ARENACEOUS ROCKS, the name ap plied to a petrographic division including loose sands and gravels, sandstone, conglomerate, quartzites and such rocks as are mainly com posed of quartz particles. They are of mechan ical origin, being derived from disintegration of pre-existing strata and removal and deposition of the materials by wind or water. The grains are generally waterworn and rounded; in some cases, however, they are more or less angular, or rounded and angular grains occur com mingled. In older deposits the grains of sand are bound together by silicious, calcareous, argil laceous, or ferruginous cements. It is seldom that a rock is composed of quartzose materials alone; grains or particles of other mineral sub stances arc frequently mingled with the grains of quartz. Silvery flakes of mica are seldom absent ; often occurring in layers parallel to the planes of stratification, thus causing the rock to split into thin slabs, and exposing a glitter ing surface. These are called micaceous sand stones. When grains of feldspar occur, the rock is a feldspathic sandstone. Often large quantities of calcareous matter, either as cement or as distinct grains, occur; and these are called calcareous sandstones. In like manner we have silicious and ferruginous sandstones, when silica and oxide of iron are conspicuously present as cementing or binding materials. Clay and carbonaceous matter, when plentifully diffused through the rock, give rise to argilla ceous, carbonaceous and bituminous sandstones.
Greensand, or glauconitic sandstone, is a rock containing abundant grains of the dirty green ish mineral called glauconite. Arkose is a sandstone composed of disintegrated granite; volcanic sandstone, trappean sandstone, etc., being composed of disintegrated igneous rocks. The presence of lime can always be detected by the effervescence which takes place on the application of hydrochloric or other acid. A sandstone of homogeneous composition, which may be worked freely in any direction, is called freestone or liver rock. Flagstone is a sand stone capable of being split into thin beds or flags along the planes of deposition. When the sandstone is coarse-grained, it is usually called grit. If it contain, more or less abundantly, grains large enough to be called pebbles, the sandstone is said to be conglomeratic; and if the pebbles or stones be angular, the rock is described as a brecciaform sandstone. Coarse grained grits and pebbly or conglomeratic sandstones pass into conglomerate or pud dingstone, which consists of a mass of various sized water-worn stones. Brecciaform sand stones frequently pass into breccia, an aggre gate of angular and sub-angular fragments. Graywacke is an argillaceous sandstone, more or less altered and sometimes semi-crystalline, met with among palmozoic formations. Lime stones and shales containing much sand are said to be arenaceous.