_Dolomite_, or _magnesian limestone_.--This is a compound of carbonate of lime and carbonate of magnesia. Its colour is usually yellow, or yellowish brown, but gray and black varieties are sometimes met with. It is generally fine-grained, with a crystalline texture, and pearly lustre. It effervesces less freely with acids than pure limestone. In many cases dolomite is merely a metamorphosed limestone.
22. _Coal_ is composed of vegetable matter, but usually contains a greater or less percentage of impurities. The varieties of this substance are very numerous, and differ from each other principally in regard to their bituminous or non-bituminous character. Coal is bituminous or non-bituminous according as it is less or more highly mineralised. Bitumen results from the decomposition of vegetable matter; but, when the mineralising process (to which the formation of coal is due) has proceeded far enough, the vegetable matter gradually loses its bituminous character, and the result is a non-bituminous coal. Varieties of coal are the following: _Lignite_ or _brown coal_; _caking coal_; _cannel_, _parrot_, or _gas coal_; _splint coal_; _cherry_ or _soft coal_; _anthracite_ or _blind coal_, so called because it burns with no flame. _Peat_ may be mentioned as another natural fuel. It is composed of vegetable matter. In some kinds it is so far decomposed, or mineralised, that the eye does not detect vegetable fibres; when thoroughly dried, such peat breaks like a good lignite, and forms an excellent fuel.
23. _Quartz-rock_, or _quartzite_, is an altered quartzose sandstone or grit; it is generally a white or grayish-yellow rock, very hard and compact. The original gritty character of the rock is distinct, but the granules appear as if they had been fused so far as to become mutually adherent. When the altered sandstone has been composed of grains of quartz, felspar, or mica, set in a siliceous, felspathic, or argillaceous base, we get a rock called _greywacké_, which is usually gray or grayish blue in colour.
24. _Clay-slate_ is a grayish blue, or green, fine-grained hard rock, which splits into numerous more or less thin laminæ, which may or may not coincide with the original bedding. Most usually the 'cleavage,' as this fissile structure is termed, crosses the bedding at all angles.