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crystallized or amorphous (nicht individualisirten) substance, sometimes glassy and sometimes microfelsitic.

Rocks really formed of compacted sediment become sometimes so close-grained that their origin may not be apparent to the naked eye. Their truly derivative character is well brought out by the microscope. In fig. 4, for example, the structure of a piece of fine greywacke is shown. It will be observed that the component particles are not crystals, but broken and more or less rounded fragments of different minerals. The larger white pieces

are quartz, the darker por- Fie. 3.-Microscopic Structure of Pitchstone. A glassy base, with numerous tions consist of granules of feathered and needle-shaped microslate, felspar, and other lites, and a sanidine crystal. substances, with a little siliceous ferruginous cement. Many exceedingly compact and even flinty argillaceous rocks are in this way shown by microscopic examination to be formed of water-worn particles.

FIG. 4.-Microscopic Structure of a fragmental rock Greywacke.

Rocks which have been so affected by subsequent changes as to acquire a new crystalline character, and to receive the name Metamorphic, exhibit many characteristic features of structure under the microscope. Limestones, for example, which have been altered into saccharoid marble are found to consist of crystalline grains of calcite, showing the characteristic cleavage of that mineral (fig. 5). The foliated rocks (schists) show a curious blending of the characters of igneous and sedimentary rocks. Thus they have often a distinctly granular structure, resembling that of sedimentary deposits, with, at the same time, an arrangement of the micaceous folia reminding us of the fluid structure of igneous rocks. In fig. 6, for instance, the quartz-grains are to be ob served in layers separated by folia of mica which curve and twist like the microlites in an obsidian (compare fig. 2).

Much light has been cast FIG.5.-Microscopic Structure of Sacchaon the origin and history of roid white Marble (Carrara). igneous rocks by microscopic investigation. It is easy, for example, to see in what order the several mineral components have crystallized out of the original glass. Thus in basalt the magnetite has appeared before the augite, in which it has been abundantly enclosed. Again, the movement of the still liquid or viscid rock, when many of its crystals had already been produced, is beautifully shown

by the "fluid structure" (fig. FIG. 6.-Microscopic Structure of Mica2), where minor crystals and


particles are drawn into curving lines which bend round

the large crystals, and also by the frequent fracture of the larger crystals and the insertion of portions of the general ground-mass of the rock between the separated pieces. That intensely saline water was present during the formation of many crystalline rocks is proved by the presence in their crystals of minute cavities filled with fluid and containing cubes of halite (common salt). Liquid carbonic acid has been observed in such cavities.

Most interesting and important information is likewise afforded by the microscope regarding the subsequent changes which rocks have undergone through the influence chiefly of percolating water. Every gradation of alteration from the fresh mineral to its complete pseudomorph may be observed. In this way many serpentines have been shown to have been originally olivine rocks. It can be seen, too, how certain minerals decay, and to what products their alteration gives rise, even when the general mass of the rock looks to the eye still tolerably fresh. There can be little doubt that the application of microscopic analysis is destined to throw much light upon both the formation and the subsequent history of the sedimentary rocks. These have not yet ben so sedulously explored as the igneous groups. The obscure subject of metamorphism is especially open to elucidation by the microscopic method.1

2. Chemical Analysis.-This method of investigation must supplement the work of the microscope. A mere chemical analysis gives the ultimate chemical constitution of a rock, but may afford little clue to its mineral structure, which can only be thoroughly examined by means of the microscopic method. On the other hand, many rocks do not allow of satisfactory determination of their constitution by means of the microscope. For these chemical analysis is, of course, indispensable. But our knowledge of no rock can be considered complete until the rock has been subjected to both processes of investigation.

II. Classification and Description.

A precise yet convenient classification of rocks is still required. We may adopt chemical characters as the basis of arrangement, and group rocks according as they may be sulphates, carbonates, silicates, &c.; but in so doing we place together rocks which, from a geological point of view, have no real affinity. Again, we may select mineralogical composition as the groundwork of the classification; but in this case also great violence may be done in the geological relationships of the rocks. In many respects the long established geological arrangement according to manner of pro. duction is a useful one-igneous, aqueous, and metamorphic rocks. There is, of course, the obvious objection to it that it starts upon a preconceived theory of the origin of the rocks, and this objection must be admitted to be serious. Every year, however, is diminishing its force by making us more certain of the mode of formation of different rocks; and, probably, some modification of it will in the end be very generally adopted. In the meantime the most eligible course seems to be to choose a scheme of arrangement which, confessedly imperfect and temporary, shall recognize at once the mineralogical, chemical, and geological relations of the rocks. With this object the following classification will be adopted here.

I. Crystalline and Glassy Non-Fragmental Rocks.

1. Simple Rocks (composed of one mineral substance), -chiefly of aqueous roeks formed from chemical precipitates.

1 The reader will find this subject fully treated in Zirkel's Mikroskopische Beschaffenheit der Mineralien und Gesteine (1878); Rosen-' busch's Mikroskopische Physiographe der Mineralien und Gesteine (1873-7), Vogelsang's Krystolliten (1874), and De la Vallée and Renard, Sur les roches plutoniennes de la Belgique (Acad. Royale de Belgique, 1876).


2. Compound Rocks (composed of two or more minerals), including (a) Massive series, embracing the various igneous rocks, as granite and lava, and (6) Schistose series, including all the crystalline schists and most of the so-called metamorphic rocks. II. Fragmental or Clastic Rocks, including (a) Gravel and Sand Rocks, (b) Clay Rocks, (c) Plant-formed Rocks, (d) Rocks formed from animal remains, (e) Volcanic Agglomerates and Tuffs.

I. Crystalline and Glassy or Non-Fragmental Rocks. The great majority of these rocks are original formations; that is, they have not been palpably derived from the destraction of pre-existing rocks, as in the case of the fragniental series. They include all chemical precipitates, whether these possess a distinctly crystalline or a dull granular texture, all rocks which have consolidated from igneous fusion, and all the schistose and metamorphic rocks which, whatever may have been their original character, now possess a crystalline or foliated structure.

1. Simple Rocks.

Limestone is a mass of carbonate of lime, either nearly pure or mixed with clay or other impurity. Few rocks vary more in texture and composition. It may be a hard flinty close-grained mass, breaking with a splintery or conchoidal fracture; or a crystalline rock built up of fine crystals of calcite and resembling loaf sugar in colour and texture (fig. 5); or a dull earthy friable chalk-like deposit; or a compact massive finely-granular rock resembling a close-grained sandstone or freestone. The colours, too, vary extensively, the most common being shades of blue-grey and creamcolour passing into white. Some limestones are highly siliceous, the calcareous matter having been accompanied with silica in the act of deposition; others are argillaceous, sandy, ferruginous, dolomitic, or bituminous. To some of these varieties particular names have been assigned:-Oolite, a granular limestone built up of small roe-like grains, each of which consists of concentric coats of lime; Pisolite, an oolitic or pisolitic limestone where the grains are as large as peas; Travertine (calcareous tufa), the material deposited by calcareous springs, usually white or yellowish, varying in texture from a soft chalk-like substance or marl to a compact building-stone; Stalactite, the calcareous pendant deposit forined on the roofs of caverns, vaults, bridges, &c. The water from which the hanging lime-icicles are derived drips to the floor, and on further evaporation there gives rise to the crust-like deposit known as stalagmite. Hydraulic limestone contains sufficient silica (and usually alumina) that, when it is burnt and subsequently mixed with water, a compound containing silicate of calcium is formed, which has the property of "setting" or hardening under water. Limestones containing perhaps as much as 25 per cent. of silica, alumina, iron, &c., which in themselves would be unsuitable for many of the ordinary purposes for which limestones are used, can be used for making hydraulic mortar. These limestones occur sometimes in beds like those in the Lias of Lyme Regis, sometimes in nodules like those of Sheppey, from which Roman cement is made. Cement-stone is the name given to many pale dull ferruginous limestones, which contain an admixture of clay, and some of which can be profitably used for making hydraulic mortar or cement. Fetid limestone (stinkstein, swinestone) gives off a fetid smell, like that of sulphuretted hydrogen gas, when struck with a hammer. In some cases, as in that at North Berwick, the rock seems to have been deposited by volcanic springs containing decomposable sulphides as well as lime. In other instances the odour may be connected with the decomposition of organic matter. In some quarries in the Carboniferous Limestone of Ireland, as mentioned by Mr Jukes, the freshly broken rock may be smelt at a distance of a hundred yards when the men are at work, and occasionally the stench becomes so strong that the workmen are sickened by it and require to leave off work for a time. Cornstone is an arenaceous or siliceous limestone particularly characteristic of some of the Paleozoic red sandstone formations. Rottenstone is a decomposed siliceous limestone from which most or all of the lime has been removed, leaving a siliceous skeleton of the rock. A similar decomposition takes place in some ferruginous limestone with the result of leaving a yellow skeleton of ochre. Marble is limestone which has acquired a granular crystalline structure. Ordinary statuary marble is a familiar example of this rock. It is white, fine-grained, composed of minute crystalline granules of calcite, and resembles loaf-sugar, whence the term saccharoid" often applied to it (fig. 5). Fine silvery scales of mica or tale may often be noticed even in the purest marble. Some limestones associated with gneiss and schist are peculiarly rich in

minerals,-tremolite, actinolite, anthophyllite, zoisite, and many other species occurring there, often in great abundance. Many may be seen in the numerous kinds of ornamental marble. varieties of colour and texture occur among these limestones, as Dolomite (Magnesian Limestone) is a massive formation of the earbonates of lime and magnesia, commonly associated with gypsum, rock-salt, and other results of the evaporation of saturated sometimes full of cavities lined with crystals of dolomite, sometimes It is dull granular to finely crystalline in texture, aggregated into botryoidal, mammillated, and other concretionary

saline waters.

forms. Dolomite also occurs as the result of a chemical transforma tion of ordinary limestone, carbonate of magnesia replacing carbonate of lime. This process, known as dolomitization, was largely insisted on by Von Buch and has been discussed by Bischof. The metamorphic variety of the rock is generally quite crystalline, resembling saccharoid marble in texture, and sometimes even in of the dolomitization or conversion of limestone into dolomite the colour, though yellowish tints are apt to predominate. As a proof fact may be noticed that fossil shells, and other organisms consisting originally of calcite, bave been altered into dolomite. On a small scale a similar change may be observed in a limestone where it is of limestone where no igneous matter has penetrated, and where traversed by some igneous dyke. Even along the vertical joints percolating water has probably been the only agent of change, the limestone is changed for some distance on either side into a dull yellow dolomite, locally termed "dunstone."

Gypsum occurs as a rock in the form of beds and concretions as well as in strings and veins. It is associated with red strata, often with dolomite, rock-salt, and anhydrite.

Rock-salt, massive chloride of sodium, occurs in beds some times 60 to 90 feet in thickness, rudely crystalline, usually stained red from an admixture of red sediment, like that of the red clays and sandstones among which the salt-beds occur.

Ironstone.-Besides the iron ores met with in veins associated with other accompaniments of metalliferous lodes, there are many which were doubtless formed as chemical precipitates on the floors of lakes and other sheets of water. Some of these deposits (either peroxide or carbonate of the protoxide of iron) arise from precipitation in water or on moist ground where organic matter, espe cially of vegetable origin, has decomposed. The hard crust of hydrous peroxide of iron which forms under wet or boggy soil (moorband pan, bog-iron-ore) is an example of such a deposit now in course of formation. Where the peroxide has been reduced and become carbonate, it occurs in beds or nodules usually mixed with a variable proportion of clay (clay ironstone), and sornetimes with a good deal of carbonaceous matter from associated vegetation (blackband ironstone). Clay iron-ore is one of the most valuable ores of the metal, and occurs largely in beds and nodules in the Carboniferous system, as well as in parts of the Jurassic series in Britain. In some of the oldest geological formations extensive beds occur of hæmatite and magnetite.

The serpen

Serpentine. This mineral occurs massive, forming large bands of rock. In some places it may have been an original deposit from oceanic water, comparable to the glauconite found filling the chambers of Foraminifera, and occurring extensively both in old geological formations and on the present ocean-floor. tines associated with the gneisses and other crystalline rocks have had this origin assigned to them by Sterry Hunt. There can be no doubt, however, that many, probably most, serpentines are the results of the alteration of pre-existing rocks. Tschermak pointed out that much serpentine has been produced by the hydration and alteration of olivine, and this view has been confirmed by illustrations from all parts of the world. In many serpentines the forms of the original crystals of olivine may still be detected. Hence the difficulty in understanding how there could be intrusive masses of serpentine-a hydrated magnesian silicate-is now removed, for we see that the original olivine-rocks may have been intruded as molten masses which would preserve their external characters as eruptive rocks though undergoing an internal conversion into serpentine. With many Paleozoic limestones, and more particularly with the crystalline beds which occur among the schistose rocks, serpentine is frequently associated. Some of this may represent the result of an alteration of dolomite, though, as above stated, it may with more probability be connected with some original oceanic deposit of a magnesian silicate.

[blocks in formation]

arrangement. Besides, a rock which, like obsidian, may appear quite simple at one place may, at a short distance, show the presence of other minerals entitling it to a place among the compound rocks.

(a) Massive Rocks.-This important division consists almost entirely of rocks which have resulted from igneous fusion. Considered from a chemical point of view, these rocks may be described as mixtures in different proportions of silicates of alumina, magnesia, lime, potash, and soda, usually with magnetic iron and phosphate of lime, and, in a great group of rocks, with an excess of silicic acid, existing as free quartz. Taking this last feature as a basis of arrangement, some petrographers have proposed to divide the igneous rocks into an acid group, including such rocks as granite, quartz-porphyry, and quartz-trachyte, where the percentage of silica ranges from 60 to 75, and a basic group, typified by such rocks as leucite-lava and basalt, where the proportion of silica is only about 50 per cent.

In the vast majority of igneous rocks the chief silicate is a felspar, the number of rocks where the felspar is represented by another silicate (as leucite or nepheline) being comparatively few and unimportant. As the felspars group themselves into two great series, the monoclinic or orthoclase, and the triclinic or plagioclase, the former with, on the whole, a preponderance of silica, and as the minerals occur under tolerably distinct and definite conditions, it has been proposed to divide the felspar-bearing massive rocks into two series, (1) the orthoclase rocks, having orthoclase as their chief silicate, and often with free silica in excess, and (2) the plagioclase rocks, where the chief silicate is some species of triclinic felspar. The former series corresponds generally to the acid group above mentioned, while the plagioclase rocks are on the whole decidedly basic. It has been objected to this arrangement that the so-called plagioclase felspars are in reality very distinct minerals, with proportions of silica, ranging from 43 to 69 per cent.; soda from 0 to 12; and lime from 0 to 20. But in the state of minute subdivision in which the minerals occur in many igneous rocks, it is often scarcely possible to determine the species of felspar.

Without attempting here any formal classification, according either to relative proportion of silica or to the distinguishing felspar, it may be sufficient to arrange the following description of the massive rocks in a continuous series, with the most typical acid or orthoclase rocks at the beginning, and the basic felsparless rocks at the end.

Granite is a thoroughly crystalline-granular admixture of felspar, mica, and quartz. The felspar is chiefly orthoclase, but striated triclinic felspars (as oligoelase and albite) may often be observed in smaller quantity. The mica in most granites seems to be the potash or muscovite variety, usually of a white silvery aspect; sometimes it is dark brown or black, and belongs to biotite (magnesian mica) or lepidomelane. Dr Heddle finds the common mica of the granites in the Scottish Highlands to be a new variety, which he has called haughtonite. The quartz may be observed to forin a kind of paste or magma wrapping round the other ingredients (see fig. 1). It is only in cavities of the granite that the component minerals occur as independent well-formed crystals, and there too the accessory minerals are chiefly found, such as beryl, topaz, tourmaline, &c.

Microscopic examination of granite by Sorby and other later observers has shown that the quartz is full of cavities containing liquid, sometimes in such numbers as to amount to a thousand millions in a cubic inch. The liquid in these cavities appears usually to be water containing chlorides of soda and potash, with sulphates of potash, soda, and lime.

Granites vary in texture from extremely coarse crystalline masses, with crystals an inch or more in length, to fine granular rocks which pass into elvanite or felsite. They are sometimes porphyritic, with large scattered orthoclase crystals. The variety known as graphic granite is distinguished by the way in which the quartz is crystallized through the felspar in imperfect or hemihedral shells arranged with their longer axes generally parallel, so as to produce on cross fracture the appearance of Hebrew characters.

The mean of eleven analyses of granites made by Dr Haughton

specific gravity of 2.66.

gave the following average composition :-Silica, 72.07; alumina, 14 81; peroxide of iron, 2-22; potash, 5'11; soda, 279; line, 1-33; magnesia, 0-33; loss by ignition, 109; total, 100'05,—with a mean Granite occurs (1) as an eruptive rock, forming huge bosses, which rise through other formations both stratified and unstratified, and sending out veins into the surrounding and overlying rocks, which usually show evidence of much alteration as they approach tho granite; (2) connected with true volcanic rocks, and forming probably the lower portions of masses which flowed out at the surface as lavas; and (3) in the heart of mountain chains and elsewhere, so intimately connected with metamorphic rocks as to suggest that it is itself a final stage of the metamorphism of rocks. Granite is thus a decidedly plutonic rock; that is, it has consolidated at some depth beneath the surface, and in this respect differs from the superficial volcanic rocks, such as lavas, which have flowed out above ground from volcanic orifices.

Quartz-porphyry, Felsite-porphyry, Felstone, Felsite.-These names are given to different conditions under which orthoclase and quartz unite to form a massive eruptive rock. When the quartz forms well-marked grains, blebs, or crystals the rock is a quartz-porphyry. when the felspar and quartz are so intimately mixed as to appear to the eye as a homogeneous matrix, the rock is called by one of the other epithets.

The base of the rock, whether in the form of quartz-porphyry or felstone, is usually exceedingly compact or even flinty in texture. Under the microscope it shows the microfelsitic character, the true nature and origin of which is still the subject of much discussion among petrographers. When the quartz occurs in forms visible to the naked eye it usually appears as round or irregular grains, varying in size from mere specks up to pieces as large as a pea or larger. Less frequently it occurs definitely crystallized, and sometimes in perfect doubly terminated pyramids.

Besides the othoclase of the diffused matrix, this mineral occurs in crystalline fragments and crystals, which sometimes reach to the length of an inch or more, so as to give a markedly porphyritic character to the rock. Triclinic felspars usually occur, though perhaps not so commonly as in granite. Mica and hornblende are among the most frequent of the minerals which accompany the two essential constituents, while apatite, magnetite, and pyrito are not infrequent accessories.

The colours of quartz-porphyry and felstone depend chiefly upon those of the felspar,-flesh-red, reddish-brown, purple, yellow, bluish or slate-grey, and even white, being in different places characteristic. The presence of much mica or hornblende gives dark grey, brown, or greenish tints. It will be observed in this, as in other rocks containing much felspar, that the colour, besides depending on the hue of that mineral, is greatly regulated by the nature and stage of decomposition. A rock weathering externally with a pale yellow or white crust may be found to be quite dark in the central undecayed portion.

The flesh-red quartz-porphyry of Dobritz, near Meissen, in Saxony, was found by Rentzsch to have the following chemical composition:-Silica, 76 92; alumina, 12.89; potash, 427; soda, 0.68; lime, 0.68; magnesia, 0.98; oxide of iron, 1.15; water, 197; total, 99 54,-specific gravity, 2:49.


Besides the differences of colour already referred to, minor varieties in composition are produced by the relative abundance and size of the felspar crystals, and by the presence of mica (micaceous quartzporphyry, micaceous felstone, or felsite), hornblende (hornblendic quartz-porphyry or felstone), or other accessory ingredient. variety called minette consists of a felsite base with crystals of orthoclase and dark mica, and may be regarded as a micaceous felstone, bearing the same relation to the acid felspar-rocks (felstones) that mica-porphyrite does to the more basic forms (porphyrites). When the base is very compact, and the felspar-crystals well defined and of a different colour from the base, the rock sometimes takes a good polish, and may be used with effect as an ornamental stone. In ordinary language such a stone is classed with the "marbles," under the name of "porphyry."

Closely related to the quartz-porphyries and felstones, of which, indeed, it can be regarded only as a variety, comes the rock known as elvan or elvanile. This is a Cornish term for a crystalline-granular mixture of quartz and orthoclase, forming veins which proceed from granite, or occur only in its neighbourhood and aro evidently associated with it. It forms an immediate stage between granite and quartz-porphyry.

Quartz-porphyry and the other varieties included under this species occur (1) with plutonic rocks, as eruptive bosses or veins, often associated with granite, from which, indeed, the elvanite, as just stated, may be seen to proceed directly; of frequent occurrence also as veins and irregularly intruded masses among highly convoluted rocks, especially when these have been more or less metamorphosed; (2) in the chimneys of old volcanic orifices, forming there the "neck" or plug by which the vent is filled up; and (3) aa truly volcanic rocks which have been erupted at the surface in the form of flows of lava, cither (a) submarine, as in the felstones of X.

- 30

Wales, associated with the marine Lower Silurian rocks, or (¿) subaerial, as probably in the quartz-porphyry of Arran, and perhaps in the series of the green-slates and porphyries" of the Silurian system in Cumberland, which Professor Ramsay has conjectured to be the products of a subaerial volcano.

The rocks enumerated up to this point are all orthoclase-rocks, and markedly siliceous, frequently showing their excess of silica the form of quartz grains or crystals. In the succeeding group free quartz is not found as a marked constituent, although occasionally it occurs in some quantity. In this series syenite may be regarded as the equivalent of granite in the quartzose series, orthoclase porphyry of quartz-porphyry and felstone, and trachyte of liparite." Syenite.-According to the modern nomenclature, this name, which was formerly given in England to a granite with horntially of a mixture of orthoclase and hornblende, to which plagioclase, quartz, and mica are occasionally added. The name syenite, first used by Pliny in reference to the rock of Syene, was introduced by Werner as a scientific designation, and applied to the rock of the Plauenscher-Grund, Dresden. Werner afterwards, however, inade that rock a greenstone. The base of all syenites like that of granites is crystalline, without a trace of any amorphous substance between the crystals, such as most igneous rocks contain. Hence the texture is of that crystalline kind commonly known as granitic.

Pitchstone (Retinite) is a glassy rock, having a pitch-like or resinous Istre, and a black or dark-green colour ranging through shades of green, brown, and yellow to nearly white. It is essentially an orthoclase rock, and may be regarded as the natural glass of many of the more granular or crystalline orthoclase rocks, such as the quartz-blende replacing mica, is now restricted to a rock consisting essenporphyries or felsites. Examined microscopically, it is found to consist of glass in which are diffused, in greater or less abundance, hair-like microlites, angular or irregular grains, or more definitely formed crystals. The pitchstone of Corriegills, in the island of Arian, presents abundant green, feathery, and dendritic microlites of a pyroxenic character (see fig. 3).

According to Durocher the mean composition of pitchstone issilica, 706; alumina, 150; potash, 16;-soda, 24; lime, 12; magnesia, 06; oxides of iron and manganese, 26; loss by ignition, 60. The mean specific gravity is given as 2:34.

Pitchstone is found either as intrusive dykes, veins, or bosses, probably in close connexion with former volcanic activity, or in Sheets which, like the porphyritic pitchstone of the Isle of Eigg, flowed out at the surface as lava-streams.

Liparite (Rhyolite, Quartz-trachyte) is an orthoclase rock con: taining an excess of silica which usually appears in distinct grains or in doubly terminated pyramids. The orthoclase, which is of the variety termed sanidine, is sparingly accompanied with triclinic felspar. Other frequent ingredients are magnesia-mica, hornblende, augite, apatite, and magnetite. Considerable diversity exists in the texture of this rock. Some varieties are coarse and granitoid in character, and are regarded by some petrographers as the equivalents in Tertiary times of the granite of older geological periods. From this crystalline aspect intermediate varieties may be obtained like the quartz-porphyries, passing by degrees into more or less distinctly vitreous rocks. Throughout all these gradations, however, a characteristic ground mass can be seen under the microscope having a glassy, enamel-like, or porcellanous character. An analysis by Vom Rath of a rhyolite from the Euganean Hills gave-silica, 7603; alumina, 13 32; soda, 5'29; potash, 3'83; protoxide of iron, 174; magnesia, 0:30; lime, 0.85; loss, 0:32; total, 101 68.-specific gravity, 2.553. Liparite is a volcanic rock of late geologi cal date occurring in the form of erupted lavas.

Obsidian is a volcanic glass representing the vitreous condition of a highly silicated sanidine-rock, such as liparite. It resembles bottle glass, having a perfect conchoidal fracture, and breaking into sharp splinters, semi-transparent or translucent at the edges. Tho color of the rock are black, brown, or greyish-green, rarely yellow, blue. or red, but not infrequently streaked or banded with paler and darker lines. When a thin slice of obsidian is prepared for the microscrope it is found to be very pale yellow, grey, or nearly colourless. On being magnified it shows that the usual dark colours are almost always produced by the presence of minute crystals, needles, and black hair-like bodies. In rare examples the obsidian appears as a perfect glass without any foreign admixture. The minute crystals and hair-like bodies sometimes so increase in abundance as to make the rock lose the aspect of a glass and assume that of a dull flint-like or enamel-like stone. This devitrification can only be properly studied with the microscope. Again little granules (spherulites) of a dull grey enamel (pearlstone) appear, and in some parts of the rock so abundantly as to alter its character and convert it from obsidian into pearlstone. The average chemical composition of the rock is-silica, 710; alumina, 138; potash, 40; soda, 52; lime, 1'1; magnesia, 06; oxides of iron and manganese, 37; loss, 06; total, 1000,--mean specific gravity, 2:40. Obsidian occurs as a product of the volcanoes of late geological periods.

Pearlstone (Perlite) is another vitreous condition of sanidine lava. As its name denotes, it consists of vitreous or enamel-like globules, occasionally assuming polygonal forms by mutual pressure. These globules sometimes constitute the entire rock, their outer portions shading off into each other so as to form a compact mass; in other cases they are separated by and cemented in a compact glass or enamel. They consist of successive very thin shells, which, in a transverse section, are seen as concentric rings, usually full of the same kind of hair-like crystallites and crystals as in obsidian. Occasionally there are found among them true spherulites where the internal structure is radiating fibrous. When such spherulites occupy the main mass they give rise to spherulite-rock.

Pumice is a general term for. the cellular and filamentous or froth-like parts of lavas. In the great majority of cases it is a form of the obsidians, showing under the microscope the usually vitreous characters, and possessing a specific gravity of 20 to 2:53, though, owing to its porous nature, it possesses great buoyancy and readily floats on water. At Hawaii, however, some of the pyroxenic or olivine lavas give rise to a pumiceous froth which has the usual outward characters of ordinary pumice.

The typical syenite of the Plauenscher-Grund, formerly described as a coarse-grained mixture of flesh-coloured orthoclase and black hornblende, containing no quartz, and with no indication of plagioclase, was regarded as a normal orthoclase-hornblende rock. Microscopical research has, however, shown that well-striated triclinic felspar, as well as quartz, occurs in it. Its composition-is shown by the following analysis:-silica, 59 83; alumina, 16-85; protoxide of ion, 701; lime, 443; magnesia, 261; potash, 6·57; soda, 2:44; water, &c., 129; total, 101-03. The average specific gravity of syenite is from 2-75 to 2.90.

Syenite occurs under conditions similar to those in which granite is found; it has been erupted in large irregular masses, especially among metamorphic rocks, as well as in smaller bosses and veins. Orthoclase Mphyry (Quartzless Porphyry) is an orthoclase rock containing no quarz, or a very sparing admixture of that mineral, but with a little plagioclase, and not unfrequently with some hornblende and dark biotite.. It con ains from 55 to 65 per cent. of silica. It differs thus from quartz-porphyry and felstone in its smaller preportion of silica, but the distinction is one which, except by chemical or microscopical analyses, must often be difficult to establish between the fine compact felstones and the orthoclase porphyries, especially when the latter contain free quartz. This rock is sometimes termed syenite-porphyry, since it is associated with syenite much in the same way that elvanite is with granite. It is like syenite a plutonic rock, and occurs in veins, dykes, and intrusive sheets: Probably, however, many of the so-called "felstones" which occur as lavas, contemporaneously ejected with the older Palæozoic formations, are really orthoclase-porphyries.

Trachyte, a term originally applied to a large series of modern volcanic rocks possessing a characteristic roughness (7paxús) under the finger, is now restricted to rocks consisting essentially of sanidine, with more or less triclinic felspar, usually with hornblende, biotite, or augite, and sometimes with magnetite and apatite. In microscopic structure the rock is distinguishable from the quartz-trachytes or liparites by the absence or feeble development of any microfelsitic ground-mass, and in general by the presence of a porphyritic base, consisting either of a pure glass or of one with devitrification products. The average composition of trachyte may be stated thus:-silica, 600; alumina, 170; protoxide of iron, 80; magnesia, 10; lime, 35; soda, 40; potash, 50; loss by ignition, 15. Average specific gravity, 265. Trachyte is a volcanic rock of Tertiary and post-Tertiary date.

Phonolite (Clinkstone), a term suggested by the metallic ringing sound emitted by the compact varieties when struck, is applied to a mixture of sanidine felspar and nepheline with hornblende and usually nosean. An average specimen contained silica, 577; almina, 2016; potash, 60; soda, 70; line, 1'5; magnesia, 0.5; oxides of iron and manganese, 35; loss by ignition, 3-2 per cent. The specific gravity may be taken as about 2:58. Phonolite is sometimes found splitting into thin slabs which can be used for roofing pur poses. Occasionally it assumes a porphyritic texture from the presence of large crystals of sanidine or of hornblende. When the rock is partly decomposed and takes a somewhat porous texture, it resembles trachyte in appearance.

Like trachyte, phonolite is a thoroughly volcanic rock and of late geological date. It occurs sometimes filling the pipes of volcanic orifices, sometimes as sheets which have been poured out in the form of lava-streams, and sometimes as dykes and veins.

In the rocks enumerated up to this point the essential felspar constituent is orthoclase; in the felspar rocks now to be described the corresponding ingredient is nearly always some triclinic form. In the volcanic rocks of this series there is usually some mineral of the hornblende or augite family present in such quantity as to give a green or even black colour to the mass.

Porphyrite may be used as the designation of rocks which consist essentially of some triclinic felspar, and show a glassy or partially devitrified ground-mass containing abundant crystals of plagioclase

with magnetite or titaniferous iron, and sometimes hornblende, augite, or mica. These rocks include many varieties which have not yet been thoroughly examined. The texture varies from coarse crystalline-granular to exceedingly close-grained, and passes occasionally even into vitreous. Porphyrite is a volcanic rock very characteristic of the later paleozoic formations; occurring there as interstratified lava-beds, and in eruptive sheets, dykes. veins, and irregular bosses.

Diorite (Greenstone in part) is a crystalline mixture of qligoclase or some allied felspar and hornblende with magnetic iron and apatite. Where free quartz occurs the rock is called quartz-diorite. The more compact dark varieties have been termed aphanite. The average chemical composition of this rock may be taken to besilica, 532; alunina, 160; potash, 13; soda, 22; lime, 63; magnesia, 6'0; oxides of iron and manganese, 140; loss by ignition, 10. The mean specific gravity is about 2:95. Diorite occurs as an eruptive rock under conditions similar to those of quartz-porphyry and syenite. It is found in paleozoic volcanic regions, as in North Wales, in "neck"-like masses which may mark tlie position of some of the volcanic orifices of eruption.

Propylite is a name given to certain Tertiary volcanic rocks consisting of a plagioclase felspar and hornblende in a fine-grained groundmass. They are subject to considerable alteration, the hornblende being converted into epidote. Some quartziferous propylites have been described by Zirkel from Clarence King's Survey of the 40th Parallel, wherein the quartz abounds in liquid cavities containing briskly moving bubbles, and sometimes double enclosures with an interior of liquid carbonic acid.

Hornblende-Andesite is a rock of late geological date consisting of a plagioclase felspar and hornblende often with a little sanidine. The ground-mass is frequently quite crystalline, or shows a small proportion of a felsitic nature, with microlites and granules. When the rock contains free quartz it is called Dacite.

In the next series of rocks augite plays a similar part to that taken by hornblende in the foregoing species.

Diabase. This name has been given to certain dark green or black eruptive rocks found in the older geological formations and consisting essentially of a triclinic or plagioclase felspar, augite, magnetic or titaniferous iron, sometimes olivine, and usually with more or less of a diffused greenish substance which has resulted from the alteration of the angite or olivine. The texture is sometimes quite crystalline; in other cases it shows a felsitie ground-inass. Except that the so-called diabases are confined to Paleozoic rocks and the basalts to Tertiary and post-Tertiary formations, there seems no essential distinction between these two groups, though, of course, as the diabases are much older and have been far longer exposed to metamorphic processes, they are in general less fresh than the basalts.

Melaphyre is a term which has been so variously used that the sense in which it is taken must always be explained. Thus Seuft described it as a rock having an indistinctly mixed character, with colours ranging from dirty greenish-brown, or reddish-grey, or greenish black-brown, to a complete black; hard and tough when fresh, and then showing crystals of reddish-grey labradorite, with magnetic titaniferous iron, and usually with carbonates of lime and iron, and ferruginous chlorite (delessite), in crystalline grains; compact or earthy, or sometimes porphyritic or amygdaloidal. Naumann defines melaphyre as a close-grained reck, very often amygda loidal, composed essentially of labradorite, with an undetermined silicate, some titaniferous iron, carbonates of lime and of iron, and sometimes crystals of augite, rubellan, and mica. it generally crypto-crystalline, sometimes porphvritic, very often amygdaloidal rock, consisting of a mixture of oligoclase and augite with magnetic iron. Lastly, Rosenbusch proposes to restrict it to those plagioclase-augite rocks which contain olivine, and possess a distinct porphyry ground-mass.. There can be little doubt that, like the so-called diabases, the melaphyres are merely older forms of the great basalt-family.

Zirkel called

Augite-Andesite is the name given to certain dark eruptive rocks of Tertiary and post-Tertiary date which consist of a triclinic felspar (oligoclase, or some species rather richer in silica than labradorite) and augite, with sometimes saniline, hornblende, biotite, magnetite, or apatite, and in some varieties quartz. The composition of an example from Santorin, erupted in the year 1866, was found to be as follows:-silica, 67 35; alumina, 15 72; magnesia, 116; line, 360; soda, 504; potash, 186; oxides of iron, 194; loss by ignition, 0.36; total, 101-06. Basalt.-Under this term are included those widespread and imMean specific gravity, 275. portant volcanic rocks which consist of a triclinic felspar, probably always labradorite, augite, olivine, and magnetic or titaniferous iron. with apatite, and sometimes sanidine or nepheline. The more Quarsely crystalline varieties are known as dolerite, while those of intermediate texture have been termed anamesite, the more close-rained black heavy kinds being distinctively basalts. The chemical composition of an average variety may be set down as silica, 450; alumina, 150; magnesia, 65; line, 105; soda. 35; potash.15; oxides of iron and manganese, 150; loss by ignition, 30. Mean specific gravity, 2.95.

Zirkel has described under the name of basalt certain rocks in which the part of the felspar is played by another mineral, in some cases nepheline, in others leucite.

Basalt occurs as dykes and veins, intrusive bosses and sheets, and successive contemporaneous flows. It often presents a columnar structure, as at Stalla and the Giant's Causeway, whence the structure has been popularly termed basaltic

Tachylite is a black volcanic glass often found in association with basalt, of which indeed it is merely the vitreous condition. Thus a basalt-dyke sometimes shows a thin crust of this pitch-like substance on its walls, where the molten rock was first suddenly cooled.

Palagonite is a volcanic glass related to basalt, and found in fragments in some tuffs.

Gabbro (Diallage-rock) is a compound of a triclinic felspar and diallage often with olivine, and also very generally with magnetic or titaniferous iron and apatite, more rarely with hornblende, biotite, or quartz. An average chemical composition is silica, 500; alumina 150; magnesia, 70; lime, 100; soda, 25; potash, 0'5; oxides of iron and manganese, 125; loss by ignition, 2.5. Mean specific gravity, 2.97

A very few crystalline massive rocks occur without felspar as an essential constituent; but they are of comparatively little importance as rock-masses, though interesting in themselves and sometimes of considerable beauty.

Pikrite is a rock rich in olivine, usually more or less serpentinized, with augito, magnetite, or ilmenite, and a little brown biotite, hornblende, or apatite.

Eulysite is a mixture of olivine, augite, and red garnet.

Garnet-olivine-rock is composed of olivine, diallage, and garnet. Enstatite-olivine-rock consists of olivine and enstatite (bronzite or hypersthene) with magnetite or chromite.

Lherzolite is a mixture of olivine, pyroxenc, picotite, and usually some magnetite.

Eclogite is a compound of garnet and omphacite, or smaragdite (hornblende).

Dunite is a mixture of olivine and chromite, found with serpentine. Limburgite is composed of crystals of olivine, sugite. and magnetite, in a base more or less vitreous.

(b.) Schistose or Foliated Rocks.-These form an exceedingly well-defined characteristic series. They are distinguished from the massive rocks by the possession of an internal arrangement into more or less closely parallel layers or folia, consisting of materials which have assumed a crystalline character along these layers. The layers may be composed of only one mineral, but are usually of two or more, which occur either in distinct, often alternate, lamine or intermingled in the same layer. Moreover, the layers are not usually continuous for more than a short space. As a rule they are strikingly lenticular, thickening out and then dying away, and reappearing after an interval on the same or a different plane. They are likewise characteristically welded, as it were, into each other, the crystalline particles of one lamina being so inter mingled with those of the layers above and below it that the whole coheres as a tough not easily fissile mass. Though arranged in distinct layers, a schistose rock is usually distinguished from an ordinary sedimentary one by the irregularity of its lamination, and by a remarkable and eminently distinctive puckering or crumpling of the folia. The vast plications which can be seen from a distance running up the face of a mountain are repeated on a smaller scale in hand specimens, and even down to such proportions as can only be seen with a microscope. As already stated, the origin of these rocks has been the subject of much discussion. That they are metamorphosed sediments, and not original See part iv. chemical precipitates, is the general opinion of geologists.

A foliated rock showing this characteristic irregular fissility in a marked degree is termed a "schist." This word, placed after the distinguishing mineral of the rock, is used as the name of the rock, as mica-schist, chlorite-schist, hornblende-schist. If the mass loses its fissile tendency owing to the felting together. of the component mineral stituted for schist, as in hornblende-rock, actinolite-rock, and into a tough coherent whole, the word rock is usually sub80 or. There are thus three kinds of fissility among rocks:

(1) that of original deposites in shale,-this is termed

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