PETROGRAPHY 387 



those having a length of 1 centimeter. As a rule, they are very numer- 

 ous and closely crowded together, but occasionally they occur very 

 sparingly in bands and patches in the groundmass. Rounded grains of 

 clear vitreous quartz are very subordinate in amount to the feldspar. 

 The groundmass weathers to a light brownish gray, and the exposed 

 rock often presents an angularly pitted appearance, due to the easy 

 removal of the feldspar individuals. 



Microscopically, the rock is composed of unstriated feldspar and 

 quartz phenocrysts, with a few grains of plagioclase, biotite, hornblende, 

 and zircon in a fine grained quartz-feldspar groundmass. Both the feld- 

 spar and the quartz occur in rounded idiomorphic and irregular forms, 

 with crenulate borders and embayments filled with groundmass. The 

 feldspar is apparently homogeneous, and presents a mottled appearance, 

 due to kaolinization. It is evidently, from the analysis, a soda-ortho- 

 clase or anorthoclase. Twinning is rare and by the Carlsbad law only, 

 with an irregular contact plane, which is usuall}^ filled with limonite. 

 Strain shadows are common, and many individuals have been broken, 

 especially where they are closely crowded together. A large number of 

 l)henocrysts were carefully freed from the base, and a mechanical separa- 

 tion made by means of Thoulet's solution. The freshest material fell at 

 a specific gravity of 2.583, which would indicate, as the analysis has, a 

 soda-potash feldspar. A few small rounded oblong individuals of clear 

 plagioclase, which is probably albite, and some small grains of partially 

 altered hornblende and biotite, are included in the feldspar. 



The quartz occurs very sparingly in large phenocr3^sts, and frequently 

 holds inclusions, both fluid and solid, the latter being opaque. Strain 

 shadows, cracked, and broken individuals appear in every slide, and, as 

 in the case of the feldspar, are no doubt due to the strain exerted during 

 the final solidification of the magma. 



The effects of resorption and secondary growth, which is the charac- 

 teristic structure in both the feldspar and quartz, are better developed in 

 the latter. In both minerals the growth is quite regular, taking the form 

 of a crenulate rather than vermiculate border around the original grain 

 and in optical continuity with it. The primary and secondary material 

 is sometimes separated by a dark line or string of magnetite granules. 

 This structure shows that, in addition to the usual resorption which 

 goes on in rocks of this class, there was a further change in conditions 

 which permitted the corroded grains to take on new material identical 

 in composition with the original. Figure 2, plate 34, is a typical exam- 

 ple of a quartz phenocryst illustrating this structure. 



Through the kindness of Mr Pirsson, the writer was enabled to exam- 

 ine a section of quartz-porphyry from Wolf butte. Little Belt mountains, 



