ASPERITES AT STEAMBOAT. 337 



They are all of the trachytic type. Similar rocks occur abundantly in the 

 surrounding region. Special mention may be made of tlie Ilufaker Butte, 

 which is an isolated volcanic mass about four miles north of Steamboat, in 

 the valley. This seems clearly to represent a single eruption or a series of 

 eruptions embraced within a short interval of time. The rock is ull tra- 

 chytic, in part highly micaceous and in part free from mica. 



While it is quite possible to distinguish varieties among these later au- 

 desites, they pass over into one another in such a manner as to indicate that 

 they form a natural group. Tlie distinctions are, at least near Steamboat 

 Springs, of little geological inii)ortanoe. As is ])ointed out in (!ha[)ter IV, 

 similar rocks occur at Mt. Shasta and from Clear Lake to the Bay of San 

 Francisco, but those of the latter area are remarkable becau.se they usually 

 contain mica and pyroxene, but no hornl)l(inde. All these; andesites seem 

 to be more recent than the clo.se of the Pliocene and all have a similar 

 })hysical chnracter. Some contain pyroxene with a very little hornblende; 

 some, pyroxene and micii, but no hornblende; some, hornblende with a little 

 pyroxene and no mica; some, much mica, a little hornblende, and a trace 

 of pyroxene. P]very possible combination of these ferromagnesian silicates 

 excepting those altogether excluding pyroxene is represented ; there is no 

 known difference in mode of occurrence and the order of succession is 

 variable. This group of rocks is the same which, before tlie reference of 

 lavas to the microscope became habitual, were regarded as tracli}tes. Tlie 

 name is indefensilde, for the rocks are plagioclastic ; but the thing to which 

 it was given is a geological entity. I liave therefore pro[)Osed for this nat- 

 ural grou)) of rocks the name aspcrttes, which is etymologically an equiva- 

 lent of trachyte, but of Latin origin. 



Basalt. — The biisalt of Steamboat is in no respect remarkiihle. 'I'liough 

 it covers a considerable area, the amount of the rock is by no means great, 

 for it is evident from some exposures tliat the sheet is only a few yards in 

 thickness. Doubtless, however, the depth of the lava is variable. Basaltic 

 breccias form a portion of the mass. The ba.salt eruption antedates the 

 dejiosition of ore, at least in part; for where it adjoins the mine it is sol- 

 fatiiricall}' decomposed and cinnabar has been deposited in crevices in the 

 lava. The spring deposits, including the cinnabar, have formed close to 



MON XIII L'L* 



