzo in 

 Sudtyrol, p. 50. 



