21. Limestone consists of carbonate of lime, but usually contains some impurities. The varieties of this rock are numerous; some of them are as follows: Chalk; oolite, a rock built up of little spheroidal concretions, whence its name, egg or roe stone (the coarser oolites are called pisolite, or pea-stone); lacustrine limestone, &c. When much silica is diffused through the rock, we have a siliceous limestone; the presence of clay and of carbonaceous matter gives us argillaceous and carbonaceous limestones. Cornstone is a limestone containing a large quantity of arenaceous matter or sand. Many limestones are distinguished by the different kinds of organic remains which they yield. Thus, we have muschelkalk or shell-limestone, nummulitic, crinoidal, &c. limestone. The crystalline limestones, such as statuary marble, are metamorphosed limestones. Not a few limestones are chemically formed rocks, and many, also, are partly of chemical and partly of organic origin, so that no hard and fast line can be drawn between these two classes of rock.
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.
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.
26. Schists.—Under this term comes a great variety of crystalline rocks which all agree in having a foliated texture—that is to say, the constituent minerals are arranged in layers which usually, but not invariably, coincide with the original bedding. Amongst the schists come mica-schist (quartz and mica in alternate layers); chlorite-schist (chlorite with a little quartz, and sometimes with felspar or mica); talc-schist (talc with quartz or felspar); hornblende schist (hornblende with a variable quantity of felspar, and sometimes a little quartz); gneiss (quartz, felspar, and mica).
27. General Character of Metamorphic Rocks.—All these rocks betray their aqueous origin by the presence of more or less distinct lines of bedding. They consist of various kinds of arenaceous and argillaceous deposits, which, under the influence of certain metamorphic actions, to be described in the sequel, have lost their original granular texture, and become more or less distinctly crystallised. And not only so, but their chemical ingredients have in many cases entered into new relations, so as to give rise to minerals which existed either sparingly or not at all in the original rocks. Frequently, it is quite impossible to say what was the original condition of some metamorphic rocks; often, however, this is sufficiently obvious. Thus, highly micaceous sandstones, as they are traced into a metamorphic region, are seen to pass gradually into mica-schist. When the bedding of gneiss becomes entirely obliterated, it is often difficult to distinguish that rock from granite, and in many cases it appears to pass into a true granite.