Part 71 (1/2)

_Chlorite-schist_ is a green slaty rock, in which chlorite is abundant in foliated plates, usually blended with minute grains of quartz, or sometimes with felspar or mica. Often a.s.sociated with, and graduating into, gneiss and clay-slate.

_Hypogene_, or _Metamorphic limestone_.--This rock, commonly called _primary limestone_, is sometimes a thick bedded white crystalline granular marble used in sculpture; but more frequently it occurs in thin beds, forming a foliated schist much resembling in colour and appearance certain varieties of gneiss and mica-schist. It alternates with both these rocks, and in like manner with argillaceous schist. It then usually contains some crystals of mica, and occasionally quartz, felspar, hornblende, and talc. This member of the metamorphic series enters sparingly into the structure of the hypogene districts of Norway, Sweden, and Scotland, but is largely developed in the Alps.

Before offering any farther observations on the probable origin of the metamorphic rocks, I subjoin, in the form of a glossary, a brief explanation of some of the princ.i.p.al varieties and their synonymies.

ACTINOLITE-SCHIST. A slaty foliated rock, composed chiefly of actinolite, (an emerald-green mineral, allied to hornblende,) with some admixture of felspar, or quartz, or mica.

AMPELITE. Aluminous slate (Brongniart); occurs both in the metamorphic and fossiliferous series.

AMPHIBOLITE. Hornblende rock, which see.

ARGILLACEOUS-SCHIST, or CLAY-SLATE. _See_ p. 465.

ARKOSE. Term used by Brongniart for granular Quartzite, which see.

CHIASTOLITE-SLATE scarcely differs from clay-slate, but includes numerous crystals of Chiastolite; in considerable thickness in c.u.mberland.

Chiastolite occurs in long slender rhomboidal crystals. For composition, see Table, p. 377.

CHLORITE-SCHIST. A green slaty rock, in which chlorite, a green scaly mineral, is abundant. _See_ p. 465.

CLAY-SLATE, or ARGILLACEOUS-SCHIST. _See_ p. 465.

EURITE and EURITIC PORPHYRY. A base of compact felspar, with grains of laminar felspar, and often mica and other minerals disseminated (Brongniart). M. D'Aubuisson regards eurite as an extremely fine-grained granite, in which felspar predominates, the whole forming an apparently h.o.m.ogeneous rock. Eurite has been already mentioned as a plutonic rock, but occurs also in beds subordinate to gneiss or mica-slate.

GNEISS. A stratified or laminated rock, same composition as granite.

_See_ p. 464.

HORNBLENDE ROCK, or AMPHIBOLITE. Composed of hornblende and felspar.

The same composition as hornblende-schist, stratified, but not fissile.

_See_ p. 376.

HORNBLENDE-SCHIST, or SLATE. Composed chiefly of hornblende, with occasionally some felspar. _See_ p. 464.

HORNBLENDIC or SYENITIC-GNEISS. Composed of felspar, quartz, and hornblende.

HYPOGENE LIMESTONE. _See_ p. 465.

MARBLE. _See_ p. 465.

MICA-SCHIST, or MICACEOUS-SCHIST. A slaty rock, composed of mica and quartz in variable proportions. _See_ p. 465.

MICA-SLATE. _See_ MICA-SCHIST, p. 465.

PHYLLADE. D'Aubuisson's term for clay-slate, from +phullas+, a heap of leaves.

PRIMARY LIMESTONE. _See_ HYPOGENE LIMESTONE, p. 465.

PROTOGINE. _See_ TALCOSE-GNEISS, p. 464.; when unstratified it is Talcose-granite.