THE AVASHOE ROCKS. 113 



The porphyry an orthoclase rock. — Messrs. Hague and Id- 

 clings assert that there are plagioclastic as well as orthoclas- 

 tic rocks among the specimens brought in by my party as 

 quartz-porphyry. For the purpose of testing the character of 

 the rock; large specimens were gathered at five localities in 

 the district last summer, and separated by the Thoulet method. 

 The localities were chosen at points as far removed from one 

 another as possible, in order that the entire area might be 

 represented as well as practicable by so small a number of 

 specimens.^' The localities are as follows: Dump of the 

 Forman shaft; quarry near toll-gate on American Flat road; 

 1,500 feet south of the Amazon mine; 1,200 feet W.S.W. of 

 Excelsior mill; 1,200 feet N.E. of Eoux' ranch. These 

 rocks are not well fitted for complete separation by the 

 Thoulet solution, the quartz, orthoclase and groundmass 

 coming down together in an almost continuous stream be- 

 tween specific gravities from 2.63 to 2.58. The material of 

 a specific gravity exceeding 2.64 consists almost exclusively 

 of ferro-magnesian silicates, iron ores and plagioclase, 

 Making allowance for mixtures, it appears from the experi- 

 ments that the specimens in the order named contain ap- 

 proximately the following percentages of plagioclase: 8, 8, 

 8, 1.5, 4.5. Although the orthoclase could not be separated, 

 it is evident that the rock contains above 25 per cent, of 

 feldspar, ^^ and that all of these specimens are to be regarded 

 as orthoclase rocks. 



That at some point or points in the district some small 

 portion of the rock may contain an excess of plagioclase, in 

 the nature of a local segregation, I cannot deny. The rock 



Note ^-. — In order that no uncouscions bias might affect the selection, I 

 chose these localities on the map without visiting them, and requested mj' 

 assistant. Mr. Lindgren, to proceed to the points chosen and take the speci- 

 mens. He collected the freshest rock he ooiild find at each spot, irrespective 

 of the frequency of quartz grains. Mr Lindgren also made the separations. 



Note i^. — On the quantitative composition of quartz porphyry, see Eoth 

 Allg. Geol. Vol. 2, p. 108. 



