664 PROCEEDINGS OF THE CORDILLERAN SECTION 



been stripped of their covering over such large areas that the axes of folding 

 are not generally determinable, but evidence so far observed consistentlj- points 

 to major axes not departing many degrees from north and south. This is iu 

 harmony with the structural lines of the pre-Tertiaries of adjoining areas. 



The great Unconformity 



After the folding and intrusion of the Bedrock complex follows a long period 

 of erosion, during which the products of disintegration and decay were carried 

 beyond the borders of this region and a country of comparatively low relief 

 was produced. The succeeding rocks are frequently deposited on a plane 

 granite floor, but in several places evidence of granite hills is obtainable. 

 This erosion interval included the Cretaceous and lower Tertiary. 



The Tertiary Lavas 



AXDESITES 



The most abundant and varied volcanics of the region are andesites, chiefly 

 hornblende andesites, varying at one extreme to augite andesites, sometimes of 

 basaltic texture and at the other to quartz-mica-horublende andesites with well 

 developed phenocrysts. 



Along the Carson block and the upper Truckee the more common sequence 

 is coarse andesitic breccia with finer ash layers lying directly on the granite 

 and capped by hornblende audesite flows. Andesites also occur on the southern 

 flank of Peavine mountain and in the low hills north and south of Reno ; also 

 in great abundance in the Virginia range. In these latter areas breccia is only 

 occasionally present. In some parts of the Virginia range the andesites are 

 amygdaloidal, and they often show the propylitic type of alteration, especially 

 near Virginia City and in the Olinghouse Canyon district. 



These various andesites are very similar to and are practically continuous 

 with those of the Sierra Nevada and are presumably of the same age — late 

 Miocene or early Pliocene. Although the most abundant of the volcanic series 

 of this region and of the Sierra Nevada, evidences of craters are entirely 

 lacking, and even the foci of eruption are not generally evident. On the upper 

 slopes of Peavine mountain the granites are cut by a number of dikes identical 

 lithologically with andesite flows lying on the lower slopes, and were appar- 

 ently loci of eruptions iu that area. 



The andesites have often suffered alteration by solfataric action or mineral- 

 ization. This generally occurs along faults which have been seats of deposi- 

 tion, the rising waters sometimes (1) impregnating a zone with sulphides and 

 very little silica, sometimes (2) with silica and very little sulphides, some- 

 times (3) with both well developed. Occasionally ealcite gangue is found. 

 In the first case the mineralized zone may now appear on the surface as a belt 

 of excessive decomposition showing the combined effects of oxidation and sul- 

 phuric acid (from the oxidized sulphides) action. The ground is soft and 

 easily eroded, is bright red from iron oxide, or leached to various shades with 

 white as the end result. Many of these belts cai-ry low (at present non- 

 commercial) values in gold and silver, but occasionally rather high. At the 

 Wedekind mine several hundred dollars a ton in silver was found at the sur- 

 face, due to secondary concentration. In general the most common original 

 sulphide is pyrite, with occasionally galena and sometimes sphalerite. 



