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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 irou. It may contain felspar, mica, quartz, chrome, nickel, &c.

H. Tuff. Ingenite in part, derivate in part [Ital. tufo, Gr. tophos].-The mechanical accompaniments of the Plutonic rocks, consisting of the dust, powder, cinders, fragments, blocks, and other rock débris 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, as 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. Hematitic.

B. WHINSTONE TUFF, Basic or Greenstone TuffA greenish, reddish, purplish, or greyish-bluish

rock, from coarse to fine and compact; often laminated, sometimes finely, at other times having alternate layers of fine and coarse materials; they may be conglomeritic or brecciated, when they often merge into Agglomerate. Varieties are, a. Hornblendic; b. Pyroxenic; c. Chloritic; d. Calcareous; e. Pyritous; f Cupriferous; and g. Hematitic.

C. CALCAREOUS TUFF.-Tuff cemented together by a limy or calcareous matrix.

D. ARENACEOUS TUFF. E. ARGILLOUS TUFF.-These respectively contain a large percentage of sand and clay. Many of these rocks probably are not true tuff, but are due to the disintegration of igneous rocks, the detritus being mixed with sand or mud. Some arenaceous tuffs are cemented together by a siliceous paste.

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F. HEMATITIC or FERRUGINOUS TUFF. - A reddish or yellowish ferruginous rock; sometimes argillous, at other times arenaceous. It may merge into an earthy limonite.

NOTE. Some varieties of the Indian rock called Lateryte, from Kutch, seem to be a variety of hematitic tuff. Very similar rocks are found associated with doleryte in Antrim, Ireland both in Ireland and Kutch the rock may be steatitic. G. PYRITOUS TUFF. H. CUPRIFEROUS TUFF. — Containing a large percentage of pyrite, marcasite, or chalcopyrite, so as to give a character to the rock. I. STEATITIC TUFF.-A greenish, greyish, bluish, or reddish mealy or flaky rock; having a greasy feel. It appears to be a pseudomorph.

Varieties due to Structure.

J. PORPHYRITIC TUFF.

K. NODULAR, with its varieties, a. Concretionary, and b. Spheroidal.

L. SHALY TUFF.-Fine-grained, compact, and deposited in thin laminæ or layers. There are two varieties,-a. Flaggy, when the layers are between half an inch and four inches thick, the rock easily splitting into tabular masses; and b. Slaty, when there is a well-developed slaty cleavage.

M. CONGLOMERITIC TUFF, PLUTONIC CONGLOMERATE. -Tuff containing blocks and fragments of different kinds of rock.

Its varieties are,-a. Brecciated Tuff, or Plutonic Breccia, and b. Plutonic Agglomerate.

Variety a includes those rocks that contain angular blocks and fragments, while Lyell has introduced the term Agglomerate to designate those large irregular unstratified accumulations of blocks and detritus thrown up by igneous eruptions.

Class III.-VOLCANIC ROCKS.

THE name of the class is derived from Vulcanus, the god of fire. They are eruptive or intrusive rocks, brought up by vulcanicity, and consolidated near or at the present surface of the globe. They are necessarily accompanied by tufa and such mechanically derived associates.

NOTE. None of the rocks allied to the volcanic have been placed among the granitic rocks; nevertheless, it appears highly

probable that some varieties of Liparyte (Trachyte porphyry) ought to be classed among the Elvanytes. Cotta says of the liparytes, which are granitoid in aspect, that in some cases it is impossible to distinguish between them and Quartz-porphyry (Elvanyte). "In these cases the only real difference consists in their geological connection with genuine trachytes or their petrographical transition into perlyte or pumice-stone."

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I. Trachytic group.-Compact or granular, or splintery or cellular, crystalline felsitic or felspathic rock some contain crystals of quartz, mica, and amphibole, or even pyroxene; the whole either confusedly united or imbedded in a felspathic paste. Some are porphyritic or even granitoid.

The Trachyte group embraces all the siliceofelspathic volcanic rocks. Among them are also included such transitional basic felspathic volcanic rocks as the Greystones of Scrope, the Trachydolerytes of Abich, or the Hybrid rocks of Durocher. A. RHYOLYTE, or RHYOLITE. "A compact enamellike, or vitreous matrix enclosing grains or crystals of sanidine" (rhyacolite), "oligoclase, mica, or even quartz."- Cotta.

In Rhyolyte free quartz appears much more frequently than in common Trachyte; while it contains no amphibole or pyroxene, or at least, those minerals are rarely found in it. From this it is apparent that Rhyolyte is highly siliceous or felsitic Trachyte.

a. Liparyte, or Liparite; Trachyte-porphyry.—“Is the name given to those rocks (prevalently felsitic and porphyritic with a compact matrix) which are geologically allied to the trachytes."—Cotta.

Liparyte has a compact felspathic matrix, containing crystals of felspar, and sometimes also

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