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predominates to the nearly total exclusior of the amphibole and other minerals. /. Mico-hornblende Rock, or Micaceous Hornblende Rock.
Black, bronze, or white mica occurs as an essential. The flakes are sometimes of a considerable size, but more often they occur in small pockets or secretions. Usually they are regularly, although not very abundantly, distributed through the mass.
A mico-hornblende rock is easily distinguished, as the mica decomposes more readily than the other constituents, and gives the rocks a pitted aspect, if the mica is in bunches or pockets; while, if it is in large flakes, the weathered surface of the rock has often a somewhat graphic character, caused by the weathering away of the edges of the mica. Other varieties also have mica as an essential, scattered through the mass; these latter are often more or less schistoid.
G. Rocks due to Pseudomorphic Action, or to Weathering.—Some of these rocks, as will be evident from their description, belong in part to Order No. II. (Derivate Rocks); as, however, they are products of the igneous rocks, it seems expedient to describe them altogether,
A. Ophyte, or Ophite; Serpentine.—A compact rock, dull in fracture, with an unctuous feel. Colour dark green to blackish or brownish, reddish or variegated.
Dana divides massive serpentine into Precious and Common Ophyte. The first "is of a rich oilgreen colour, of pale or dark shades, and translucent even when in thick pieces." The common ophyte is of dark shades of colour, and sub-translucent. "The former has a hardness of from 2"5 to 3; and the latter often of 4 or beyond, owing to impurities."
Ophyte may originally have been either whinstone or felstone. It weathers into an impure ferruginous meerschaum of a dirty yellowish colour, giving the rock a peculiar burnt or baked aspect.
A variety is 0phi-hornblende rock, or a hornblende rock in which part of the felspar or part of the amphibole is changed into ophyte.
B. Steatyte, Soap-done.—An unctuous, soft, fissile, but, at the same time, tough rock, of a light greenish, greyish, or bluish colour.
Ophyte and steatyte often occur together in intrusive masses or dykes, associated with steatitic felstone or ophi-hornblende rock, also with hornblende schist or felsitic schist. Friable felstone, felstone tuff, whinstone tuff, &c, may change into steatyte.
Varieties are,—a. Felspathic; b.Pyroxenic; and c. Amphibolic.
Steatyte, as formed from felstone or whinstone, may contain a large percentage of felspathic tuff, or of pyroxene or amphibole.
0. Bkloqyte, orEKtoGiTE.—An aggregate of green smaragdite and red garnets. The smaragdite forms a finely crystalline matrix, in which the crystals of garnet are disseminated. It is often micaceous. Some varieties undoubtedly are pseudomorphs of hornblende rock.
Note.—Some of the fissile rocks, usually called Eklogyte, are metamorphic sedimentary rocks, and their description is given under the head of Smaragdite Schist (see page 93). The rock here described is an intrusive rock.
D. Epidosyte, Epidote Bock. — An aggregate of
felspar, amphibole, epidote, pyrite, or marcasite, &c. It occurs as dykes and intrusive masses, or as nodules and lentils in the metamorphic rocks.
E. Kaolin, Porcelain Clay [Chinese, Kau-ling, high
ridge, the name of the place where the clay was first manufactured].—A white, greyishwhite, yellowish, sometimes brownish, bluish, or reddish, clay-like or mealy or compact substance, due to the decomposition of a very felsitic rock.
Baron Von Bichthofen has proved that kaolin was originally manufactured by the Chinese from Petrosilex; but though the name at first indicated an artificial felsitic clay, now the term has generally been adopted for a clay due to the decomposition of a felsitic rock. Kaolin sometimes occurs in dykes, but more often it seems to have a bedded structure, when it more properly belongs to the second order of rocks (Derivate rocks).
F. Fuller's Earth. — "A substance resembling
clay, somewhat greasy, but not in the smallest degree plastic, but falling to pieces in water; usually of a yellowish-green colour; is probably a product of the decomposition of basic igneous rocks."—Cotta.
Fuller's Earth sometimes occurs in dykes, at other times, similarly to kaolin, it seems to belong to the Derivate rocks.
G. Meerschaum, Sepiolyte. — Compact, with a
smooth feel, and fine earthy texture. Colour greyish-white, or with a faint yellowish or reddish tinge; opaque. An earthy hydrated silicate of magnesia.
Note.—Ophyte weathers into an impure meerschaum. Dana points out that sepiolyte, in Asia Minor, occurs "in masses in stratified earthy or alluvial deposits ;" and Cotta states it "forms separate beds, which are the result of a process of transmutation, probably of magnesite."
H. Magnesyte.—An aggregate of the carbonates of magnesia and iron. It may contain felspar, mica, quartz, chrome, nickel, &c.
H. Tuff. Ingenite in part, derivate in part [Ital. tafo, Gr. tophos].— The mechanical accompaniments of the Plutonic rocks, consisting of the dust, powder, cinders, fragments, blocks, and other rock debris ejected during an igneous eruption. Subsequently these were more or less stratified by water or air, and afterwards consolidated by pressure, heat, or cement; or, perhaps, by two or more combined.
Note.—The name tuff was originally applied to the mechanical accompaniments ejected during an igneous eruption, but afterwards it was erroneously used for any porous vesicular stone, often a purely sedimentary rock, while the term ash was used to denote the true tuffs. Ash, however, is an objectionable name, as an ash is the residue of any substance left after that substance has been burnt; while tuffs are never a pure ash, although portions of ash may form minor constituents. Tuff may be used as the name for the mechanical accompaniments of the Plutonic rocks, and tufa for those of the Volcanic rocks (see page 75).
Correctly speaking, no rock should be called a tuff unless the materials forming it were ejected in fragments from an igneous vent, and subsequently fell either on land or in water. In practice, however, this distinction cannot always be followed, ■".s many tuffose sandstones or tuffose shales, whose origin is mainly due to the abrasion and disintegration of ingenite rocks, are undistinguishable in aspect or composition from true tuff. Forbes has pointed out, that when outbursts of ingenite rock are forced or have flowed into the sea, they may be "at once broken up into a state of division, more or less fine, in proportion to the greater or lesser cooling power of the water mass in immediate contact, and may be spread out into beds by the action of the waves." Many rocks thus formed, although not true tuffs, are yet not only undistinguishable from them, but in some cases (except under the microscope), are also undistinguishable in aspect and composition from igneous rocks. Tuffs may be stratified or unstratified; sometimes they even occur in dykes and pipes. As they, in part, are sedimentary rocks, fossils may occur in them.
Varieties due to Composition.
A. Felstone Tuff, Felsyte or Felsitic Tuff.—
An aggregate of felspathic or felsitic parts usually more or less flaky or mealy; colour reddish, greenish, greyish, or bluish, weathering yellowish-white. Often very greasy to the touch, which appears to be due to particles or portions changing into steatite.
As previously mentioned, portions of some felstones are very flaky or mealy in aspect; these it is difficult, except when studied in situ, to distinguish from felsyte tuff.
Varieties are, — a. Quartzose; b. Calcareous; c. Pyritous; d. Cupriferous; and e. Hcmatitic.
B. Whinstone Tuff, Basic or Greenstone Tuff.—
A greenish, reddish, purplish, or greyish-bluish