052 DESCRIPTIVE GEOLOGY. 



original beds of porphyry. The porphyry is of a dark-gray or dark-purple 

 color, containing only macroscopical crystals of hornblende. In the fresher 

 varieties, the hornblende is black and glistening, but, in most of the rock, 

 it is decomposed, of a light-green color, and almost earthy texture. In one 

 variety of the porphyry, the hornblende exists as light-green, almost white 

 crystals, which might be mistaken for feldspar, their outlines being rudely 

 rectangular, and the crystals being frequently half an inch in length. 

 The same parallel arrangement of these hornblende prisms is observed 

 in the porphyries as in the andiesites which have succeeded them. In the 

 hand-specimens, it is difficult to distinguish the fresher porphyries from the 

 andesite. That at the head of Clan Alpine Canon, in the hand-specimen, 

 might as readily be taken for hornblende-porphyry as an andesite. A 

 microscopical examination of these porphyries shows, in even more striking 

 manner, their close resemblance to the Saxon occurrence already mentioned. 

 Like this, they contain both plagioclase and orthoclase feldspar, the latter, 

 however, decidedly predominating, and occurring both in single individuals 

 and in Karlsbad twins. The hornblende plays a very unimportant part in 

 the groundmass, which consists mostly of a yellowish-gray, amorphous 

 and somewhat globulitic substance, including small feldspar prisms and 

 aggregations of numerous black grains. It occurs mostly in larger crystals, 

 which are generally broken and fragmentary, and do not present a regular 

 crystallographical shape, but are encircled by a border of black grains, a 

 phenomenon which is also frequently observed in andesites and trachytes, 

 of the former of which it is a characteristic feature. This black, granular 

 border, which is a result of superficial alteration of the crystal, is well 

 shown in Plate IV, fig. 2, of Professor Zirkel's report, which presents a thin 

 section of this rock. A further alteration has produced a bright-green 

 substance, Avliich is called viridite; and in the alteration of the black border, 

 which occurs less frequently, a certain amount of calcite is formed, whose 

 jDresence is shown by a slight effervescence in the hornblendes when treated 

 with dilute acid. The porphyry contains some apatite but no augite. 



The prevailing andesite of the region of Crescent Peak and Augusta 

 Canon is a light greenish-gray rock, containing large prismatic crystals of 

 black hornblende porphyritically imbedded in a somewhat earthy ground- 



