GRANITE-POKPHYKY. 339 



Granite Porphyry.— The micFoscopical study of the graiiite-|)oq>hyry of this district, 

 though somewhat limited, is <^f great interest as showing the modifications ]irodiiced 

 in the final crystallization of a p'anitic magma through the chilling caused by the 

 inclosing rocks, and consequently the relation of the quartz porphyries to the coarse 

 grained granite ; and also as pointing out the correspondence iu microscopical structure 

 of the metamorphosed sandstone of the district to certain forms of micro-granite. The 

 most important occurren(!e of this rock is in the granite-poqihyry dike and its 

 apophyses south of Wood (Jone, near Fish Creek Wells, which is represented by thin 

 sections 10, 11, 12, 16, 19, 2(», 21, 22, 24, 25, 20, 27, 28, 29. It is found to be a wholly 

 crystalline rock of most varying structure, fi-om coarse grained grauite and porphyritic 

 granite to dense porphyry with an aphauitic groundmass. It is composed of quartz 

 and feldspar, both orthoclase and plagioclase, with a small amount of biotite and 

 hornblende; and since the character of these minerals is the same throughout the 

 different thin sections, and only their relative abundance and structural combination 

 vary, it seems best to give a general description of each of the minerals first, and 

 afterwards the special features which characterize the different modifications of the 

 rock. 



The most noticeable component is quartz, occurring both in macroscopic pheno- 

 crysts and in microscopic grains. In the former instance it is usually well developed in 

 the form of dihexahedral crystals, sometimes having short piism faces; but it also 

 occurs in rounded and irregular grains of varying size, the largest being about as large 

 as a pea. In the granitic portions of the rock the grains are wholly irregular in form. 

 The quartz substance is colorless and perfectly fresh, and is filled with minute fiuid 

 inclusions, mostly with a single gas liidible, sometimes in motion. Frequently there ai'e 

 double bubbles, the inner of which is sometimes briskly moving; the fluids in this case 

 are water and liquid carbon dioxide. There are also rounded bays of groundmass pene- 

 trating the crystals, and more rarely minute portions of groundmass in dihexahedral 

 cavities. The habit of the quartz diflers from that of the quartz in quartz-porjjhyry 

 by the abundance of liquid carbon dioxide and the absence of any isotropic glass, but 

 corresponds closely to it in other respects. The microscopic grains of (juartz which 

 form a large part of the groundn)ass, have a granitic habit, being in part irregularly 

 outlined, in part conjointly crystallized with the feldspar, producing micropegmatitic 

 structure, to be described later on. 



The feldspar is mostly orthoclase, but a triclinic species also is always preseut, 

 the large phenocrysts of the former are often well crystallized with tlie ordinary faces, 

 ooP-^, OP, QcP, 2Pa;, the cliuopinacoid being the most strongly developed, forming 

 tabular Carlsbad twins. The cleavage parallel to the base is very perfect, that paral- 

 lel to the cliuoi)iuacoid less so, and in numerous individuals a fine striping is noticed, 

 which is remarkably regular, but occasionally deviates from right lines and loses its 

 parallelism. It at first suggests the polysynthetic twinning of plagioclase, l)ut on 

 closer examination appears to be an iuterlamiuation of albite iu orthoclase parallel to 



