In these rocks the principal component is quartz in fragments of greater or less size, either angular, or more or less rounded by wear. Of the common rock-forming minerals quartz is the hardest and the one which best resists chemical change. Small quantities of other minerals, such as magnetite, mica, felspar, garnet, etc., are generally present.

Sand is made up of fine grains of quartz, not compacted to-gether, but forming a loose, incoherent mass. River sands and those formed by the atmospheric disintegration of rocks commonly have angular grains, due to the splitting up of the quartz fragments along preexisting flaws, though desert and wind-blown sands are apt to be fine-grained, rounded and pitted by abrasion. Beach sand is somewhat rounded, due to the constant wash of the surf.

Sandstone is a rock of varying degrees of hardness, the grains' of sand being held together by a cement. The most important cementing substances are carbonate of lime, the oxides of iron and silica. The sandstones with calcareous cement usually yield quickly to the action of the weather, because of the solubility of the cement. Those with ferruginous cement are much more durable and more highly coloured, being of various shades of red, yellow, and brown. Most durable of all are the siliceous cements.

Varieties of sandstone are produced by the conspicuous admixture of other minerals; thus, micaceous sandstone has: abundant flakes and spangles of mica deposited along the planes of stratification. Argillaceous sandstone is composed of a more finely grained sand than the more typical sandstones, contains considerable quantities of clay, and is, in general, more thinly bedded. The flagstones, so largely used for pavements, are examples of such a rock, and split readily into slabs of almost any desired size.

Arkose is a sandstone containing considerable quantities of felspar in a mechanically subdivided but undecomposed state.

Gravel is composed of rounded, water-worn pebbles, varying in size from a pin-head up to cobblestones and boulders. The coarser kinds are often called shingle. Gravel may be composed of almost any kind of rock material, but the commonest pebbles are of quartz, because of its greater resistance to wear. Masses of quartz will be only rounded into pebbles, when other substances are ground into fine silt, or chemically disintegrated, and so washed into deeper water.

Conglomerate is a cemented gravel. Different names are given to conglomerate, according to the character of the pebbles, as quartz conglomerate, flint conglomerate, limestone conglomerate, granite conglomerate, etc.

Hand specimen of conglomerate, natural size.

Fig. 146. - Hand specimen of conglomerate, natural size.