282 BASALTIC ROCKS OF THE ROCKY MOUNTAINS. 



free from triclinic felspar, is rich in biotite, and shows augite, nephe- 

 line, and sanidine. Sometimes triclinic felspar is present with 

 nepheline, as at Hantz Peak and Fortification Peak. 



Modes of Occurrence of American Basalt. There is frequently 

 evidence that the basalt was extremely liquid, and flowed for long 

 distances, spreading out in thin sheets, which are superimposed on 

 each other. Felspar basalt extends over hundreds of miles in Nor- 

 thern California, Oregon, and Idaho, and the surface of the country 

 appears to be made up of continuous sheets. 



In the Fish Creek Mountains basalt is seen to have come to the 

 surface through true craters, in the rhyolitic cones. This rock con- 

 tains large crystals of sanidine an inch long, and its base cannot be 

 resolved into its constituent minerals. 



In most cases the basalt appears to have been poured out from 

 fissure eruptions, as in the Pahute range. The sheets, frequently 1000 

 feet thick, are exposed on table-lands ; while in some canons, like 

 those of Clarke's Station, the thickness amounts to 2000 or even 3000 

 feet. 



The degree to which the rock is crystallised and the minerals 

 developed in it vary much with locality. In the Kugby chain, basalt 

 breaks with a curved fracture, and rings like bottle-glass, and is a 

 dark-brown glass, which is representative of obsidian, though the 

 glassy conditions are less developed than in the island of Hawaii. 

 After crystals of felspar, augite, and olivine have formed, the amount 

 of glass which remains is very variable ; and the basalts of the hills 

 to the north of Sou Springs have the whole of the ground mass 

 crystallised, and are remarkable for absence of glassy material. On 



1 U. S. Geol. Surv. Fortieth Parallel, p. 676 ; Table XII. 



