ANDESITES IN AMERICA. 



261 



American Andesites. In tlie United States andesites are more 

 widely distributed than propylites. They extend in scattered ex- 

 posures from the Cedar Mountains, in the great Salt Lake desert, to 

 California, and generally appear as massive eruptions. The relics of 

 an enormous crater at Lassen's Peak indicate an immense andesite 

 volcano ; and andesite volcanoes extend at intervals along the axis of 

 the Sierra Nevada and Cascade ranges. As in Europe, the rock is 

 divided into three groups, termed hornblende-andesite, dacite, and 

 au<nte-andesite. 



The hornblende-andesite described by Clarence King, frequently 

 exhibits on weathering a lavender tint, but, freshly broken, is dark 

 brownish, with a compact, homogeneous, half-glassy matrix, including 

 small white crystals of plagioclase, occasional brown micas, the 

 lamellar form of hornblende distinctive of andesite, and a few rounded 

 grains of quartz. This rock is seen on the Divide between Gosiute 

 Valley and Deep Creek. Andesites are found in the Cortes range break- 

 ing through the propylite of the Tuscarora region where the rock is 

 dark, containing black hornblende and bright plagioclase in a greenish- 

 grey ground mass. At Carlin Peaks in the Cortes range, the dark- 

 grey andesite rises in a mass 1200 feet above the surrounding rhyolites. 

 At the head of Clan Alpine Canon the summits of the Augusta range 

 are formed of andesite, which shows a rudely columnar structure. 

 The long hornblende prisms have a rude parallel arrangement, and 

 under the microscope the crystals of hornblende are often seen to be 

 fractured, with the pieces displaced. Sometimes there is a percentage 

 of sanidine in the rock, which gives a roughness to its weathered 

 surface. At the upper end of the canon of Truckee Kiver the andesites 





1 U. S. Geol. Surv., Fortieth Parallel, vol. i. p. 576 ; Table IX. 



