Slate is a fine-grained, dense, and hard rock, which, when metamorphosed by compression, is cleaved. It results from the transformation of clay shales, fine arkose, and sometimes of volcanic tuffs. Crushed fragments of felspar change into interlocking crystals of quartz and felspar, or quartz and mica. The mineral particles, both original and newly developed, have a parallel arrangement of their long axes and cleavage planes, which determines the cleavage of the rock. In colour, slates are usually drab, or dull dark blue, but they may be brick-red, green, or purple. When fine-grained and regularly cleaved, they are extensively quarried for roofing purposes. Great areas of them occur in Vermont, eastern Pennsylvania, Virginia, and Georgia, south of Lake Superior, and on the western flank of the Sierra Nevada.
Phyllite is slate in a more advanced stage of metamorphosis, in which the mica spangles are more abundant, and visible to the naked eye, giving lustrous surfaces to the cleavage-planes. Like micaceous quartzite, phyllite may often be traced into mica schist.
Marble is a metamorphic limestone, in which the fragments and particles of organic origin have been converted into crystalline calcite. Magnesian limestones yield crystalline dolomites, which are likewise included under marble. In the process of reconstruction, the fossils and even the bedding-planes of the original limestone are usually entirely obliterated. The grain of the rock varies much, from the fine, dense, loaf-sugar-like statuary marble to a very coarse texture of large crystals. Pure limestone gives rise to a white marble, but the presence of organic matter is betrayed by veins of graphite, which may indicate the lines of mashing and flow, along which the rock yielded to the compressing force. Iron and organic matters present in the limestone produce a great variety of coloured and variegated marbles, some of which are of extraordinary beauty. The sand and clay present in many limestones will, on metamorphosis, give rise to a variety of silicated minerals. Not all crystalline limestones are to be called marbles, for crystallization may be the work of surface waters at ordinary temperature, and even modern coral-rocks may be crystalline.
Such non-metamorphic crystalline limestones differ from marbles in being less hard and in retaining the fossils and stratification planes which they originally had. Other crystalline limestones, like stalagmite and travertine (see p. 307), were deposited from solution.
Marble is an exceptional case of a completely crystalline rock derived from sediments by dynamic metamorphism, which is not foliated or schistose. This is believed to be due to the capacity of calcite to recrystallize freely after it has been subjected to compression and mashing.
The economic value of the marbles makes them largely sought after; in this country they are extensively developed along the Appalachian region, from Vermont to Georgia, in the Rocky Mountains, and the Sierra Nevada.
The Ophicalcites are crystalline magnesian limestones and dolomites, with varying amounts of included serpentine, which gives them a mottled appearance. They are not thoroughly understood, and it appears that they may be formed in various ways. Some ophicalcites are almost certainly marbles, in which inclusions of olivine, pyroxene, or hornblende have been formed and afterward altered into serpentine (see p. 19). Others would appear to be broken and fissured serpentines, having the crevices filled up with calcite deposited from solution.
Anthracite is usually regarded as a metamorphic form of coal, and, as we have seen in a preceding paragraph of this chapter, it is formed from bituminous coal by contact metamorphism. On a large scale it occurs chiefly in-areas of folded and disturbed rocks, though not invariably so. A more intense metamorphism of carbonaceous material gives rise to graphite (or black lead), a semi-crystalline form of carbon, which, however, is a mineral rather than a rock.