GRANITE-PORPHYRY. 341 



with partially decomposed titanic iron, which, is characterized by strongly marked, 

 rhombohedral cleavage, it is most likely that these minute, secondary crystals are 

 rutile or anatase. The decomposition starts from the surface of the crystal, sections 

 of partially altered mica being found with portions of the mineral still fresh in the 

 center. A further stage of alteration produces a green, fibrous chlorite, and in one 

 instance (12) quartz and epidote. The colorless potash-mica, scattered through the 

 groundmass in shreds and fan-like aggregations, which appears brilliantly colored 

 between crossed nicols, and shows a small angle between the optic axes, is undoubt- 

 edly of secondary origin, arising from the decomposition of orthoclase, as already 

 mentioned, or from that of the brown mica ; for in every thin section where it occurs 

 both the brown mica and feldspar are more or less altered, and in those where they 

 are both perfectly fresh it is wanting. 



The hornblende is by no means a constant ingredient, being absent from all the 

 more porphyry-like varieties and present in only part of the granitic ones. It is of a 

 dark brown color, sometimes green, with strong absorption and pleochroism, and is 

 seldom in fully developed crystals, though some cross sections with the characteristic 

 cleavage show the presence of the prism and both the pinacoidal faces. The crystals 

 are short and stout, their outline broken by intruding grains of the surrounding 

 groundmass, which are also abundantly included in the hornblende, together with 

 apatite, iron oxide, and more rarely mica. It would seem to be one of the later crys- 

 tallizations, contemporaneous with that of the ground mass. 



There are a great number of accessory minerals, which are not all present, 

 however, in any one thin section, and are more abundant in the granitic than in the 

 porphyry-like forms of the rock. The most exceptional of these is augite, found in 

 only one thin section (22). It is of pale green color, with high index of refraction and 

 characteristically great extinction angle. Titanite or sphene in wedge-shaped crystals 

 and irregular grains is common to the hornblende-bearing varieties, with which min- 

 eral it is usually in close association. The iron oxide appears to be for the most part 

 titaniferous, many of the larger grains showing a most pronounced rhombohedral 

 cleavage, the decomposition in several cases resulting in lencoxene and a chemical 

 test giving the reaction for titanic acid. Apatite and zircon in sharply defined 

 crystals are everywhere present in small quantities, and garnet is found in a thin sec- 

 tion from a closely related dike. Allanite is present in those sections rich in hornblende 

 and biotite; it is especially abundant in No. 11, where ten grains of it were noted. 



The groundmass of this rock, in all of its modifications, is wholly crystalline, 

 no isotropic glass being found anywhere in it. It is composed of quartz and ortho- 

 clase feldspar, very little plagioclase having been recognized. To these is sometimes 

 added hornblende, biotite, and titanic iron, besides the colorless potash -mu-a of 

 secondary origin. The quartz and feldspar are either in an aggregation of irregularly 

 outlined grains of nearly uniform size, which is the ordinary structnie of granite, or 

 they form orderly arranged groups of triangular or rhombic figures, and others clou- 



