328 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



been developed. They occur both as lava flows and as dikes, but no special 

 characteristics can be connected with either mode of occurrence, except 

 that the more highly crystallized forms are found as dikes. Not all the 

 dikes, however, are more crystalline than all the lava flows. 



Rocks of the third class are known mostly in the form of dikes, and 

 in only a few localities. The rocks are highly feldspathic, with small 

 amount of ferromagnesian minerals, and these chiefly biotite with sub- 

 ordinate augite. The phenocrysts are labradorite in a groundmass rich in 

 orthoclase. Chemically they have 51 to 61 per cent of silica, 16.7 to 19.6 

 per cent of alumina, 3.5 to 6 per cent of lime, 1 to 4 per cent of magnesia', 

 3.8 to 4.5 per cent of soda, and 4.4 to 5.7 per cent of potash. The ratio of 

 alkalies to silica is 0.13 and 0.14. Since much of the calcium and sodium 

 goes into the phenocrysts of labradorite, the feldspathic groundmass is rich 

 in potash and is largely orthoclase. The rocks stand at the end of the 

 series, representing the variation reached when all the phenocrysts are 

 labadorite and when biotite occurs instead of olivine and augite. 



The division of the series into three parts is wholly artificial and for 

 convenience. There is gradation from one end to the other, and from the 

 middle of the series into the normal basaltic rocks of the region, which will 

 be pointed out later. The three classes will be described under the names 

 absarokite, shoshonite, and banakite. 



ABSAROKITE. 



All of the rocks here classed as absarokites carry abundant phenocrysts 

 of olivine and augite, except two (1282 and 1624). These are classed with 

 them on the ground of chemical identity. The rocks occur in the Absaroka 

 Range, and also in other parts of the Yellowstone Park, being found upon 

 Mirror Plateau (1151, 1152), within the region of the Crandall volcano (1282, 

 1277, 1306, 1307), at Signal Point, Yellowstone Lake (1617, 1618\ at 

 Two Ocean Pass (1719, 1720), on Coulter Creek (1743), about the head- 

 waters of Conant Creek (1745, 1751), and in the Ishawooa Canyon (1698). 

 The absarokite is found as bowlders on the west shore of Yellowstone Lake, 

 south of Bridge Bay, probably coming from Signal Point, on the east 

 shore. 



