ANDESITES. 147 



of wliicli liave entirel}' replaced tlie ampliibole), a very little pyroxene, and 

 a moderate number of porpliyritic plagioclases embedded in a fine-grained, 

 holocrvstalline groundmass of magnetite and feldspar, a portion of the latter 

 being granular and a portion microlitic. In No. 2ort, a light-gray rock with 

 small porpln-ritic hoi-nblcndes, the amphibole is entirely replaced by black 

 border, the porpliyritic feldspars are small and few in number, and the 

 groundmass is a more luiiform, fine-grained material. No 18 is a specimen 

 of the prevalent fine-grained variety and presents under the microscope no 

 considerable difference from No. 25a, excepting that the hornblendes are 

 smaller. The glassy variety of this rock shows under the microscope sniall 

 brown hornblendes with heavy black borders, an occasional pyroxene, a 

 few small porphj-rilic feldspars, and a groundmass consisting of feldspathic 

 microlites and magnetite embedded in a glass base. Fluidal structure is 

 common. 



The younger andesites of Steamboat Springs are all of the trachytic 

 type, gi'ay or reddish or yellowish rocks, rough and soft. Though these 

 rocks stand in such close relations that I shall venture to propose a single 

 name to embrace them all, they are divisible into' three groups. In one 

 large area the rock is extremely uniform and is essentiall}- a pyroxene- 

 andesite containing abundant augite and hypersthene. A few very minute, 

 black-bordered hornblendes are usually visible under the microscope, but 

 they certainly do not form 1 per cent, of the entire quantity of bisilicates. 

 This rock contains no mica. Large poi'phyritic plagioclases, which appear 

 to be andesine, are embedded with tlie bisilicates in a groundmass of 

 feldspar microlites and magnetite. A second pretty well defined variety is 

 a hornblendic I'ock in general appearance similar to the first variety. It 

 always contains more or less pyroxene. It also often contains mica. This 

 last mineral seems to be entirely absent in some croppings and even over 

 small areas. A few flakes only occur in other masses, while in others still it 

 is fairly abundant, and in one area brown mica with a variable angle be- 

 tween the optical axes forms a large part of the rock. This rock appears to 

 be substantially identical with that which I called later hornblende-andesite 

 in the Washoe district, whei'e also mica is present in variable quantities 

 and is sometimes absent. Messrs. Hague and Iddings prefer to rename 



