In typical karstlands, water sinks in, and emerges again on low ground, where the surface-forms cut the level of the subterranean water-table. Streams that manage to hold their own for a time on the uplands often disappear into the clefts. Marshes may occur in hollows, but may have no outlet, except in vertical directions, upwards by evaporation and downwards through the dolinas. The dolinas correspond, as the Slavonic shepherds so aptly perceived, to the river-valleys of more normal areas. The landscape of flowing streams has to be sought for in a mysterious underworld, of which we can gain only a few glimpses. What we know is largely due to explorers of singular enterprise and resource, notably E. A. Martel and the "spelæologists" whom he has inspired.
A view over the plateau of Hercegovina shows us how deep gorges, rather than ordinary river-valleys, are prevalent where important streams run across a karstland ([Fig. 4]). The roads are carried, where possible, along the ravines, and the country possesses a double life, that of the broad uplands, where tanks have to be made to preserve the water, and that along the commercial highways, four or five thousand feet below. Even beside the rivers there is a sense of desolation in the barren whiteness of the rocks. The sunlight strikes on the wall of some theatre of the limestone, carved out in old times by a side-swirl of the stream, and the hollow glares like a white furnace in the hills. The river in summer shrinks among broad stony reaches, to which thin-flanked sheep are driven for a scanty pasture. Its clear green water gives no promise of alluvium for its banks. Limestone, even in temperate Europe, may create the features of a desert land.
The most extraordinary rock-scenery in Europe is due to limestone in the dolomitic state. It is not clear if the crags and pinnacles of Tyrol are caused by the change from calcium carbonate into dolomite, whereby a granular mass has arisen, weathering freely along its vertical joints. It may well be that these compact limestones have developed an exceptionally jointed structure under earth-stresses, and that faulting has intensified their tendency to break up into fort-like blocks. Stratified masses of more normal Rhætic limestones often provide a terraced structure near the mountain-crests; but in thousands of feet of underlying dolomite vertical clefts prevail entirely over planes of bedding. If, as is extremely probable, these dolomite-rocks arose from the composite masses that we style coral-reefs, stratification was none the less a marked feature as their limestone grew in thickness. This structure is still plainly visible; but the joints have been widened, and the mass is cut up into stupendous pinnacles and dominating towers. The Drei Zinnen near Landro, the deeply notched wall of the Langkofel and the Plattkofel, rising four thousand feet above a grassy upland of normal Lower Triassic strata, and the overhanging crests of the Sett Sass above Buchenstein, are types of a country where dolomite is pre-eminent, and where the zone of steep rock-weathering is marked by the most fantastic forms.
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.
CHAPTER III
THE SANDSTONES
THE ORIGIN OF SANDS
The essential characteristic of Sandstone is that it consists mainly of detrital grains of quartz, or occasionally of grains of chalcedonic silica (flint); these are found to scratch the steel blade of a knife, and are not affected by boiling in ordinary acids. The grains usually become cleaner in the boiling process, since the cement that has bound them together is liable to be destroyed. This cement may cause effervescence, being often formed of chemically deposited calcium carbonate.