ON MARBLES
Any limestone the markings or colour of which render it suitable for ornamental purposes passes as a Marble. "Fossil marbles" are often mere grey limestones, in which the stems of crinoids, or the curved sections of shells, or the radiating patterns due to corals, please the eye with their variety on a polished surface. The Purbeck Marble that was so much used as a grey foil to the massive white columns of cathedrals throughout England is simply a freshwater limestone, of no great merit as a building stone, crowded with the shells of Paludina. The black marbles are limestones coloured by one or two per cent. of carbon, derived from the decay of organisms, and white shells may stand out in them conspicuously, in contrast with the ground. The red marbles of Plymouth and of Cork have become iron-stained, and at the same time secondary crystallisation has destroyed many of their original features. In Little Island, near Cork city, earth-movements have crushed the mass, which in consequence shows signs of solid flow. The breaking of a crystalline limestone under such stresses furnishes us with many handsome marble Breccias. The abrupt juxtaposition of angular masses of various colours, torn from beds originally distinct, renders some of these rocks almost too startling for the decoration of rooms of moderate size.
There seems no such thing in nature as amorphous carbonate of lime, and all limestones are therefore formed of crystalline particles; but the further crystallisation of this material produces a true marble, in which all traces of fossils may be lost. Heat and pressure underground probably facilitate this change, since even soft chalk is converted by igneous dykes into granular marble. But where the pressure is accompanied by the possibility of movement, the shearing action breaks down the grains, and a more delicate structure results.
We have already seen ([p. 35]) how dolomite may undergo striking mineral changes through advanced metamorphic action. Lime-garnets, wollastonite, diopside, and other silicates similarly develop in ordinary limestones exposed to the intrusion of an igneous magma. The extreme changes in such rocks will be described when amphibolites are dealt with.