DIOEITE INTRUSIVES. 225 



invariably more or less decomposed, and shows innumerable minute dark 

 specks scattered through it. The quantit}^ of orthoclase varies in these 

 dioritic rocks considerably; at times it almost equals or even exceeds the 

 plag-ioclase, when the rocks approach the granites, and at times it sinks to 

 a few large plates in each section, when the rocks are a normal diorite. 



Microciine. — This mineral is not abundant. It is in individuals which 

 frequently, though not in all cases, are automorphic with respect to the 

 orthoclase and quartz. It is remarkably fresh. 



Quartz. — Quartz, at times, is an essential constituent, and again it dimin- 

 ishes in amount until it is present only in a few grains, or even disappears 

 altogether. Like the orthoclase, it is completely xenomorphic, and with the 

 orthoclase constitutes the mesostasis. Undulatory extinction in the quartz 

 gives indication of slight pressure. 



Biotite. — The original biotite in the granular dioritic rocks is automorphic 

 with respect to all minerals but the hornblende. In the ophitic forms it 

 has a development about equal to that of the hornblende. It shows a dark 

 rich chocolate-brown or greenish-brown color for Tj and c, and a light 

 yellowish-brown for a. The biotite includes small epidote crystals with 

 pleochroic courts and some grains of sphene. Both of these are original. 

 The biotite is almost invariably more or less altered, bleaching in some 

 cases to a very light colored mica with exceedingly high polarization colors. 

 This bleaching follows along the laminje of the biotite and results in giving 

 sections parallel to the vertical axis a banded appearance resembling parallel 

 intergrowths of muscovite and biotite laminse. More commonly it alters 

 to chlorite, rutile (often present as sagenite), sphene, epidote-zoisite, and 

 calcite. There is also a distinct banding of the biotite and the chlorite in 

 places. In the alteration of the biotite we very commonly find lenses of 

 calcite produced between the laminse. In some cases the epidote-zoisite is 

 clearly a product of the alteration of the biotite, for in many cases it is found 

 in the rectangular shape of the biotite section, and in other instances in spaces 

 between the feldspars in the ophitic rocks, which in fresher specimens are 

 found to be occupied by the biotite. Moreover, in the epidote-zoisite are 

 minute grains of sphene similar to those contained in the original biotite. 



Where it is present as a secondary product, it occurs with the musco- 

 vite and is xenomorphic with respect to it. The green tone is absent from 

 the secondary biotite. 

 MON xxxvi 16 



