Sandstone—indurated sand usually composed of grains of quartz, but not necessarily so; sometimes formed of calcareous grains or of grains of the various silicates.
Schist—a crystalline rock having a foliated or parallel structure, splitting easily into slabs or flakes, less uniform than slate; they are mainly composed of the silicate minerals.
Scoriæ—light, cellular fragments of volcanic rock, coarser than pumice; cinders.
Septaria—concretions the interior of which have parted, and the gaping cracks become filled with calcite or other mineral deposited from solution ([Figs. 375–77]).
Serpentine—a rock consisting largely of serpentine; derived in most cases by alteration from magnesian silicate rocks.
Shale—a more or less laminated rock, consisting of indurated muds, silts, or clays.
Slate—an argillaceous rock which is finely laminated and fissile, either due to very uniform sedimentation or (more properly) to compression at right angles to the cleavage planes; e.g., common roofing-slate ([Fig. 362]).
Soapstone (steatite)—a soft unctuous rock, composed mainly of talc.
Stalactites—pendant icicle-like forms of calcium carbonate deposited from dripping water.
Stalagmite—the complement of stalactites formed by calcareous waters dripping upon the floors of caverns.