Part 57 (2/2)
The more compact lavas are often porphyritic, but even the scoriaceous part sometimes contains imperfect crystals, which have been derived from some older rocks, in which the crystals pre-existed, but were not melted, as being more infusible in their nature.
Although melted matter rising in a crater, and even that which enters rents on the side of a crater, is called lava, yet this term belongs more properly to that which has flowed either in the open air or on the bed of a lake or sea. If the same fluid has not reached the surface, but has been merely injected into fissures below ground, it is called trap.
There is every variety of composition in lavas; some are trachytic, as in the Peak of Teneriffe; a great number are basaltic, as in Vesuvius and Auvergne; others are andesitic, as those of Chili; some of the most modern in Vesuvius consist of green augite, and many of those of Etna of augite and Labrador-felspar.[374-A]
_Trap tuff, volcanic tuff._--Small angular fragments of the scoriae and pumice, above mentioned, and the dust of the same, produced by volcanic explosions, form the tuffs which abound in all regions of active volcanos, where showers of these materials, together with small pieces of other rocks ejected from the crater, fall down upon the land or into the sea. Here they often become mingled with sh.e.l.ls, and are stratified.
Such tuffs are sometimes bound together by a calcareous cement, and form a stone susceptible of a beautiful polish. But even when little or no lime is present, there is a great tendency in the materials of ordinary tuffs to cohere together.
Besides the peculiarity of their composition, some tuffs, or _volcanic grits_, as they have been termed, differ from ordinary sandstones by the angularity of their grains. When the fragments are coa.r.s.e, the rock is styled a volcanic _breccia_. _Tufaceous conglomerates_ result from the intermixture of rolled fragments or pebbles of volcanic and other rocks with tuff.
According to Mr. Scrope, the Italian geologists confine the term _tuff_, or tufa, to felspathose mixtures, and those composed princ.i.p.ally of pumice, using the term _peperino_ for the basaltic tuffs.[374-B] The peperinos thus distinguished are usually brown, and the tuffs grey or white.
We meet occasionally with extremely compact beds of volcanic materials, interstratified with fossiliferous rocks. These may sometimes be tuffs, although their density or compactness is such as to cause them to resemble many of those kinds of trap which are found in ordinary dikes. The chocolate-coloured mud, which was poured for weeks out of the crater of Graham's Island, in the Mediterranean, in 1831, must, when unmixed with other materials, have const.i.tuted a stone heavier than granite. Each cubic inch of the impalpable powder which has fallen for days through the atmosphere, during some modern eruptions, has been found to weigh, without being compressed, as much as ordinary trap rocks, and to be often identical with these in mineral composition.
The fusibility of the igneous rocks generally exceeds that of other rocks, for there is much alkaline matter and lime in their composition, which serves as a flux to the large quant.i.ty of silica, which would be otherwise so refractory an ingredient.
It is remarkable that, notwithstanding the abundance of this silica, quartz, that is, crystalline silica, is usually wanting in the volcanic rocks, or is present only as an occasional mineral, like mica. The elements of mica, as of quartz, occur in lava and trap; but the circ.u.mstances under which these rocks are formed are evidently unfavourable to the development of mica and quartz, minerals so characteristic of the hypogene formations.
It would be tedious to enumerate all the varieties of trap and lava which have been regarded by different observers as sufficiently abundant to deserve distinct names, especially as each investigator is too apt to exaggerate the importance of local varieties which happen to prevail in districts best known to him. It will be useful, however, to subjoin here, in the form of a glossary, an alphabetical list of the names and synonyms most commonly in use, with brief explanations, to which I have added a table of the a.n.a.lysis of the simple minerals most abundant in the volcanic and hypogene rocks.
_Explanation of the names, synonyms, and mineral composition of the more abundant volcanic rocks._
AMPHIBOLITE. _See_ Hornblende rock, amphibole being Hauy's name for hornblende.
AMYGDALOID. A particular form of volcanic rock; _see_ p. 372.
AUGITE ROCK. A kind of basalt or greenstone, composed wholly or princ.i.p.ally of granular augite. (_Leonhard's Mineralreich_, 2d edition, p. 85.)
AUGITIC-PORPHYRY. Crystals of Labrador-felspar and of augite, in a green or dark grey base. (_Rose_, _Ann. des Mines_, tom. 8. p. 22. 1835.)
BASALT. Chiefly augite--an intimate mixture of augite and felspar with magnetic iron, olivine, &c. _See_ p. 371. The yellowish green mineral called olivine, can easily be distinguished from yellowish felspar by its infusibility, and having no cleavage. The edges turn brown in the flame of the blow-pipe.
BASANITE. Name given by Alex. Brongniart to a rock, having a base of basalt, with more or less distinct crystals of augite disseminated through it.
CLAYSTONE and CLAYSTONE-PORPHYRY. An earthy and compact stone, usually of a purplish colour, like an indurated clay; pa.s.ses into hornstone; generally contains scattered crystals of felspar and sometimes of quartz.
CLINKSTONE. _Syn._ Phonolite, fissile Petrosilex; a greenish or greyish rock, having a tendency to divide into slabs and columns; hard, with clean fracture, ringing under the hammer; princ.i.p.ally composed of compact felspar, and, according to Gmelin, of felspar and mesotype. (_Leonhard_, _Mineralreich_, p. 102.) A rock much resembling clinkstone, and called by some Petrosilex, contains a considerable percentage of quartz and felspar.
As both trachyte and basalt pa.s.s into clinkstone, the rock so called must be very various in composition.
COMPACT FELSPAR, which has also been called Petrosilex; the rock so called includes the hornstone of some mineralogists, is allied to clinkstone, but is harder, more compact, and translucent. It is a varying rock, of which the chemical composition is not well defined, and is perhaps the same as that of clay. (_MacCulloch's Cla.s.sification of Rocks_, p. 481.) Dr.
MacCulloch says, that it contains both potash and soda.
CORNEAN. A variety of claystone allied to hornstone. A fine h.o.m.ogeneous paste, supposed to consist of an aggregate of felspar, quartz, and hornblende, with occasionally epidote, and perhaps chlorite; it pa.s.ses into compact felspar and hornstone. (_De la Beche_, _Geol. Trans._ second series, vol. 2. p. 3.)
DIALLAGE ROCK. _Syn_. Euphotide, Gabbro, and some Ophiolites. Compounded of felspar and diallage, sometimes with the addition of serpentine, or mica, or quartz. (_MacCulloch. ibid_. p. 648.)
DIORITE. A kind of greenstone, which see. Components, felspar and hornblende in grains. According to _Rose_, _Ann. des Mines_, tom. 8. p. 4., _diorite_ consists of albite and hornblende.
DIORITIC-PORPHYRY. A porphyritic greenstone, composed of crystals of albite and hornblende, in a greenish or blackish base. (_Rose_, _ibid._ p. 10.)
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