RICHMOND MOUNTAIN. 241 



as shown by the microscope do they enter into the composition of the 

 groundmass. Associated with the hornblendes are a few flakes of biotite, 

 the latter mineral occasionally appearing scattered through the rock with- 

 out the presence of the former. It is this association of hornblende and 

 biotite in the pyroxene-audesite that relates it to the earlier hornblende- 

 audesite. These relatively acid lavas are well shown in the neighbor- 

 hood of Trail Hill. In all these rocks of Richmond Mountain the 

 groundmass is made up of innumerable lath-shaped feldspars and micro- 

 lites of pyroxene, producing that peculiar felt-like structure first described 

 by Prof. Zirkel and since recognized by others as a characteristic of 

 pyroxene-andesites. Two typical varieties occur here, extreme forms 

 of the same lava : one, a rough, porous rock, dark purple in color and 

 having what has frequently been called a trachytic texture ; the other, a 

 compact rock, bluish black in color and possessing an oily resinous luster 

 which has been described as a characteristic of many pyroxene-andesites 

 elsewhere. Both of these sharply contrasted rocks pass by insensible transi- 

 tions, the one into the other, preventing the tracing out of separate flows 

 in the field. There is, however, this marked peculiarity between them: 

 the former has a laminated and fissile appearance, whereas the latter 

 nowhere exhibits any tendency to such lines of parting. 



Owing to the geological and petrographical importance of the Rich- 

 mond Mountain rocks, Mr. Iddings has devoted much time to an investi- 

 gation of the lime-soda-feldspars and ferro-magnesian-silicates, the results of 

 which will be found in detail in his chapter on microscopical petrography. 

 Mr Iddings has determined anorthite by its extinction angles and other 

 optical properties as one of the prevailing feldspars which at the time of his 

 work was the first recognition of this species as an essential constituent of 

 the volcanic rocks of the Great Basin. 



Within recent years investigations have demonstrated that hypersthene 

 is the prevailing ferro-magnesian silicate of many pyroxeue-andesites in 

 volcanic regions throughout the world. The importance of hypersthene 

 as an essential and controlling constituent of pyroxene-andesites in Colorado 

 was first shown by Whitman Cross. 1 Soon after these observations were 



1 Bulletin of the U. S. Geological Survey, No. 1, 1883. 

 MON XX 10 



