146 QUICKSILVER Ui:POSlTS OF TIIK PACIFIC SLOPE. 



lies on the opposite tlank of the same range. A portion of this series also 

 possesses a close analogy to some of the andesites of the Coast Eanges. 

 A detailed account of tlie occurrence of these rocks will be given in the 

 descriptive geology of the locality. Here it is sufficient to state that the 

 latest eruption is normal basalt and the earliest a normal, dense hornblende- 

 andesite, while between the two comes a series of pyroxenic eruptions 

 which form a natural group. All of this group have the rough fracture and 

 porous texture which a few years ago would have led to its being called 

 trachyte.' The exposures, as is so usual in the Great Basin, are admirable, 

 a large proportion of the entire neighborhood being bare rock. 



Andesites of Steamboat Springs. — Tlio predominant Variety of the (dder horn- 

 blende-andesite of Steamboat Springs is a grayish-blue, dense, fine-grained, 

 thin-bedded rock, with a few porphyritic feldspars of small size. It is 

 en'.irelv similar to one variety of the earlier hornblende-andesite of the 

 Washoe district. Intermingled with this rock in the eastern portion of the 

 mass are patches of coarser-grained and more porphyritic modifications. 

 The relations of these patches to the fine-grained material were studied 

 with great care, and no evidence was found that they were separate erup- 

 tions; on the contrary, it was clear at several points that they represented 

 merely local modifications of structure. These coarser-grained rocks are in- 

 distinguishable from the prevalent older hornblende-andesite of the Washoe 

 district. In the western area of the earlier hornblende-andesite the fine- 

 grained, fissile rock is also abundant, but here it is associated with consid- 

 erable masses of what is plainly glassy rock. Transitions were distinctly 

 traced here also. 



Under the microscope no very definite lines can be drawn between 

 these older rocks. Thus, No. 313 is a gray rock showing many porph}-ritic, 

 black hornblendes Under the microscope this s])ecimen is found to be 

 composed of Ijrownish-green hornblendes with heavy, black borders (many 



'Previous to my cxjiiiiiuatiou of the Comstock, tbe United States geologists aud Professor Zirkcl 

 regarded tlio lutcr Iiorublcndo-audesito of Waslioe and many similar occurrences clsewbere as traciiytc. 

 I showed that tlie Washoe rock contaiued uo sauidiuo and stated that, from a cursory examinatiou of 

 the trachyte slides of tlio fortieth parallel collection, there was " much reason to helieve that trachyte 

 occurs less often than had been supposed in the Ore it Basin area " (Second Ann. Rept. U. S. Gool. Sur- 

 vey, 1880-'31, i>. :iO0; Geology of the Comstock Lode, Mon. U. S. Geol. Survey No. 3. p. 374). A year 

 later Messrs. Hague a;id I Idings announced that tlirr.) were no trachytes in the collections or tlie 

 forfictli parallel from th J Great B isin (Tliird Ann. Rept. U. S. Geol. Survey, lH31-'aa, p. 12). 



