304 University of California Publications. [Geology 



iferous and were not exposed by erosion, it must have appeared 

 at the surface and would partake of the characters of a volcanic 

 extravasation. But it has none of these characters and is thor- 

 oughly plutonic in its habit. We are, therefore, constrained to 

 regard the time of the intrusion as post-Carboniferous. 



Contact Phenomena. — Along the contact of the monzonite 

 with the Nevada limestone and with the White Pine Shale, it has 

 been mentioned that considerable bodies of garnet rock occur as 

 a product of contact metamorphism. These are not without 

 interest from an economic point of view since at their outcrops 

 they afford indications of the presence of copper ores. These 

 indications are usually in the form of malachite and azurite 

 stains and deposits in more or less decomposed and rusty rock. 

 Several prospect pits and tunnels have been cut into this contact 

 zone and have in several cases resulted in finding chalcopyrite 

 interlaminated or interspersed through the garnet mass. As yet, 

 however, no important deposits of ore have been found in this 

 situation, the most favorable being at the Taylor claim, near 

 the apophysis of the monzonite already mentioned to the north- 

 northwest of Pilot Knob, a little beyond the limits of the map. 

 Here very good ore is found in the garnet rock, but the question 

 as to its quantity had not been settled at the time of the writer's 

 visit in 1904. 



The Nevada limestone on the contact of which the garnet 

 rock is most abundantly developed is, in several cases observed, 

 thoroughly marmorized. A typical sample of the garnet rock 

 from a pit to the north of Pilot Knob was examined microscop- 

 ically. It consists of a mesostasis of garnet partly isotropic and 

 partly doubly refracting. In this are imbedded rather numerous 

 irregularly bounded, but more or less elongated, colorless crys- 

 tals with a high refractive index and a strong double refraction. 

 These crystals have a cleavage parallel to the elongation and 

 the angles of extinction measured against this cleavage have a 

 high value. It is very probable, therefore, that the mineral is 

 diopside. With the diopside there are also numerous stout hex- 

 agonal prisms of apatite. There are also areas of ealcite, of quite 

 irregular boundaries, and areas of limonite in the garnet. The 

 rock effervesces freely with dilute acid. 



