LINDGEEN.] 



ROCK OF SILVER WREATH MINE. 



709 



ish-green anhedral grains, and biotite as irregular, yellov/ish-brown 

 foils. The quartz occurs in abundant anhedral grains, frequently 

 exhibiting undulous extinction, due to pressure. The feldspar, also 

 in irregular grains, is quite predominantly a soda-lime feldspar, gen- 

 erally an oligoclase or andesine, though some labradorite was found 

 in a specimen from the Payette River Canyon. Small plagioclase 

 crystals are ' sometimes embedded in the biotite. Orthoclase and 

 microcline both occur in the specimens from the Silver Wreath mine 

 and near the Checkmate, but are practically absent in other speci- 

 mens from the Payette River Canyon, 3 miles east of Marsh. A lit- 

 tle magnetite and apatite always occurs. Titanite is present in larger 

 quantities, and sometimes, as at the Silver Wreath mine, makes up a 

 notable percentage of the rock. In this rock it occurs in idiomorphic 

 wedge-shaped crystals protruding in feldspar grains and also includ- 

 ing small prisms of the same mineral. An analysis of the rock from 

 the Silver Wreath mirie is given below, and a calculation of the 

 analysis may be found on page 641 : 



Analysis of rock from the Silver Wreath mine. 

 [Analyst, George Steiger.] 



According to these data, it is clear that the granitic rock is more 

 closely related to a quartz-inica-diorite than to a granite, and shows 

 great similarity to the granodiorite of the Sierra Nevada in California. 

 Being, however, only a local modification of a large area of normal 

 granite, with which it is connected by transitions, the name granite 

 has provisionally been retained. More basic dioritic rocks, contain- 

 ing an abundance of hornblende, sometimes appear as irregular 

 streaks and masses near Pearl. This is well shown in the tunnel at 



