Petrographical Relations qf-Laurentian Limestones. 15 



constituents quartz, plagioclase, microcline, and biotite. 

 A very few grains of an untwinned feldspar, probably 

 orthoclase, a little iron ore, probably hematite, and a few 

 small, well-formed crystals of apatite, usually embedded in 

 the quartz, are accessory constituents. 



The quartz is abundant ; it is clear and glassy, and 

 often shows distinct strain-shadows. Microcline is present 

 in considerable amount and presents its typical cross- 

 hatching in polarized light, it is quite clear and fresh. 

 The plagioclase also is abundant ; it is twinned in broad 

 and narrow bands, and often shows a slight turbidity, 

 probably due to kaolinization, along certain lines which 

 doubtless represent cleavage planes ; some, however, is 

 perfectly clear. It is oligoclase, having a specific gravity 

 under 2.64, and an extinction angle of 10°. A considerable 

 amount of biotite is present, in grains of very irregular 

 outline. It is very deep brown, strongly pleochroic, and 

 shows good cleavage, with extinction parallel. 



The structure of the rock is typically granular. 



Quartz Diorite Gneiss, Township of Glamorgan, Range VIII., 

 southern part of Lot 7 . (Section 1275). 



This is a fine-grained, gray gneiss, rather distinctly 

 foliated, the foliation being especially well brought out on 

 the weathered surface. 



Under the microscope it is found to consist essentially 

 of quartz, orthoclase, plagioclase, biotite, and hornblende ; 

 individuals of microcline are not uncommon. As acces- 

 sory constituents it holds numerous grains of sphene, a 

 grain or two of black iron ore, and a few poorly developed 

 crystals of apatite. 



The quartz is in large and small irregular grains, clear, 

 but often showing undulatory extinction. The orthoclase 

 is untwinned, shows cleavage, with which extinction is 

 frequently parallel, and though generally fresh, is some- 

 times decomposed along the cleavage lines. The plagio- 



