Petrographical Relations of Laurentian Limestones. 13 

 GRAY GNEISSES. 



These rocks occur in patches throughout the granite 

 mass, and are often found near the basic, highly calcareous 

 contact rocks or inclusions. They more closely resemble 

 the granites, however, and are distinguised from it by 

 their gray color. 



Quartz Diorite Gneiss, Township of Dysart, Lot :'Jf, between 

 Ranges LX. anal X. {Section 1268). 



This is a fine-grained, gray, and very quartzose gneiss, 

 of uneven and interrupted foliation. The acid constituents 

 are nearly white. 



AVhen examined microscopically, its essential con- 

 stituents are seen to be quartz, plagioclase, biotite, and 

 hornblende. Orthoclase, a very few grains of microcline, 

 magnetite, pyrite, apatite, and a few small crystals of 

 zircon embedded in the quartz make up the accessory 

 constituents. 



The quartz is very abundant, in grains of varying size, 

 some of which show uneven extinction ; the larger grains 

 are much elongated in the plane of the foliation of the 

 rock, in fact, they largely determine the direction of the 

 foliation. Some are clear, but others are clouded with 

 inclusions, which, however, are so minute that little can 

 be determined concerning them, even with a high power. 

 They are doubtless inclusions of some basic material, for 

 considerable of the quartz separated from Thoulet's solu- 

 tion in which an amethyst crystal of specific gravity 2.651 

 just floated. The plagioclase is present in large amount, 

 as large angular grains, fairly fresh and clear; cleavage 

 can often be seen. It shows beautiful albite twinning, in 

 broad and narrow bands, and is sometimes also twinned 

 according to the Pericline law. It is probably andesine, 

 since it gives an extinction angle of 15° and has a specific 

 gravity of about 2.65. The ferro-magnesian constituents 

 are not nearly so abundant as the more acid ones. The 



