AUGUSTA MOUNTAINS. 651 



• inclusions that are of peculiar branching shape, with dark bubbles at the 

 ends of the branches. 



In the deep canon-cuts to the east of Crescent Peak, in the central 

 portion of the Augusta Mountains, is found a great variety of older erup- 

 tive rocks, associated with limestones and some few quartzites. The sedi- 

 mentary rocks are apparently wrapped around a body of granite, which is 

 exposed at Granite Point, and of which traces are seen in the detritus 

 of Augusta Canon. Through these have poured successive outbursts of 

 hornblende-porphyry, which have been succeeded by extensive eruptions 

 of andesite forming the main body of the range, which in turn are covered 

 by flows of trachyte, and the whole succeeded by rhyolite. 



The granite of Granite Point is a fine-grained rock, containing quartz, 

 feldspar, and biotite, with a little hornblende. Of the feldspars, orthoclase 

 decidedly predominates, the plagioclase being present in small quantity. The 

 mica is irregularly distributed through the mass in large hexagonal plates, 

 while the hornblende is present in comparatively small quantities, and 

 always accompanied by apatite. 



The sedimentary rocks, as exposed in Crescent Canon, and along the 

 foot-hills as far as Augusta Canon, have a dip of 55° to the northeast, and 

 strike north 40° west. They consist of greenish clay-slates and dark-blue 

 calcareous shales, with some development of limestone, but, as far as 

 observed, contain no fossils. They probably represent the upper portion of 

 the Stiar Peak Triassic. At Granite Point, these slates are found apparently 

 resting against the granite body, though the structure is somewhat obscure. 



The porphyry presents a remarkable resemblance to the well-known 

 hornblende-porphyry from Potschappel near Dresden, both in external 

 appearance and in interior structure and composition. It also presents 

 remarkable points of resemblance with the andesite, which has followed 

 it. evicfently through the same channels, since small bodies of the por- 

 phyry are found also under the western body of andesite in Antimony 

 Canon, though the outcrops are too limited to be indicated on the map. 

 It is probable that in this part of the range the pre-existing elevation of 

 Triassic rocks has been traversed and covered by flows of this porphyry, 

 which have been succeeded by the andesites, that now mostly conceal the 



