onymous with slate; but it may be very
useful to distinguish between a schistose and a slaty structure. The
hypogene or primary _schists_, as they are termed, such as gneiss,
mica-schist, and others, cannot be split into an indefinite number
of parallel laminae like rocks which have a true slaty cleavage. The
uneven schistose layers of mica-schist and gneiss are probably
layers of deposition, which have assumed a crystalline texture. See
"Cleavage." _Etym._, _schistus_, adj. Latin, that which may be
split.

SCHISTOSE ROCKS. See "Schist."

SCORIAE. Volcanic cinders. The word is Latin for cinders.

SEAMS. Thin layers which separate two strata of greater magnitude.

SECONDARY STRATA. An extensive series of the stratified rocks which
compose the crust of the globe, with certain characters in common,
which distinguish them from another series below them called
_primary_, and from a third series above them called _tertiary_.

SECULAR REFRIGERATION. The periodical cooling and consolidation of
the globe from a supposed original state of fluidity from heat.
_Saeculum_, age or period.

SEDIMENTARY ROCKS are those which have been formed by their
materials having been thrown down from a state of suspension or
solution in water.

SELENITE. Crystallized gypsum, or sulphate of lime--a simple
mineral.

SEPTARIA. Flattened balls of stone, generally a kind of iron-stone,
which, on being split, are seen to be separated in their interior
into irregular masses. _Etym._, _septa_, inclosures.

SERPENTINE. A rock usually containing much magnesian earth, for the
most part unstratified, but sometimes appearing to be an altered or
metamorphic stratified rock. Its name is derived from frequently
presenting contrasts of color, like the skin of some serpents.

SHALE. A provincial term, adopted by geologists, to express an
indurated slaty clay. _Etym._, German _schalen_, to peel, to split.

SHELL MARL. A deposit of clay, peat, and other substances mixed with
shells, which collects at the bottom of lakes.

SHINGLE. The loose and completely water-worn gravel on the
sea-shore.

SILEX. The name of one of the pure earths, being the Latin word for
_flint_, which is wholly composed of that earth. French geologists
have applied it as a generic name for all minerals composed
entirely of that earth, of which ther