OF CENTRAL CANADA PART III. 



193 



cally known as a " selvage." 



FIG. 104. 



It usually facilitates the working of the 

 vein. A broad selvage of this charac- 

 ter lines the south wall of the Fron- 

 tenac vein, referred to above. In 

 inclined veins, the upper wall is 

 generally termed the " hanging 

 wall," and the lower, the "foot wall" 

 or floor. A and B, in Fig. 104, il- 

 lustrate these positions respectively. 



Mineral veins occur chiefly in mountainous or geologically-disturbed 

 districts ; and although present in certain localities among unaltered 

 strata, they prevail mostly in metamorphic regions, especially where 

 these are broken through by eruptive masses and dykes of granitic or 

 trappean rock. In the Provinces of Ontario and Quebec, they occur 

 chiefly in four districts : First, in the Laurentian country lying 

 between the Ottawa and Lake Huron, as, more especially, in the 

 counties of Carleton, Lanark, Leeds, Frontenac, Hastings, Peter- 

 borough, and Victoria; secondly, in the allied Huronian strata on 

 the north shore of Lake Huron ; thirdly, in somewhat higher rocks on 

 the shores and islands of Lake Superior; and fourthly, in metamorphic 

 strata of apparently Pre-Cambrian age, in the Eastern Townships 

 and adjoining region south of the St. Lawrence. These districts, as 

 regards their geological relations, are described in Part Y. 



In reference to form and geological position, four different kinds of 

 veins have been recognized. These comprise: (1) Independent or 

 ordinary veins, consisting of well-defined fissures which pass through 

 rocks of various kinds, and generally hold a more or less straight 

 course, whilst extending at the same time to great depths. In mining 

 localities, several veins of this kind are commonly found to run in the 

 same direction at greater or less distances apart. If crossed by another 

 series of veins, the latter are usually found to carry ores of a different 

 nature. The course of these veins may often be traced by trench-like 

 depressions in the ground, arising from the atmospheric decomposition 

 to which the surface of the vein has been subjected; but in some cases, 

 especially when the gangue consists essentially of quartz, the vein has 

 weathered to a less extent than the surrounding rocks, and thus stands 

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