DISSECTED VOLCANO OF C RANDALL BASIN. 607 



basalt like the first. This was succeeded by a period of 

 extensive erosion ; reducing the country to nearly its present 

 form. Then came the eruption of a vast flood of_rhyolite con- 

 stituting the Park plateau, which was followed in this region by 

 smaller outbreaks of basalt. The last phase of volcanic activity 

 is found in the geysers and fumaroles which have rendered this 

 region famous. 



The volcano of Crandall Basin consists chiefly of the first 

 series of basic andesites and basalts. The earlier acidic andesite, 

 which occurs beneath these rocks, appears to be the remnants of 

 eruptions from neighboring centers. 



Nothing remains of the original outline of the volcano. The 

 district is now covered by systems of valleys and ridges of 

 mountain peaks that rise from two thousand to five thousand feet 

 above the valley bottoms. The geological structure of the 

 country, however, makes its original character evident. 



The outlying portions of the district to the south, west, and 

 north consist of nearly horizontally bedded tuffs, and subaerial 

 breccias of basic andesite and basalt. With these are intercalated 

 some massive lava flows, which are scarce in the lower parts of 

 the breccia, but predominate in the highest parts, above an 

 altitude of ten thousand feet. Here they constitute the summits 

 of the highest peaks. 



In contrast to the well-bedded breccias around the margin of 

 the district, the central portion consists of chaotic and orderless 

 accumulations of scoriaceous breccia with some massive flows. 

 These breccias carry larger fragments of rocks and exhibit 

 greater uniformity in petrographical character. 



A still more noticeable feature of the central portion of the 

 district is the occurrence of dikes which form prominent walls, 

 and may be traced for long distances across the country. The 

 greater part of them are found to converge toward a center in 

 the highest ridge in the middle of the drainage basin of Crandall 

 creek. A small number converge toward a second center three 

 or four miles east of the first. In the southern part of the 

 district there are many dikes trending toward a center near the 



