

AMERICAN TRACHYTES. 267 



Much of this overflow consists of breccias, in which the fragments 

 range from the size of a pebble to a diameter of 20 feet. The 

 breccias are capped with sanidine trachyte, varying from 100 

 to 1000 feet in thickness. The older trachytes are rich in plagio- 

 clase, though sanidine slightly preponderates ; and they contain more 

 brown hornblende than the newer rocks, and thus approximate to 

 andesites. Such trachytes are seen in Mount Rose and Sugarloaf. 

 The newer trachytes are poor in plagioclase, richer in sanidine, 

 contain much less hornblende, and are usually rich in laminae of 

 biotite. The felspar frequently contains kernels of yellowish-brown 

 glass, and the hornblende is intermediate in colour between green 

 and brown. Aggregations of tridymite occur like those in the 

 trachytes of the Siebenbiirgen in Hungary, the Euganean Hills in 

 North Italy, Mont Dore, and the Puy de D6me in Central France. 

 In the north and middle parks, cretaceous rocks are dislocated, and 

 have blocks and fragments wedged in the flood of lava. Some of 

 the hills and cones of this district are termed by Zirkel granite 

 porphyries, and by Clarence King trachytoid porphyry. They have a 

 fine-grained and nearly homogeneous ground mass, which consists of 

 orthoclase quartz and a little hornblende, with occasionally plagio- 

 clase, apatite, titanite, magnetite, and pyrite. The hornblende is 

 green, and the quartz contains fluid inclusions, but no glass ; while in 

 Steve's ridge the hornblende is brown, and the quartz contains glass. 

 In the Henry mountains, Gilbert found both green and brown horn- 

 blende together in trachyte, where the quartz contained fluid in- 

 clusions. 



Zirkel remarks of the quartz trachyte of Steve's ridge in the 

 Elkhead Mountains, that the sanidine crystals are more than an inch 

 long, and the rock closely resembles the trachyte of the Drachenfels ; 

 but the felspar, though behaving like sanidine, has the crystal faces 

 which characterise the old compact and dull orthoclase of porphyritic 

 granite and felsite 'porphyry. The rock also contains grains of 

 quartz of the size of peas, which are broken by many cracks ; but 

 there is no microscopical quartz. In other localities, as at Camel 

 Peak in the Elkhead Mountains, olivine occurs with the quartz, and 

 the rock has augite for the predominant mineral. But at the Little 

 Snake River in Colorado the cracked quartz occurs in grains as large 

 as hazel nuts, with large glassy sanidine crystals, and large plates of 

 mica ; but shows much augite in the ground mass. At City Creek in 

 the Wahsatch range, microscopic cavities in the rock are encrusted 

 in tridymite. Tridymite occurs at Silver Creek, near Kimball's 

 station. 



The rocks to t-he west of the Elkhead Mountains, and which extend 

 from Hantz Peak to Camel Peak and south from Steve's Ridge in a 

 broad field 35 miles long, are all sanidine trachytes. Some have a 

 rough porous crystalline ground mass in which cracked grains of quartz 

 occur like those of rhyolite, but there is no quartz in the ground 

 mass. 



Besides these minerals the trachyte there contains hornblende, a 



