ARENACEOUS ROCKS, rocks com posed entirely, or to a large extent, of grains of quartz. Beds of loose sand oc cur extensively in the more recent de posits. Silvery flakes of mica are sel dom absent; and they often occur in lay ers parallel to the planes of stratification, causing the rock to split into thin slabs, and exposing a glittering surface. These are called micaceous sandstones. When grains of feldspar occur, it is a feldspathic sandstone. Often large quantities of cal careous matter, either as cement or as distinct grains, occur; and these are called calcareous sandstones. In like manner we have siliceous and ferrugi nous sandstones, when silica and oxide of iron are conspicuously present as cement ing or binding materials. Clay and car bonaceous matter, when plentifully dif fused through the rock, give rise to argillaceous, carbonaceous, and bitumi nous sandstones. Greensand, or glau conitic sandstone, is a rock con taining abundant grains of the dirty greenish mineral called glauconite. Arkose is a sandstone composed of dis integrated granite; volcanic sandstone, trappean sandstone, etc., being composed
of disintegrated igneous rocks. A sand stone of homogeneous composition, which may be worked freely in any direction, is called freestone or liver rock. Flag stone is a sandstone which is capable of being split into thin beds or flags along the planes of deposition. When the sand stone is coarse-grained, it is usually called grit. If it contain, more or less abun dantly, grains large enough to be called pebbles, the sandstone is said to be con glomeratic; and if the pebbles or stones be angular, the rock is described as a brecciiform sandstone. Coarse-grained grits and pebbly or conglomeratic sand stones pass into conglomerate or pudding stone, which consists of a mass of various sized water-worn stones. Brecciiform sandstones frequently pass into breccia, which is an aggregate of angular and sub-angular fragments. Graywacke is an argillaceous sandstone, more or less al tered and sometimes semi-crystalline, met with among palazozoic formations.