Ill 



any are present show as glistening grains, usually | inch 

 or less in diameter. On weathered surfaces they stand out 

 in relief, giving a rough surface to the serpentine, like 

 raised nail heads, or knots in a worn floor. This is well 

 shown in the rock near the summits of the serpentine and 

 peridotite hills above Black Lake village. 



The granite in this area is composed of feldspar, 

 quartz and hornblende or biotite, or both. It is light 

 grey in color, and occasionally shows a pinkish tint. Types 

 without hornblende or mica, — principally dykes, are 

 also present, but are more properly classed as aplite. The 

 granite is considered to be the end product of the process 

 of differentiation and nearly contemporaneous with the 

 peridotite. The peridotite retained at least sufficient 

 heat at the time of the intrusion of the granite dykes to 

 enable them to cool without a fine grained border. 



Occurrence and Relations of Asbestos Veins. 



The asbestos is of the chrysotile variety — hydrous 

 silicate of magnesium — and has the same chemical compo- 

 sition as the serpentine which contains it, but is disti- 

 guished from it by a fibrous form. The asbestos occurs 

 almost wholly in veins which are usually 2^2 inches or 

 less in width, the greater number being less that ^2 inch. 

 The fibres lie usually at right angles to the walls of the veins, 

 hence the length of the fibre is limited by the width of the 

 vein; but it rarely equals it, for there is usually a parting 

 in the vein which is marked by a film of iron ore, generally 

 magnetite. The veins are invariably bordered by a band 

 of pure serpentine on either side of the vein, whether the 

 country rock is wholly or partially serpentinized, or even a 

 slightly altered peridotite. These serpentine bands border- 

 ing the veins are usually as well defined as the vein itself, 

 and in width are proportionate to it, each being nearly 

 three times the width of the asbestos vein. 



From a consideration of these facts, and of the number, 

 size, and directions of the veins, it is believed that they 

 were formed, not by the filling of once open fissures, but 

 by the replacement or crystallization — more or less perfect 

 — of the pure amorphous serpentine of the side walls. This 

 process is thought to have begun at a fracture, now indi- 

 cated by the parting or film of iron ore within the vein, 



