336 QUICKSILVER DEPOSITS OF THE PACIFIC SLOPE. 



sometimes present iu most uiuisual (|uautities, tormiiiga large percentage of 

 the entire mass, and such flows appear to have been the latest of all. Else- 

 where the rock is scantily supplied with mica and over considerable areas 

 the most diligent search failed to reveal a single flake. 



The non- micaceous portions of the later hornblende-andesites are dis- 

 tinguishable from the pyroxene -andesite only by the relative abundance uf 

 the bisilicates, texture and habitus being identical and both of them trachytic. 

 The hornblendic and the pyroxenic moditications of this rock are in con- 

 tact at the eastern portion of the map. Neither is eroded to any considera- 

 ble extent, though both areas are more or less covered with loose fragments. 

 I was unable to make sure whether the eruptions had been actually simul- 

 taneous or not. On the whole, the pyroxenic rock seemed a little earlier 

 than the hornblendic lava. On comparing the occiwrences here with those 

 at Virginia I found that transitions occurred at the latter locality also, which 

 I had overlooked when I mapped the geology of that region. The transi- 

 tion rocks in the Washoe district occur along the ridge of which Mt. Kate 

 forms a part and near that peak, and are present in small quantities only.^ 

 At Virginia the pyroxenic portion of the later andesites is older than that 

 which carries mica. On the other hand, Mr. Lindgren found near Mono 

 Lake flows of pyroxenic andesites, entirely similar to that of Steamboat, 

 overlying micaceous, hornblendic andesites. The order of succession is thus 

 not a fixed one. At Steamboat the two are very nearly and perhaps abso- 

 lutely contemporaneous. Bjth of them are later than the earlier dense 

 liornblende-andesite and have overflowed it. The area of the older ande- 

 sites at the east of the map is also intersected by dikes of the later rocks. 

 These dikes are in part micaceous and in part free from mica. Those por- 

 tions of them which carry no mica are sometimes highly charged with horn- 

 blende and sometimes carry comparatively large quantities of pyroxene. 



' The occurreuce of these tr.insitious shows that the pyroxeue-.-imiesite of the Mt. Kate Kauge iui- 

 luoJiately preceded the later horubleude-audesite adjoiuing it. The pjToxcue-andesite of the Washoe 

 district (laid dowu on the map as aiigite-audesite) is divisible into two eruption.-!, one much older than 

 the other, thon^h without any intervening ontbnrst. The Wiishoe district contains three pyroxenic 

 rocks of dirtVront ages: diabase, which was followed by a hornblende-andesite, and two snccessive 

 oiitdows of pyrox^ne-andesite, both later than this earlier horn blende-andesite. The pyroxene-andesites 

 were again followed by later hornblende-andesite. Dr. Whitmau Cross pointed ont the prevalence of 

 hypersthone in andesite after my discussion of the lithology of WasbJe was completed. Compare my 

 paper, " Washoe rocks," cited above. 



