The character of the rock varies considerably, and in some places it is even now apparently in the transition stage between the original rock from which it is derived and a true serpentine, having still almost the hardness of felspar, while it yet retains the general aspect and colour of the serpentine in which it is found. Large masses of dioritic rock, having the aspect of dykes, are found in most of the quarries, possibly representing portions of the original rock not yet altered to serpentine.[6]
It may be taken as a general rule that wherever the true serpentine occurs asbestos will be found, though it is difficult to say how the veins have been formed, or how the fact of their existence is to be known with any degree of certainty, except by the light superficial indication already mentioned, which seems at present to be the only, but by no means infallible, guide. The asbestos traverses the serpentine in irregular veins ranging from mere threads or stringers to a thickness of three or four, and in some cases it is asserted of as much as six inches; the fibre always, unless affected by the dislocation of the containing rock, lying at right angles to the sides of the fissure.
The rock is often impure, the impurity arising mostly from the admixture of particles, occasionally of small irregular thread-like veins, of magnetite or of chromic iron, which break the continuity of the fibre and cause very careful cobbing of the ore to be necessary in order to get rid of these impurities. This is particularly the case at Thetford, as we shall see when speaking of Messrs. King's mine there.[7]
In other districts the fibre is discoloured (and the value of it consequently much reduced) by the infiltration of water impregnated with the oxide of iron. This is especially the case in the Black Lake district, more particularly on the property of the Anglo-Canadian Company, where the serpentine is a good deal shattered by the action of the weather, or possibly from other causes. This discoloration ceases as a general rule, or at any rate becomes considerably diminished in intensity, in proportion as the containing rock becomes more solid. It is, however, a most serious matter as affecting the character and pecuniary value of these mines.