A Manual of Elementary Geology Part 57

Felspar, for example, may be, first, common felspar, that is to say, potash-felspar, in which the alkali is potash (see table, p. 377.); or, secondly, albite, that is to say, soda-felspar, where the alkali is soda instead of potash; or, thirdly, Labrador-felspar (Labradorite), which differs not only in its iridescent hues, but also in its angle of fracture or cleavage, and its composition. We also read much of two other kinds, called gla.s.sy felspar and compact felspar, which, however, cannot rank as varieties of equal importance, for both the albitic and common felspar appear sometimes in transparent or _gla.s.sy_ crystals; and as to compact felspar, it is a compound of a less definite nature, sometimes containing both soda and potash; and which might be called a felspathic paste, being the residuary matter after portions of the original matrix have crystallized.

The other group, or _hornblende_, consists princ.i.p.ally of two varieties; first, hornblende, and, secondly, augite, which were once regarded as very distinct, although now some eminent mineralogists are in doubt whether they are not one and the same mineral, differing only as one crystalline form of native sulphur differs from another.

The history of the changes of opinion on this point is curious and instructive. Werner first distinguished augite from hornblende; and his proposal to separate them obtained afterwards the sanction of Hauy, Mohs, and other celebrated mineralogists. It was agreed that the form of the crystals of the two species were different, and their structure, as shown by _cleavage_, that is to say, by breaking or cleaving the mineral with a chisel, or a blow of the hammer, in the direction in which it yields most readily. It was also found by a.n.a.lysis that augite usually contained more lime, less alumina, and no fluoric acid; which last, though not always found in hornblende, often enters into its composition in minute quant.i.ty. In addition to these characters, it was remarked as a geological fact, that augite and hornblende are very rarely a.s.sociated together in the same rock; and that when this happened, as in some lavas of modern date, the hornblende occurs in the ma.s.s of the rock, where crystallization may have taken place more slowly, while the augite merely lines cavities where the crystals may have been produced rapidly.

It was also remarked, that in the crystalline slags of furnaces, augitic forms were frequent, the hornblendic entirely absent; hence it was conjectured that hornblende might be the result of slow, and augite of rapid cooling. This view was confirmed by the fact, that Mitscherlich and Berthier were able to make augite artificially, but could never succeed in forming hornblende. Lastly, Gustavus Rose fused a ma.s.s of hornblende in a porcelain furnace, and found that it did not, on cooling, a.s.sume its previous shape, but invariably took that of augite. The same mineralogist observed certain crystals in rocks from Siberia which presented a hornblende _cleavage_, while they had the external form of augite.

If, from these data, it is inferred that the same substance may a.s.sume the crystalline forms of hornblende or augite indifferently, according to the more or less rapid cooling of the melted ma.s.s, it is nevertheless certain that the variety commonly called augite, and recognized by a peculiar crystalline form, has usually more lime in it, and less alumina, than that called hornblende, although the quant.i.ties of these elements do not seem to be always the same. Unquestionably the facts and experiments above mentioned show the very near affinity of hornblende and augite; but even the convertibility of one into the other by melting and recrystallizing, does not perhaps demonstrate their absolute ident.i.ty. For there is often some portion of the materials in a crystal which are not in perfect chemical combination with the rest.

Carbonate of lime, for example, sometimes carries with it a considerable quant.i.ty of silex into its own form of crystal, the silex being mechanically mixed as sand, and yet not preventing the carbonate of lime from a.s.suming the form proper to it. This is an extreme case, but in many others some one or more of the ingredients in a crystal may be excluded from perfect chemical union; and, after fusion, when the ma.s.s recrystallizes, the same elements may combine perfectly or in new proportions, and thus a new mineral may be produced. Or some one of the gaseous elements of the atmosphere, the oxygen for example, may, when the melted matter reconsolidates, combine with some one of the component elements.

The different quant.i.ty of the impurities or refuse above alluded to, which may occur in all but the most transparent and perfect crystals, may partly explain the discordant results at which experienced chemists have arrived in their a.n.a.lysis of the same mineral. For the reader will find that a mineral determined to be the same by its physical characters, crystalline form, and optical properties, has often been declared by skilful a.n.a.lyzers to be composed of distinct elements. (See the table at p. 377.) This disagreement seemed at first subversive of the atomic theory, or the doctrine that there is a fixed and constant relation between the crystalline form and structure of a mineral, and its chemical composition.

The apparent anomaly, however, which threatened to throw the whole science of mineralogy into confusion, was in a great degree reconciled to fixed principles by the discoveries of Professor Mitscherlich at Berlin, who ascertained that the composition of the minerals which had appeared so variable, was governed by a general law, to which he gave the name of _isomorphism_ (from +isos+, _isos_, equal, and +morphe+, _morphe_, form).

According to this law, the ingredients of a given species of mineral are not absolutely fixed as to their kind and quality; but one ingredient may be replaced by an equivalent portion of some a.n.a.logous ingredient. Thus, in augite, the lime may be in part replaced by portions of protoxide of iron, or of manganese, while the form of the crystal, and the angle of its cleavage planes, remain the same. These vicarious subst.i.tutions, however, of particular elements cannot exceed certain defined limits.

Having been led into this digression on the recent progress of mineralogy, I may here observe that the geological student must endeavour as soon as possible to familiarize himself with the characters of five at least of the most abundant simple minerals of which rocks are composed. These are, felspar, quartz, mica, hornblende, and carbonate of lime. This knowledge cannot be acquired from books, but requires personal inspection, and the aid of a teacher. It is well to accustom the eye to know the appearance of rocks under the lens. To learn to distinguish felspar from quartz is the most important step to be first aimed at. In general we may know the felspar because it can be scratched with the point of a knife, whereas the quartz, from its extreme hardness, receives no impression. But when these two minerals occur in a granular and uncrystallized state, the young geologist must not be discouraged if, after considerable practice, he often fails to distinguish them by the eye alone. If the felspar is in crystals, it is easily recognized by its cleavage: but when in grains the blow-pipe must be used, for the edges of the grains can be rounded in the flame, whereas those of _quartz_ are infusible. If the geologist is desirous of distinguishing the three varieties of felspar above enumerated, or hornblende from augite, it will often be necessary to use the reflecting goniometer as a test of the angle of cleavage, and shape of the crystal.

The use of this instrument will not be found difficult.

The external characters and composition of the felspars are extremely different from those of augite or hornblende; so that the volcanic rocks in which either of these minerals decidedly predominates, are easily recognized. But there are mixtures of the two elements in every possible proportion, the ma.s.s being sometimes exclusively composed of felspar, at other times solely of augite, or, again, of both in equal quant.i.ties.

Occasionally, the two extremes, and all the intermediate gradations, may be detected in one continuous ma.s.s. Nevertheless there are certain varieties or compounds which prevail so largely in nature, and preserve so much uniformity of aspect and composition, that it is useful in geology to regard them as distinct rocks, and to a.s.sign names to them, such as basalt, greenstone, trachyte, and others, already mentioned.

_Basalt._--As an example of rocks in which augite greatly prevails, basalt may first be mentioned. Although we are more familiar with this term than with that of any other kind of trap, it is difficult to define it, the name having been used so vaguely. It has been very generally applied to any trap rock of a black, bluish, or leaden-grey colour, having a uniform and compact texture. Most strictly, it consists of an intimate mixture of augite, felspar, and iron, to which a mineral of an olive green colour, called olivine, is often superadded, in distinct grains or nodular ma.s.ses.

The iron is usually magnetic, and is often accompanied by another metal, t.i.tanium. Augite is the predominant mineral, the felspar being in much smaller proportions. There is no doubt that many of the fine-grained and dark-coloured trap rocks, called basalt, contained hornblende in the place of augite; but this will be deemed of small importance after the remarks above made. Other minerals are occasionally found in basalt; and this rock may pa.s.s insensibly into almost every variety of trap, especially into greenstone, clinkstone, and wacke, which will be presently described.

_Greenstone_, or _Dolerite_, is usually defined as a granular rock, the const.i.tuent parts of which are hornblende and imperfectly crystallized felspar; the felspar being more abundant than in basalt; and the grains or crystals of the two minerals more distinct from each other. This name may also be extended to those rocks in which augite is subst.i.tuted for hornblende (the dolorite of some authors), or to those in which albite replaces common felspar, forming the rock sometimes called Andesite.

_Syenitic greenstone._--The highly crystalline compounds of the same two minerals, felspar and hornblende, having a granitiform texture, and with occasionally some quartz accompanying, may be called Syenitic greenstone, a rock which frequently pa.s.ses into ordinary trap, and as frequently into granite.

_Trachyte._--A porphyritic rock of a whitish or greyish colour, composed princ.i.p.ally of gla.s.sy felspar, with crystals of the same, generally with some hornblende and some t.i.taniferous iron. In composition it is extremely different from basalt, this being a felspathic, as the other is an augitic, rock. It has a peculiar rough feel, whence the name +trachys+, _trachus_, rough. Some varieties of trachyte contain crystals of quartz.

[Ill.u.s.tration: Fig. 437. Porphyry.

White crystals of felspar in a dark base of hornblende and felspar.]

_Porphyry_ is merely a certain form of rock, very characteristic of the volcanic formations. When distinct crystals of one or more minerals are scattered through an earthy or compact base, the rock is termed a porphyry (see fig. 437.). Thus trachyte is porphyritic; for in it, as in many modern lavas, there are crystals of felspar; but in some porphyries the crystals are of augite, olivine, or other minerals. If the base be greenstone, basalt, or pitchstone, the rock may be denominated greenstone-porphyry, pitchstone-porphyry, and so forth.

_Amygdaloid._--This is also another form of igneous rock, admitting of every variety of composition. It comprehends any rock in which round or almond-shaped nodules of some mineral, such as agate, calcedony, calcareous spar, or zeolite, are scattered through a base of wacke, basalt, greenstone, or other kind of trap. It derives its name from the Greek word _amygdala_, an almond. The origin of this structure cannot be doubted, for we may trace the process of its formation in modern lavas. Small pores or cells are caused by bubbles of steam and gas confined in the melted matter.

After or during consolidation, these empty s.p.a.ces are gradually filled up by matter separating from the ma.s.s, or infiltered by water permeating the rock. As these bubbles have been sometimes lengthened by the flow of the lava before it finally cooled, the contents of such cavities have the form of almonds. In some of the amygdaloidal traps of Scotland, where the nodules have decomposed, the empty cells are seen to have a glazed or vitreous coating, and in this respect exactly resemble scoriaceous lavas, or the slags of furnaces.

[Ill.u.s.tration: Fig. 438. Scoriaceous lava in part converted into an amygdaloid.

Montagne de la Veille, Department of Puy de Dome, France.]

The annexed figure represents a fragment of stone taken from the upper part of a sheet of basaltic lava in Auvergne. One half is scoriaceous, the pores being perfectly empty; the other part is amygdaloidal, the pores or cells being mostly filled up with carbonate of lime, forming white kernels.

_Scoriae_ and _Pumice_ may next be mentioned as porous rocks, produced by the action of gases on materials melted by volcanic heat. _Scoriae_ are usually of a reddish-brown and black colour, and are the cinders and slags of basaltic or augitic lavas. _Pumice_ is a light, spongy, fibrous substance, produced by the action of gases on trachytic and other lavas; the relation, however, of its origin to the composition of lava is not yet well understood. Von Buch says that it never occurs where only Labrador-felspar is present.

_Lava._--This term has a somewhat vague signification, having been applied to all melted matter observed to flow in streams from volcanic vents. When this matter consolidates in the open air, the upper part is usually scoriaceous, and the ma.s.s becomes more and more stony as we descend, or in proportion as it has consolidated more slowly and under greater pressure.

At the bottom, however, of a stream of lava, a small portion of scoriaceous rock very frequently occurs, formed by the first thin sheet of liquid matter, which often precedes the main current, or in consequence of the contact with water in or upon the damp soil.

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.)