mica or quartz. As a rule it is much richer in silica than the trachytes hereafter described. Cotta says it only very rarely and exceptionally contains some traces of amphibole. In some the matrix is compact and somewhat shining; in others it is enamel-like, while in others it is dull. They may be platy or vesicular or pumiceous. Usually lightcoloured; some are granitoid and undistinguishable except in their geological position and age from Elvanyte; the latter variety ought probably to be classed among the Granitic rocks. As among the Plutonic rocks, so also among the Volcanic rocks, there must be passage-rocks into true granite; among the Volcanic, however, these cannot be so prevalent as among the Plutonic, as the latter rocks have been usually exposed by the force of denudation, while rarely do we find the roots of the more recent eruptions exposed.

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b. Perlyte, or Perlite, Pearlstone, Pearlstone-porphyry. "An enamel-like matrix containing round grains, several of which structed with concentric layers.”—Cotta. Perlyte may be granular, or sphærulitic (with compact or radial striped felsyte balls), or porphyritic, or pumiceous, or vitreous (with resinous lustre), or argillaceous.

c. Obsidian (so named after its discoverer, the Roman named Obsidianus).- Trachytic or volcanic glass.

Obsidian may be compact, or a mere glass, or porphyritic, with sanidine crystals, and sometimes also mica plates, or sphærulitic, in which case it is a passage-rock into perlyte.

d. Pumice, Vesicular Obsidian (Ital. pomice, akin

to spuma, froth).—Vesicular volcanic glass. Some varieties are so porous that they will float on water.

e. Phonolyte or Phonolite Clinkstone (Gr. phonè, sound, and lithos, stone).- Compact, in places vitreous; due to cleavage-surfaces of felspar; usually dark greenish-grey. Often platy in texture, and rings when struck with a hammer. Weathers with a whitish crust, similarly to many of the felstones.

Phonolyte may be compact, vesicular, flaky, variolotic, porphyritic, or amygdaloidal.

B. TRACHYTE (Gr. trachys, rough)." A compound of sanidine, oligoclase (or even albite and labradorite), with some amphibole or pyroxene, and dark-coloured mica. A rough principal mass in which, as matrix, some of its mineral constituents are frequently distinctly and separately developed and imbedded."— Cotta.

Varieties in Composition.

a. Sanidine Trachyte.—An aggregate of sanidine crystals, with some amphibole or mica. From coarse to fine, and from porphyritic to compact.

b. Drachenfels Trachyte.-Containing oligoclase in addition to the sanidine, with some magnesia-mica, and amphibole, also pyroxene, magnetite, and titanite.

c. Domyte, or Domite (after Puy de Dôme, Auvergne). — Oligoclase trachyte, having no sanidine. It also contains some amphibole or pyroxene, and dark-coloured mica.

d. Andesyte, or Andesite (after the Andes).-Fine

or compact trachyte, sometimes vitreous; dark colour, with imbedded crystals. e. Trachydoleryte, or Trachydolerite; Greystone."A compound of oligoclase (or labradorite) with amphibole or pyroxene, some magnetite, and frequently also mica. These minerals lie imbedded in a grey or brown matrix."-Cotta.

Trachydoleryte is the passage-rock between the Trachytes and Augytes.

Varieties in Texture.

f. Granular Trachyte; g. Compact; h. Porphyry; i. Trachyte-lava or Vesicular Trachyte; and j. Alum-stone.

The last does not appear to be a normal rock, but to be due to decomposition.

J. Augitic Group.-Aggregates of felspar with pyroxene and amphibole; they frequently contain mica and magnetite, while quartz is rarely present.

The Augitic group includes all the basic volcanic rocks; they may be compact, crystalline, granular, porphyritic, amygdaloidal, vesicular, or variolitic. A. AUGYTE or AUGITE, DOLERYTE or DOLERITE, BASALT.-A crystalline granular aggregate of labradorite or nepheline and pyroxene, with some titaniferous magnetite; usually blackish or dark-coloured. In the compact mass there often occur prominently distinct grains or even crystals of olivine, labradorite, pyroxene, and magnetite.

NOTE. The name Basalt is given above, as it is in such common use; it ought, however, to be solely confined to the compact varieties. See Doleryte among the Plutonic rocks.

Varieties in Composition.

a. Nephelite Augyte, when this mineral is the felspar.

b. Hauynophyre (Rammelsberg); hauyne being in the place of labradorite.

c. Allogovyte, or Allogovite (Winkler), a reddish variety of labradoritic augyte; and d. Common Augyte, Labradoritic Augyte.

Varieties in Texture.

e. Anamesyte, or Anamesite (Leonhard). f. Basalt, or Compact Augyte.

e and f seem to be different names for the same kind of rock, as both refer to those augytes that are so fine-grained and compact that the constituents are undistinguishable.

g. Porphyritic, containing felspar crystals.

h. Amygdaloidal, having almond-shaped concretions.

i. Basalt Lava, or Vesicular Augyte, being scoriaceous or full of minute holes; and

j. Variolitic, when there are dark grains in a lighter-coloured mass.

h and i always merge one into the other, as the latter was the original condition of all amygdaloids.

B. LEUCITYTE, or LEUCITITE, LEUCITE-ROCK.— A more or less distinct aggregate of leucite and pyroxene, with some magnetite.

Varieties are,-a. Compact; b. Leucitophyre, or Porphyritic, having felspar crystals; c. Amygdaloidal; and d. Leucityte Lava, or Scorious or Vesicular Leucityte.

K. Volcanic Tufa and Peperino (Ingenite in part, Derivate in part).—According to Scrope, the Italian geologists restrict the term tufa to the felspathic or trachyte aggregates, which are grey or whitish; while the name peperino denotes the augitic varieties, which usually are brownish.

A. TUFA. — A trachytic aggregate of slag, ash, pieces of pumice and lava, with fragments of various other rocks. It may be arenaceous, argillaceous, conglomeritic, or brecciated. The principal fragments and particles are of trachyte.

Varieties are,-a. Trachyte Tufa; b. Pumiceous Tufa or Pumiceous Sand, which have received the following local names: Trass (Rhine), Tosca (Sicily), and Pausilippo Tufa (Teneriffe); c. Phonolyte; and d. Pozzuolana, a volcanic sand, very useful in the construction of mortar for hydraulic works. B. PEPERINO.—A light porous rock; augitic sand, scoria, cinders, &c., cemented together, the grains having a peppercorn-like appearance. Varieties are,-a. Augyte Peperino; b. Leucityte; and c. Palagonyte, called after Palagonia, in Sicily.

C. VOLCANIC CONGLOMERATE, with its varieties, a. Volcanic Breccia, and b. Volcanic Agglomerate. Some of the mechanical accompaniments of the volcanic rocks consist of more or less stratified accumulations of blocks and fragments of volcanic and other rocks, usually more or less loose, but sometimes cemented together. If the contained blocks are round or roundish, the rock may be called Volcanic

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