448 



ARTHUR BEVAN 



elevation of this plateau is approximately 12,000 to 12,400 feet, or 

 about 1,500 to 2,000 feet above the general surface of the sub-summit 

 plateau. As observed from an altitude of 12,200 feet in the vicinity 

 of Granite Peak it appears to have a southwesterly dip of about 5° 

 (Fig. 6). 



Fig. 6. — The summit peneplain southeast of East Rosebud Valley. The eleva- 

 tion is about 12,400 feet. The rocks are pre-Cambrian crystallines. Looking south- 

 east from the vicinity of Granite Peak. 



STRATIGRAPHY^ 



The pre-Cambrian core. — The main mass of the range consists 

 chiefly of pre-Cambrian granite and granite-gneiss, with subor- 

 dinate amounts of basic gneisses and schists. A considerable mass 

 of anorthosite-gneiss exists on the east side of Boulder Valley, and 

 extends southeast for an unknown distance.^ Numerous basic 

 intrusions, mainly dikes and small stocks, partly of pre-Cambrian 

 age and in part possibly much younger, invade this crystalline core 



' The areal geology of the northwestern portion of the range is shown by the 

 Livingston, Montana, folio (No. i), and that of the southwestern slope south of the 

 state line on the CrandaU sheet of the Absaroka, Wyoming, folio (No. 52). 



^ C. H. Clapp, oral communication. 



