210 TRANSACTIONS OF THE [A PR. 24 



In other cases the different areas of quartz in the feldspar are 

 wholly independent of each other, producing the structure to 

 which the term micropoikilitic* has been applied. 



Beyond the development of microperthite and a varying 

 amount of cloudiness, the feldspar, as a rule, shows no decided 

 alteration. Muscovite and chlorite have been rarely produced 

 at the expense of feldspar, but not in sufficient quantity to exert 

 any influence upon the character of the rock. 



Like the quartz, the feldsj^ars show much evidence of 

 mechanical strains. Undulatory extinction is quite general, 

 often accompanied by granulation, either jjeripheral or extend- 

 ing through entire individuals. In the plagioclase there is 

 much, bending and breaking of twin lamella, while the develop- 

 ment of twinning by pressure is often seen. 



In the norixial granite, biotite is the only ferro-magnesian 

 mineral, and varies greatly in quantity-, often being entirely 

 absent. It is dark brown, strongly pleochroic, and fairly fresh 

 as a rule, though sometimes bleached. Pleochroic halos are 

 abundant, often extending through an entire fragment of the 

 mineral, and showing no dependence upon inclusions. Besides 

 the original biotite the very basic phases of the granite contain 

 small amounts of biotite that is clearly of secondary origin. 



The distribution of hornblende in the granite is very limited 

 being confined to the basic segregations which have the com- 

 position of diorite. In these, hornblende is the prevailing 

 ferromaguesian mineral. It never shows any decided crystal 

 outline, though occasionally in the form of an impei^fect prism. 

 It is dark green, strongly pleochroic and usually very fresh. 

 In one section it is associated with a considerable amount of 

 light green monoclinic pyroxene. A parellel growth with 

 biotite is common. 



Tourmaline characterizes, by its presence in considerable 

 quantities, a few limited areas of granite. It occurs in the ordi- 

 nary prism with triangular cross section, or in irregular masses. 

 The color is brown or green, sometimes in zonal arrangement, 

 and the pleochroism intense. 



The ordinary minor constituents of granite, such as apatite, 

 zircon, ilmenite, leucoxene, etc., are present in varying quanti- 

 ties, but need not be described. 



The granite shows several phases which differ considerably 

 from the normal type of the rock. Coarse pegmatitic varieties 

 are abundant, both associated with the ordinary rock and form- 

 ing independent masses. In the latter tourmaline is sometimes 



* Williams. I. H.. " Od the Use of the Terms Poikilitic and Micropoikilitic in 

 Petrography," Jouruiil of Geology, I., p. 170. 



