66 GEOLOGY OF THE COMSTOCK LODE. 



is greatly decreased. While the changes in direct contact with tlie air ai-e 

 markedly different fi'om tliose wliich take place in hornblende-andesite, the 

 process of decomposition under ground seems to be identical in the two 

 rocks; nor are the jDroducts of decomposition distinguishable after the pro- 

 pylitic stage has been reached. As is the case with the hornblende-andesites, 

 too, solid augite-andesite disintegrates, while brecciated masses retain their 

 consistency, and are consequently exposed as bold croppings by the erosion 

 of adjoining disintegrated rocks. I do not know of any cases of unbrec- 

 ciated augite-andesite retaining its C' nsistenc}- in spite of considerable 

 decomposition, as the hornblende-andesite of the South Twin Peak has 

 done. 



LATER HORNBLENDE-ANDESITE. 



General character. — TLis Tock, most of wliich lias liithcrto bocn regarded 

 as trachyte, varies greatly in appearance in different parts of the field. The 

 more trachytic varieties, such as those of the quarries a couple of thousand 

 feet northeast of Sntro shaft No. Ill , are purplish or reddish soft rocks, 

 loose in structure, and thickly stndded with large feldspar crj'stals, horn- 

 blendes, and flakes of mica. Near the Utah mine the color is gray, and 

 the texture firmer and finer-grained, while further north the rock is dense, 

 black, and glassy. It also occurs largely as tufii. 



Fe-Mg suicates. — All tlic youHger lioniblende-audesite contains mica, though 

 in some cases the amount of this mineral in comparison -witli the bisilicates 

 is small. Hornblende, too, is alwa^'s present, and augite generally forms a 

 subordinate constituent. The feldspars are of course triclinic, and no deter- 

 minable sanidin has been detected. Much of the rock is thoroughly crystal- 

 line excepting inclusions, but the extent of the occurrences showing a glassy 

 base is considerable. The hornblende is entirely similar to that of the older 

 andesite, but there seems to be a relation between the physical structure of the 

 rock and the development of black border. In the coarser, more trachytic- 

 looking masses, the black liorder of both hornblende and mica almost wholly 

 replaces the origiiuil mineral, as maybe seen to some extent in Fig. 32, Plate V. 

 In the dense glassy rocks, on the other hand, the border is narrow, or alto- 



