280 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



is dense in the middle, hut vesicular for 2 or 3 feet at the bottom (1717). 

 It is about 20 feet thick and is filled with zeolites (1722) and calcite, which 

 line cavities and cracks. 



This sheet rests directly on the slaggy, scoriaceous top of the next 

 lower one, which is a basalt with only a few phenocrysts of pyroxene 

 (1718). It is distinctly vesicular for some depth (1719). The lowest sheet 

 is a dark, dense basalt with numerous phenocrysts of augite and olivine 

 (1720). The top and bottom of the sheet are vesicular (1721). This 

 basalt extends across the valley south. It rests on assorted basic breccia, 

 and may be traced west along the northern side of the valley for 2 miles 

 (1724), where it is similar to the upper two sheets at Two Ocean Pass. 

 The more crystalline forms of these basalts have a slightly resinous luster. 



The petrographical character of the rocks constituting the early basic 

 breccia is variable within limits, and is slightly different in the two princi- 

 pal localities mentioned, Sepulchre Mountain and Crandall volcano. In all 

 cases it is pyroxene-andesite in large part, grading into hornblende- 

 pyroxene-andesite on the one hand and into olivine-bearing andesite and 

 basalt on the other. At Sepulchre Mountain the hornblendic end of the 

 series is more pronounced and the basaltic end is subordinate. But it is 

 to be remembered that the size of the mass of this mountain is insig-nincant 

 when compared with that of the groups of mountains embraced in the 

 Crandall volcano. The breccias more directly connected with the Crandall 

 volcano are largely pyroxene-andesites, onl}^ a very small proportion of 

 which carry hornblende. Basaltic forms are very abundant, and true 

 basalts preponderate in the upper parts of the volcano. The bulk of this 

 breccia lies within the district alread}- described as the dissected volcano of 

 Crandall Basin, and its petrographical characters have been given in 

 Chapter VII. 



The basaltic lava flows or streams connected with this breccia, as 

 already pointed out, occur partly near its base, as do the flows exposed in 

 the lower Lamar River Valley and in the valley of Soda Butte Creek. The 

 greater part constitutes the thick accumulation of lava sheets forming Mirror 

 Plateau and the summits of the mountains immediately south of Lamar 

 River. The petrographical character of these rocks is somewhat variable. 

 A large number of sheets consist of normal andesitic basalt — that is, basalts 

 with abundant phenocrysts of lime-soda feldspar (labradorite-bytownite), 



