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."

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

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.

b. Perlyte, or Perlite, Pearlstone, Pearlstone-porphyry.-"An enamel-like matrix containing round grains, several of which are constructed 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

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or compact trachyte sometimes vitreous; dark colour, with imbedded crystals. A e. Trachydoleryte, or Trachydoterte: 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 Dolery te 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 ƒ 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 concre

tions.

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.