150 OriCKSlLVER DP^POSTTS OF THE PACIFIC SL(3PE. 



would iinquestiouably have been classified as trachytes by nearly every 

 American geologist a few years since. They are nearly all soft, rough, 

 light-colored rocks, possessing great similarity in external appearance and 

 in their mode of occurrence. This similarity cannot be regarded as acci- 

 dental, for the series of younger andesites found at Steamboat is repeated 

 at Mt. Shasta, which I visited for the purpose of instituting a comparison, 

 and in part also near Clear Lake, while the area regarded as trachyte by 

 all field observers at Washoe previous to my study of that district em- 

 braces highly pyroxenic, non-micaceous rocks as well as micaceous, horn- 

 bleudic andesite. If the macroscopical resemblance and the intimate asso- 

 ciation of these rocks be not accidental, it must be due to common features 

 in their origin or history, and they may therefore properly be I'egarded as 

 forming a natural group, recognized by earlier observers, though wrongly 

 named. They are all more or less pyroxenic rocks, which may contain 

 both mica and hornblende or one of these minerals or neither. Horn- 

 blende occurs in this series over large areas without associated mica, and 

 mica (near Clear Lake) in large areas without associated hornblende. The 

 members of this series are connected by transitions wherever I have studied 

 them. There is certainly a marked distinction between this series and the 

 older liornblende-andesite, both at Steamboat and at Washoe, as there is 

 also at the latter locality between it and the earlier dense pyroxene-ande- 

 site. Xear Clear Lake also the earlier pyroxene-andesite is distinct. The 

 causes of these difi'erences are not as yet known. I do not believe that 

 they depend simply upon the rate of cooling or upon the pressure under 

 which the rock has cooled. As has been mentioned, a portion of the older 

 liornblende-andesite of Steamboat is glassy, while directly associated with 

 the glass is ordinar}", dense, older andesite. That a glassy magma cooled 

 slowly under considerable pressure will crystallize appears almost certain, 

 and it is to be inferred that the earlier andesite at this locality has not been 

 very deeply eroded. Consequently dense andesites may consolidate near 

 the surface. On the other hand, there are manv exposures of the vounger 

 andesites at depths of hundreds of feet, and in the Sutro tunnel at a 

 depth of some 2,000 feet below what must be supposed to have been the 

 surface of the rock at the period of eruption. Such exposures are indeed 



