148 QUICKSILVER DEPOSITS OF THE PACIFIC SLOPE. 



this rock hornblende-raica-andesite, a term which would he misleading as 

 applied to Steamboat Springs, where later hornblende-andesite is certainly 

 appropriate, since five well defined dikes of it cut the earlier hornblende- 

 andesite. A portion of these dikes show mica ; the others, none. The 

 greater part of this rock is holocrystalline, but some croppings contain glass, 

 sometimes in imperfect spherolitic forms. In two cases hornblendes with 

 concentric, double, black borders were noticed. 



More interesting than either of these varieties of the younger ande- 

 sites is the third, which, for lack of a better name, I will call provisionally 

 transition andesite. This variety occurs in a number of areas shown on the 

 western portion of the map, some of which are small, isolated, and excel- 

 lently exposed. They are as trachyticin texture as the associated varieties 

 and are frequently laminated, the sheets being half an inch to an inch in 

 thickness and not very sharply divided from one another. The andesites 

 of Clear Lake show the same structure. The transition andesites are of 

 exceedingly variable composition, even in the smaller areas. Some speci- 

 mens are almost purely pyroxenic; others, at a distance of only a few feet, 

 carry much more hornblende than pyroxene. Sometimes mica is present, 

 but oftener absent, though it has been found in all of the principal areas. 

 In one area olivine also has been detected in several specimens. One of 

 these comes from a portion of the rock which appears to lie beneath the 

 remainder and is much denser than usual, but other olivinitic specimens 

 are of the ordinary trachytic type. 



The younger andesites are comparatively recent, though older than 

 the basalt. They show few signs of erosion and cap a large part of the Vir- 

 ginia Range near Steamboat Springs. They also form the Ilufaker Buttes, 

 between Steamboat and Reno, where, too, micaceous and non-micaceous 

 rocks are found in company. The younger andesites are nearly- contem- 

 poraneous. The highly micaceous, later hornblende-andesite overlies and 

 appears to have followed the less micaceous portion of the same variety. 

 The relative ages of the pyroxenic and hornblendic varieties are not abso- 

 lutely certain. ( )n the map will be found a dike of later hornblende-andesite, 

 which should cut pyroxene-andesite were it younger than the latter. It is 

 not thus shown, but it is not impossible that it may do so, for at the points 



