« PreviousContinue »
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, DoLEEYTE Or DoLEEITe,
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
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/ 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, Ledcite-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 Feperino (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 (TeneriS'e); r.. 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.
G. 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 Conglomerate; if they are more or less angular, a Volcanic Breccia; while if the accumulation is massive and without stratification, Lyell's name of Volcanic Agglomerate may be adopted.
Class IV.—TRANSITION Or METAMORPHIC SEDIMENTARY ROCKS.
These rocks, similarly to the metamorphosed igneous rocks, may occur in various degrees of change, from a rock scarcely altered to a rock undistinguishable from granite, in accordance with the intensity of the metamorphic action to which they were subjected.
The lowest degree of metamorphism seems to be, principally, induration, with the planes of the most conspicuous structure (whether lamination, cleavage, or jointing) glazed or micacized, while, at the same time, peculiar structures are developed. In finely laminated, or cleaved rocks, a crumpling takes place; in others a nodular or concretionary development; in some the joint-lines and the rock in their immediate vicinity, are silicified or hardened, so that on the weathered surfaces of the rocks are formed well-marked rectangular, rhombic, or oblique depressions. In the second degree, the rocks become typical schist; in the third gneiss; while in the fourth they become granitoid, and from that pass into typical granite.
Rocks usually pass in this order to the highest degree of change; however there are exceptions, as some rocks are more susceptible of change than others, on account of their mineral constituents. Thus some sandstones, or even shales that contain the constituents of gneiss, may have a gneissoid aspect, while the associated rocks have only been changed into schist.
Note.—The following changes in sedimentary rocks were observed in co. Mayo, Ireland :—Felspathic and micaceous sandstones, changing into mica-schist and gneiss. This gneiss, however, was not typical; that is, the quartz, mica, and felspar were not arranged in leaves ; nevertheless the rocks contained these three ingredients, and had an incipient foliation;—quartzose sandstones and grits, changing through quartz-schist, or quartzitic mica-schist into gneiss; green tuffose shale and slate, through hornblende schist, chlorite schist, or talc schist into basic gneiss ; argillaceous shale and slate, through argillyte into mica-schist or chloritic mica-schist, and from that into gneiss; felstone or felsitic tuff, through felsitic schist into steatitic schist, garnetiferous schist, or talcose schist.
L. Gneiss.—When typical, a crystalline granular aggregate of quartz, felspar, and mica; occurring in leaves or plates, more or less parallel to one another. Typical gneiss is rather uncommon, as usually various other minerals besides the quartz, felspar, and mica occur, disarranging the regularity of the leaves.
Varieties in Composition.
A. Felspathic and Felsitic Gneiss, having one or more felspars as the predominant ingredients.
These may be divided into,—a. Orthoclase, b. Oligoclase, c. Alpinyte (Simler), andd. Adulariagneiss.—Cotta.
c. is named after the Alps. It is principally made up of a variety of oligoclase, and is a gneissoid aggregate of quartz, oligoclase, and an undetermined flaky green mineral. The latter Cotta suggests " probably belonging to the mica species."