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. METAMORPHIC ROCKS. 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. 25. _Crystalline limestone_ is an altered condition of common limestone. _Saccharoid marble_ is one of the fine varieties: it frequently contains flakes of mica. _Dolomite_, or magnesian limestone, already described, is probably in many cases an altered limestone; the carbonate of lime having been partially dissolved out and replaced by carbonate of magnesia. _Serpentine_ is also believed by some geologists to be a highly metamorphosed magnesian limestone.