ANDESITE. 89 



south of Weston's Pass. They consist of a normal hornblende-andesite, 

 which is the cap rock, with a black vitreous rock which was at first consid- 

 ered an augite-andesite, and a great development of tufaceous and breccia 

 beds. A careful study of the darker rocks led Mr. Cross to the conclusion 

 that their characteristic mineral was hypersthene, and to the establishment 

 of hypersthene-andesite as a normal pyroxenic variety of this class. These 

 rocks are described briefly in Appendix A, and more fully in No. 1 of the 

 Bulletins of the United States Geological Survey. 



