Returning to asbestos, however, its formation or actual origin is at present unknown. In its pure state it is as heavy as the rock in which it is found, so closely are its fine elastic crystalline fibres compressed together. These have a beautiful silky lustre, varying in colour from pure white to a dusky grey or green, sometimes of a yellowish green; the direction of the fibres being transverse to the walls of the vein. The essential point in which it differs from any other known mineral consists in its being at once fibrous and textile. Its quality is determined by the greater or less proportion of silicious or gritty matter with which its fibres are associated. When crushed out from the rock, these fibres, which vie in delicacy with the finest flax or the most beautiful silk, can be corded, spun, and woven into cloth in precisely the same way as any other textile fibre. Of good quality it is only found in serpentine. One instance of its having been found in quartz is mentioned; but, even in that case we are told, when six feet of the superficial quartz rock had been blasted away, the inevitable serpentine was found cropping through. According to Mr. Ells,[2] the serpentines in which it is found are intimately associated with masses of dioritic or doloritic rocks, of which rocks certain varieties, rich in olivine or some allied mineral, the serpentine is, in many cases, an alteration product. They are frequently associated with masses and dykes of whitish rocks, which are often composed entirely of quartz and felspar, but occasionally with a mixture of black mica, forming a granitoid rock. They occur generally not far from the axes of certain anticlinals which exist in the group of rocks called by Sir William Logan the "altered Quebec group." For centuries asbestos was regarded merely as a mineral curiosity. Indeed, it is only within the last few years that it has developed into a valuable article of commerce, the first modern experiments in the use of it practically extending no farther back than 1850. Its uses in the arts and manufactures are of a very important character, and now that it is clearly demonstrated that a fairly abundant supply can be obtained at a moderate cost, there seems no reasonable limit to be put to the demand, new uses for it being continually found. These will, of course, rapidly increase as its value becomes more clearly and widely known. It is found in most parts of the world, but in only a few places of a sufficiently valuable kind or in quantities large enough to give it any commercial value. The main sources of supply at present are Canada and Italy.