H3 



surrounding a core of peridotite. Where all of these 

 classes of veins are found together, a very intricate net- 

 w<~ results, but by careful observation many of them can 

 lm- *■*. ired to one or other of these clasess. 



r ihe determination of other causes of the shattering 

 of the rock, such as hydration, rapid cooling, and, possibly, 

 original gneissic structure near the edge of an intrusion, 

 requires a full investigation covering the entire process 

 of serpentinization. 



Mining and Milling of the Asbestos-Bearing Rock. 



All the mines are worked by open-cut methods. The 

 ground at the bottom of the pit is usually cut into a series 

 of benches, generally about 8 to 15 feet (2-4 to 4-5 m.) 

 high, which afford a number of faces from which the rock 

 can be quarried at the same time. At the Bell mine, 

 Thetford, extensive underground work has been carried on 

 in winter with apparent success. Generally, the mines 

 are operated only by day. Several of the pits have 

 reached a depth of about 200 ft. (61 m.), with two or three 

 times greater horizontal extension. 



In some of the mines the asbestos-bearing portion 

 is separated from the barren rock in the pit, and in part, 

 the "crude" from the mill stuff. A certain amount of hand 

 cobbing is also done in some pits. In most, however, all 

 hand separation is done at the surface. There, the 

 separate products are emptied into tramcars, drawn 

 usually by small locamotive engines; the dead rock is 

 then taken to the waste dump, and the rock which will 

 afford crude asbestos, to the cobbing sheds, where it is 

 separated by hand work and put in bags. The remain- 

 der, usually 35 to 60 per cent of all the rock handled, 

 goes to the ore bins, or directly to the mill for mechanical 

 concentration. 



Except for minor differences in handling, the principles 

 used in the treatment of the asbestos-bearing rock are the 

 same at all the mills. The dried rock is crushed by stages. 

 The crushing is effected by means of gyratory or jaw 

 crushers, or both, followed by rolls, and then cyclones or 

 jumbos, the particular style of machine employed being 

 largely a matter of the personal taste of the manager. 

 Between each stage of crushing, the material is passed 



35064—8 



