Quartzite, which fractures into small angular blocks under earth stresses, yields an intractable surface of bare rock and taluses of shifting stones. The latter sometimes crumble down into white sand, which provides some basis for the growth of heather. The numerous joints, independent of the bedding-planes, cause the rock to break up almost equally on any exposed slope, and the crests of quartzite hills become typically converted into cones ([Fig. 7]). Viewed from a distance, the white taluses, streaming down evenly from the crests, resemble caps of snow.
Fig. 7. Quartzite Cone. Croagh Patrick, Co. Mayo.
The absence of soil and the smoothness of weathered surfaces render quartzite mountains hard to climb. The uniform cementing of the rock leaves the bedding with little influence on the surface-features, and rock-ledges and shelves are rare. The traveller ascends over taluses of angular and obstinate blocks towards slippery and inhospitable domes. But the wildness of the scenery will be his sure reward. It is of interest to reflect that the material of these bold outstanding mountains may in certain cases have originated, in all its hardness, in the levels of a sun-parched plain.
CHAPTER IV
CLAYS, SHALES, AND SLATES
CHARACTERS OF CLAY AND SHALE
The question of what is a true Clay has been much discussed, especially by agriculturists, in recent years[939]. The material, as a rock, is regarded as a massive kaolin, and, if pure, should have the following percentage composition:—silica 46·3, alumina 39·8, water 13·9. Some Pipe-clays, white and uncontaminated, closely approximate to this ideal. True clays are very plastic when moistened, and shrink on drying, forming a compact mass the particles of which do not fall apart. When thoroughly dried, however, and placed in water, lumps of clay break up readily; the water creeps in along their capillary passages and expels trains of air-bubbles as it goes. This fact has been utilised in the extraction of fossils from a matrix of stiff clay. If the clay thus reduced to powder is now "puddled" by the finger, it again forms a closely adherent plastic mass.
The individual spaces between adjacent particles in a clay are very minute, and this accounts for its practical impermeability to water; but the total pore-space or "porosity" may amount to more than fifty per cent. of the volume of the rock. Unless earth-pressures have brought the mass into the condition of shale or slate, the tiny flaky kaolin particles, and the associated very small grains of other minerals, have not shaken themselves down into a closely aggregated state. When moistened, however, and again dried, the surface-tension of the film of water about any group of grains, increasing as evaporation thins the film, draws the grains nearer to one another, and a considerable shrinkage of the mass results. Alternate wetting and drying tends to make a clay less obdurate and sticky, by increasing the number of separate aggregates of grains. The passages between these aggregates are no longer so minutely capillary, and a clay soil becomes by this process distinctly "lighter" from the farming point of view.
The larger cracks caused by shrinkage greatly increase the evaporation of water, by exposing new surfaces, which penetrate deeply into the clay. Often the mass shrinks so as to develop hexagonal structure, from the drying surface downwards [Fig. 8]).
The natural "flocculation" of clays, the process by which compound grains are formed in place of individual soil-particles, is assisted by the action of water bearing certain salts in solution. Calcium carbonate is an excellent flocculator, and this fact has long led farmers to place burnt lime or powdered limestone on their lands. Sodium carbonate, on the other hand, is brought up in some dry regions by capillary action, and exercises a reverse effect, keeping the minute particles apart from one another, and thus promoting thorough clayiness in the clay.