AUGUSTA MOUNTAINS. 653 



mass. This earthy character of the groundmass is all that distinguishes it 

 in external appearance from the neighboring liornblende-porphvries. It 

 resembles closely the andesite from the Walkenberg in the Siebengebirge. 

 The hornblende prisms are remarkably ruptured, and are seen to be filled 

 with the green viridite substance, which is also prominent in the ground- 

 mass. The phenomenon of the rupturing of the hornblende prisms is even 

 more remarkable in the andesites of this region than in those at the head of 

 Clan Alpine Canon. Under the microscope, they present also the charac- 

 teristic black border, and the ruptured fragments of a single crystal can be 

 traced with such distinctness that often one of the larger crystals is seen to 

 be broken into thirty or forty pieces ; a thin section of this rock, showing 

 some fragments of crystals, has been illustrated by Professor Zirkel, in Vol. 

 VI, Plate V, fig 2. The groundmass is an aggregation of minute color- 

 less feldspar crystals, brown hornblende microlites, and black grains. 

 Large masses of this earthy andesite are made up of breccia-like fragments 

 of the same rock, which are only distinguishable from the enclosing andesite 

 by slight differences of color. 



At the head of the South Fork of Augusta Canon is found a different 

 character of andesite overlying the earthy andesite, which shows macroscop- 

 ically small white crystals of plagioclase-feldspar imbedded in a dark-gray 

 compact groundmass. Its hornblende is not easily distinguished by the 

 naked eye, but under the microscope is seen to be largely altered into a 

 fibrous substance, of green color, though still preserving its original outline 

 and the characteristic black border. The groundmass is made up of feld- 

 spar-microlites, which show a fluidal structure, and is very rich in minute 

 brown grains of ferrite. The rock contains no augite, but the apatite 

 which occurs in the rock is not colorless, as is generally the case, but of a 

 light brownish-yellow. 



Still another andesite, which may probably be of a later flow than this 

 which occurs at the head of Crescent Canon, contains augite as well as 

 hornblende, though both in small proportions. It is a compact gray rock 

 having an even, conchoidal fracture, and is rich in plagioclase-feldspar. The 

 groundmass has a very uniform texture, and is seen, under the microscope, 

 to contain a certain amount of amorphous base. This variety of andesite 



