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lamination; (2) that of cleavage, as in clay slate; (3) that of feliation, as in the schists. There is a fourth kind of divisional planes, that of joints, sometimes so closely placed together as almost to rival the others, as will be pointed out in part iv.

Mica-schist (Mica-slate) is a schistose aggregate of quartz and mica. The relative proportions of the two minerals vary widely even in the same mass of rock. Each is arranged in lenticular wavy lamina. The quartz shows greater inconstancy in the number and thickness of its folia. Frequently a layer of this mineral may often be seen to swell out to a thickness of an inch or more, and, dwindling rapidly down to a mere thread, disappear. The quartz may often be observed to retain a granular character like that of quartz-rock, no doubt indicative of its originally sedimentary origin (see fig. 6). The mica lies in thin plates, sometimes so dovetailed into each other as to form long continuous irregular crumpled folia, separating the quartz layers, and often in the form of thin spangles and meinbranes running in the quartz. Among the accessory minerals, garnet, felspar, and hornblende are not infrequent. Mien-schist forms extensive regions in Norway, Scotland, the Alps, and other parts of Europe, in connexion with other members of the schistose family of rocks. It is also found encircling granite masses in Scotland and Ireland as a metamorphic zone a mile or so broad, which shades away into the unaltered strata of greywacke or slate outside. Though the possession of a fissile structure, showing abundant divisional surfaces covered with glistening mica, is characteristic of mica-schist, we must distinguish between this structure and that of many micaceous sandstones which can be split into thin scams each splendent with the sheen of its mica-flakes. A little examination will show that in the latter case the mica has not crystallized in situ, but exists merely in the form of detached worn scales, which, though lying on the same general plain, are not welded into each other as in a schist; also that the quartz does not exist in folia but in rounded separate grains.

Gneiss is a crystalline schistose aggregate of the same minerals as in granite-felspar, quartz, and unica. The relative proportions of these minerals, and the manner in which they are grouped with each other, give rise to numerous varieties of the rock. As a rule the folia are coarser and the schistose character less perfect than in mica-schist. Sometimes the quartz lies in tolerably pure bands a foot or even more in thickness with plates of mica scattered through it. These quartz layers may be replaced by a crystalline mixture

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of quartz and felspar, or the felspar will take the form of independent lenticular folia, while the mica runs abundantly through the rock, Sometimes the mic is mostly or wholly replaced by hornblende, in and by its own ready cleavage imparts a fissile structure to the whole, other cases by tale protogine-gueiss). Like mica-schist, gneiss occupies a large space in regions where the older geological forma. tions come to the surface. Varieties of it are also found in the metamorphic zone encircling some masses of granite. So course is the texture of many gneisses that they cannot, in hand-specimens nor even in large blocks, be certainly discriminated from granite. In such cases it is only by examination in the field and the detection of clear evidence of foliated structure that their true character can be determined.

An interesting and important variety is met with in some regions of gneiss and schist, viz., conglomerate bands in which pebbles of quartz and other materials from less than an inch to more than a foot in diameter are imbedded in a foliated matrix. Examples of this kind are found in the piss of the Tete Noir between Martigny and Chamouni, in N. W. Îrdland, in the islands of Bute and Islay, and in different parts of Argyllshire. These enclosures are not to be dis tinguished from the ordinary water-worn blocks of true conglomer ates; but the original matrix which encloses them has been so altered as to acquire a micaceous foliated structure, and to wrap the pebbles round as with a kind of glaze. These facts are of considerable value in regard to the theory of the origin of the crystalline schists. Granulie (Leptynite) is a crystalline schistose aggregate of orthoclase and quartz, with some garnet and kyanite.

Chlorite-schist (or Chlorite-slate) is a schistose aggregate of green chlorite, often with some quartz, felspar, inica, or tale. The more massive forms (lapis ollaris, potstone) can be cut as building stone, or for the manufacture of articles for domestic use.

Talc-schist is a schistose aggregate of whitish-green or yellowish tale often combined with felspar or quartz. Dr Heddle has recently shown that many so-called tale-schists contain no tale, but one their unctuous character to a variety of mica (margarodite).

Horablade-schist is a schistose mass of black or dark-green hornblonde, but often interleaved with felspar, quartz, or mica. Whe the schistose character disappears, the mass becomes a hornblende rock (amphibolite). When the variety actinolite occurs instead of cominon hornblende it forms actinolite-schist.

Numerous other varieties of schists have been described, but they occupy very subordinate places among the foliated rocks. Th following analyses show the chemical composition of the more important of those which have been enumerated:

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As the disappearance of the schistose structure produces a crystalline amorphous compound like that of a massive or ordinary igneous rock, we are brought at last round again to rocks which we cannot distinguish from those to which elsewhere an igneous origin is assigned. In gneiss, for example, the same minerals occur which form granite, and possess a crystalline character. Any process, such as irregular internal motion of the mass, which could destroy the schistose structure and produce a thoroughly granite-like texture, would give rise to a rock which, whatever its previous history might have been, could not be distinguished from granite. That such internal transformations have taken place among the crystalline gneissose masses can hardly be doubted. And thus, at the one end of the schistose series, we may have ordinary unaltered sediment; at the other, after many intermediate stages, a thoroughly crystalline amorphous rock like grauite or syenite.

II. Fragmental (Clastic) Rocks. This great series embraces all rocks of a secondary or derivative origin; in other words, all formed of par

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ticles which had previously existed on the surface of the earth in another form, and the accumulation and consolidation of which gave rise to new compounds. Some of these rocks have been produced by the mechanical action of running water, such as gravel, sand, and mud; others have arisen from the gathering together of the remains of once living plants or animals; others have been formed by the consolidation of the loose debris thrown out by volcanoes.

(a.) Gravel and Sand Rocks.-Ordinary gravel and sandl are produced by the action of running water on every seacoast and river-course. These sedimentary materials, being mere mechanical formations, vary indefinitely in composition, according to the nature of the source from which they are derived. As a rule they consist of the detritus of siliceous rocks, these being among the most durable materials. Quartz, in particular, enters largely into the composition of sandy and gravelly detritus. Fragmentary materials tend to. group themselves according to their size and relative density. Hence they are apt to occur in layers, and to show the characteristic stratified arrangement of sedimentary rocks. They may enclose the remains of any plants or animals entombed on the same sea-floor, river-bed, or lake-bottom.

Blown sand is saul which has been produced by previous waveaction, and is blown into long ridges or dunes by prevailing winds. It varies in composition as ordinary sandstones do, being sometimes entirely siliceous, sometimes calcareous where derived from triturated shells or other calcareous organisms. Layers of finer and coarser particles often alternate as in water-formed sandstone. Grasses and other plants bind the surface of the shifting sand, but are apt to be covered by fresh encroachments of the loose material, and then by their decay they give rise to dark peaty layers in the sand. Calcareous blown sand is compacted into hard stone by the action of rain-water which alternately dissolves a little of the lime and re-deposits it on evaporation as a thin crust cementing the grains of sand together. Clif-debris consists of angular rubbish disengageil by frost and ordinary atmospheric waste from the ace of cliffs, crags, and steep slopes. It slides down the declivities of hilly regions, and accumu lates at the base of slopes and precipices, until washed away by rain or by brooks. It naturally depends for its composition upon the nature of the solid rocks from which it is derived. The material constituting glacier moraines is of this kind.

Rain-wash is a loam or earth which accumulates on the lower parts of slopes or at their base, and is due to the gradual descent of the finest particles of disintegrated rocks by the transporting action of rain. Brick-earth is the name given in the south-east of England to thick masses of such loam which are extensively used for making bricks.

Subsoil is the broken-up part of the rocks immediately under the soil. Its character of course is determined by that of the rock out of which it is formed by subaerial disintegration.

Soil is the product of the subaerial decomposition of rocks and of the decay of plants and animals. Primarily the character of the soil is determined by that of the subsoil, of which indeed it is merely a further disintegration. The formation of soil is treated in part ii, pages 265, 269. Conglomerate (Puddingstone) is a name given to any rock formed of consolidated gravel or shingle. The component pebbles are rounded and water-worn. They may consist of any kind of rock, though usually of some hard and durable sort, such as quartz or quartz-rock. A special name may be given according to the nature of the pebbles, as quartz-conglomerate, limestone-conglomerate, graniteconglomerate, &c. The pasto or cementing matrix may consist of a hardened sand or clay, and may be siliceous, calcareous, argillaceous, or ferruginous. In the coarser conglomerates, where the blocks may exceed 6 feet in length, there is often very little indication of stratification. Except where the flatter stones show by their general parallelism the rude lines of deposit, it may be only when the mass of conglomerate is taken as a whole, in its relation to the rocks below and above it, that its claim to be considered a stratified rock will be

conceded.

Breccia is a rock in which the stones are angular and not rounded, and usually with less trace of stratification than in conglomerate. Intermediate stages between this rock and the preceding, where the stones are partly angular and partly subangular and rounded, are known as brecciated conglomerate.

Sandstone is a rock formed of consolidated sand. The component grains are for the most part of quartz-a most durable mineral, which must here be regarded as the residue left after all the more decomposable minerals of the original rocks have been carried away in solution or in suspension as fine mud.

The colours of sandstones arise, not so much from that of the quartz, which is commonly white or grey, as from. the film or crust which often coats the grains and holds them together as a cement. As already stated iron is the great colouring ingredient of rocks. In sandstones it gives rise to red, brown, yellow, and green hues, according to its degree of oxidation and hydration. In ordinary red sandstones, for example, each grain of sand is coated with red carthy hæmatite. In yellow sandstone the oxide has become hydrous in the form of limonite.

There is as much variety of composition among sandstones as among conglomerates. Though they consist for the most part of siliceous grains, they include others of clay, felspar, mica, or other mineral; and these may increase in number so as to give a special character to the rock. Thus sandstones may be argillaceous, felspathic, micaceous, calcareous, &c. By an increase in the argillaceous constituents, a sandstone may pass into one of the clay-rocks, just as modern sand on the sea-floor shades imperceptibly into mud. On the other hand, by an augmentation in the size of the grains a sandstone may become a grit, or a pebbly or conglomeratic sandstone, and pass into a fine conglomerate. A piece of fine-grained sandstone seen under the microscope looks like a coarse conglomerate, so that the difference between the two rocks is little more than one of relative size.

Among the varieties of sandstones may be mentioned Flagstone, a thin-bedded sandstone capable of being split into slabs or flags; Freestone, a sandstone which can be cut freely in any direction (the term is popularly applied to some limestones and other rocks); and Buhrstone, a highly siliceous, exceedingly compact, though ceilular,

rock (with Chura seeds, &c.), found alternating with unaltered Tertiary strata in the Paris basin, and forming froui its hardness and roughness an excellent material for the grindstones of flour-mills.

Greywacke is a compact aggregate of rounded or subangular grains of quartz, slate, felspar, or other minerals or rocks cemented by a paste which is usually siligcous but may be argillaceous, felspathic, or calearcous. Grey, as its name denotes, is the prevailing colour; but it passes into brown, brownish-purple, and sometimes, where anthracite occurs, into black. The rock is distinguished from ordinary sandstone by its darker hue, its hardness, the variety of its component grains, and above all by the compact cement in which the grains are imbedded. In many varieties so pervaded is the rock by the siliceous paste that it possesses great toughness, and its grains seem to graduate into each other as well as into the surrounding matrix. Such rocks when fine-grained can hardly, at first sight or with the unaided eye, be distinguished from some compact igneous rocks, though a microscopic examination at once reveals their fragmental character. In other cases, where the greywacke has been formed mainly out of the debris of granite, quartz-porphyry, or other felspathic masses, the grains consist so largely of felspar, and the paste also is so felspathie, that the rock might be mistaken for some close-grained granular porphyry. Greywacke occurs extensively among the Paleozoic formations in beds alternating with shales and conglomerates. It represents the sand of the Paleozoic sea-floor, retaining often its ripple-marks and sun-cracks. The metamorphism it has undergone has generally not been great, and for the most part is limited to induration, partly by pressure and partly by permeation of a siliceous cement.

That

Quartz-rock (Quartzite) is a close-grained granular aggregate of quartz cemented by a highly siliceous inatrix. Originally it consisted of a tolerably pure quartz-sand, which has been metamorphosed by pressure and the transfusion of a siliceous cement into an exceedingly hard mass. This cement was probably produced by the solvent action of heated water upon the quartz grains, which very generally seem to shade off into each other, or into the intervening silica. It is owing no doubt to the purely siliceous character of the grains that the blending of these with the surrounding cement is more intimate than in greywacke, so much so that the rock often assumes an almost flinty homogeneous texture. quartz-rock as here described is an original sedimentary rock and not a chemical deposit is shown, not only by its granular texture, but by the exact resemblance of all its leading features to ordinary sandstone-false-bedding, alternation of coarser and finer layers, worm-burrows, and fucoid-casts. It occurs in the forin of large masses interstratified with limestones, slates, and schists. It is also met with locally as an altered form of sandstone, where this rock is traversed by igneous dykes and indurated into quartz-rock for a distance of a few inches or feet from the intrusive mass. Bands of highly silicated sandstones, having the lustrous aspect, fine grain, and great hardness of quartz-rock, occur among the unaltered shales and other strata of the Carboniferous system. In such cases, the supposition of any general metamorphisin being inadmissible, we must suppose either that these quartzose bands have been indurated, for example, by the passage through them of thermal silicated water, or that the quartz-rock is there an original formation.

(b.) Clay-rocks.-These are composed of the finer argillaceous sediments or muds derived from the waste of previously formed rocks. Perfectly pure clay, hydrated silicate of alumina, may be seen where some granites and other felspar-bearing rocks decompose. But, as a rule, the clay is mixed with various impurities.

Pipe-clay is white, nearly pure, and free from iron. Fire-clay is a deposit largely found in connexion with coal-scams, contains little iron, and is nearly free from lime and alkalies. Some of the most typical fire-clays are those long used at Stourbridge, Worcestershire, for the manufacture of pottery. The best glasshouse pot-clay, that is, the most refractory, and therefore used for the construction of pots which have to stand the intense heat of a glass-house, has the following composition-silica, 73-82; alumina, 15 88; protoxide of iron, 295; lime, trace; magnesia, trace; alkalies, 90; sulphuric acid, trare; chlorine, trace; water, 6:45; specific gravity, 2:51. A very siliceous close-grained or flinty variety, termed Gannister, occurs in the Lower Coal-measures of the north of England, and is now largely ground down as a material for the hearths of iron furnaces." Brick-clay is properly rather an industrial than a geological term, since it is applied to any clay, leam, or earth, from which bricks or coarse pottery are made. It is an impure clay, containing a good deal of iron, with other ingredients. An analysis gave the following composition of a brick-clay-silica, 49'44: alumina, 34 26; sesquioxide of iron. 774; lime, 148: magnesia, 5·14; water, 194.

Mudstone is a fine, usually more or less sandy, argillaceous rock, having no fissile character, and of somewhat greater hardness than any form of clay. The term Cloy-rock has been applied by some

writers to an indurated clay requiring to be ground and mixed with water before it acquires plasticity.

When clay has been deposited intermittently so as to assume a thinly stratified or fissile structure, it receives the general name of Shale. Under this terin are included all laminated and indurated clays which are capable of being split along the lines of deposit into hard leaves. They present almost endless varieties of texture and composition, passing on the one hand into clays, on the other into flagstones and sandstones, or again, through calcareous gradations into limestone, or through ferruginous varieties into clayironstone, and through bituminous kinds into coal. An important variety, known as Oil-shale, and containing so much bituminous matter that it is now extensively used as a source for the manufacture of solid paraffin and mineral oils is described in the next section.

Flinty-slate (Lydian-stone, Hornstone) is siliceous shale or mudstone which has been indurated into an exceedingly compact flinty mass, breaking with a conchoidal or splintery fracture, and usually of dark colours, black, brown, and red, more rarely white.

Clay-slate is a compact close-grained, very hard, fissile argillaceons rock, dull lead-blue, grey, green, red, purple, or black in colour, splitting into thin leaves which are not those of original deposit but those produced by a superinduced cleavage. In this case the rock has been affected by great lateral pressure, whereby its particles have been forced to adjust themselves with their longer axes perpendicular to the direction of pressure. This rearrange ment has imparted to the rock a fissility wholly independent of original lamination. The possession of this cleavage is the distinctive character of a true slate.

(c.) Rocks formed of the Debris of Plants.-These have sometimes been produced by the decay and entombment of vegetation on the spot where it grew, sometimes by the drifting of the plants to a distance and their consolidation there. In the latter case, they may be mingled with inorganic sediment, so as to pass into carbonaceous shale.

Peat is vegetable matter, more or less decomposed and chemically altered, found in boggy places and elsewhere where marshy plants grow and decay. It varies from a pale yellow or brown fibrous, substance, like turf or compressed hay, in which the plant remains are abundant and conspicuouts, to a compact dark-brown or black material, resembling black clay when wet and some varieties of lignite when dried. The nature and proportions of the constituent elements of peat, after being dried at 100° C., are illustrated by the analysis of an Irish example which gave-carbon, 60 48; hydrogen, 610; oxygen, 32 55; nitrogen, 088; while the ash was 3 30.

There is always a large proportion of water which cannot be driven off even by drying the peat. In the manufacture of compressed peat for fuel this constituent, which of course greatly lessens the value of the peat as compared with an equal weight of coal, is driven off to a great extent by chopping the peat into fine pieces, and thereby exposing a large surface to evaporation. The ash varies in amount from less than 100 to more than 65 per cent., and consists of sand, clay, ferric oxide, sulphuric acid, and minute proportions of lime, soda, potash, and magnesia.

Lignite is compressed and chemically altered vegetable matter, often retaining a lamellar or ligneous texture, and stems with woody fibre crossing each other in all directions. It varies from pale brown or yellow to deep brown or black. Some shade of brown is the usual colour, whence the name brown coal, by which it is often known. It occurs in beds chiefly among the Tertiary strata, under conditions similar to those in which coal is found in older formations. It may be regarded as a stage in the alteration and mineralization of vegetable matter intermediate between peat and true coal. Coal, the most completely mineralized form of vegetable matter, occurs as a black (sometimes dark-brown), brittle, usually lustrous substance, intercalated in beds between strata of sandstone, shale, fireclay, &c., in geological formations of Palaeozoic, Secondary, ami Tertiary age. The word coal is rather a popular than a seienific term, as it is indiscriminately applied to any mineral substance capable of being used as fuel. Strictly employed it onght only to be used with reference to beds of fossilized vegetation, the result either of the growth of plants on the spot or of the drifting of them thithor.

The following analyses show the chemical constituents in some of the principal varieties of coal :—

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Diatom-earth is a siliceous deposit formed chiefly of the frustules of diatoms. It is laid down both in salt and in fresh water. Wide tracts of it are now being deposited on the bed of the South Pacific. In Virginia, United States, an extensive tract occurs covered with diatom-earth to a depth of 40 feet. It is used as a polishing or tripoli powder.

Vil shale (Brandschiefer) is shale containing such a proportion of hydrocarbons as to be capable of yielding mineral oil on slow distillation. This substance occurs as ordinary shales do, in layers or beds, interstratified with other aqueous deposits, as in the Scottish coal-fields. It is in a geological sense true shale, and owes its peculiarity to the quantity of vegetable (or animal) matter which has been preserved among its inorganic constituents. It consists of fissile argillaceous layers, highly impregnated with bituminous matter, passing on one side into common shale, on the other into cannel or parrot coal. The richer varieties yield from 30 to 40 gallons of crude oil to the ton of shale. They may be distinguished from non-bituminous or feebly bituminous shales (throughout the shale districts of Scotland) by the peculiarity that a thin paring curls up in front of the knife, and shows a brown lustrous streak. Some of the shales in the Lothians are crowded with the valves of ostracol crustaceans, besides scales, corprolites, &c., of ganoid fishes (Palæoniscus, Amblypterus, Megalichthys, &c.); and it is possible that the bituminous matter may in some cases have resulted from animal organisms, though the abundance of plant remains indicates that it is probably in most cases of vegetable origin. Under the name "pyroschists" Sterry Hunt classes the clays or shales (of all geological ages) which are hydrocarbonaceous, and yield by distillation volatile hydrocarbons, inflammable gas, &c.

(d) Rocks formed of Animal Remains.-These may be formed on land, as in bone caves, but most abundantly under water, as on the bottom of lakes and of the sea. They may be calcareous, siliceous, or phosphatic.

Limestone.-Besides the limestones resulting from the deposition of chemical precipitates of carbonate of lime, there is another important series derived from the remains of organisms, either by growth on the spot, or by accumulation as mechanical sediment. Limestone so originating has often been so altered that it cannot always be distinguished from that which has been chemically produced, especially when it has been exposed to the action of percolating acidulated water, for in that case a crystalline texture is gradually superinduced, by which the original organic structures in the mass are wholly or in great part obliterated. Limestone composed of the remains of living organisms forms thin layers and massive beds. In some instances, as in that of the English and Irish Mountain Limestone, it occurs in hiasses several thousand feet thick, which extend for hundreds of square miles, and form the rock out of which picturesque valleys, gorges, hills, and table-lands have been excavated. Limestone may be either of fresh water or of marine origin. Some of the more common and important varieties may be here enumerated :

lime.

Coral-rock is linestone formed by the continuous growth of coralbuilding polyps. This substance affords an excellent illustration of the way in which organic structure may be effaced from a limestone entirely formed from the remains of once living animals. Though the skeletons of the reef-building corals remain distinct on the upper surface, those of their predecessors beneath them are gradually obliterated by the passage through them of percolating water dissolving and redepositing carbonate of This same action may be observed among the stalactites of a damp vault, in which, though the successive rings of growth are preserved, a crystalline divergent structure is superinduced, which traverses these rings from the centre outward. We can thus understand how a mass of crystalline limestone may have been produced from one formied out of organic remains without the action of any subterranean heat, but merely by the permeation of water from the surface. Crinoidal (Encrinite) Limestone is a rock composed in great part of joints of encrinites, with Foraminifera, corals,' and mollusks. It varies in colour from white or pale grey, through shades of bluish-grey (sometimes yellow or brown, less commonly red) to a dark-grey or even black colour. It is abundant among Paleozoic formations, being especially characteristic of the lower part of the Carboniferous system. Chalk as a lithological term is applied to a white soft rock, meagre to the touch, soiling the fingers, formed of a fine calcareous flour derived from the remains of Foraminifera, echinoderms, mollusks, and other marine organisms. It occurs in massive beds, and covers a great part of the south-east and east of England. In Ireland and elsewhere it assumes a firmer grain and various colours, so as to pass into some of the numerous varieties of compact white limestone Shell-Marl, a soft white, earthy, or crumbly deposit, is formed in lakes and ponds by the accumulation of the remains of shells and Entomostraca on the bottom. When such calcareous deposits become solid compact stone they are known as fresh-water (lacustrine) limestone. These are generally of a

emooth texture, and either dull white or pale grey, their fracture only slightly conchoidal, rarely splintery. Ooze is a mud of organic origin found covering vast areas of the floor of the Atlantic and other oceans. Some of it is calcareous and formed wholly or mostly of the remains of Foraminifera, particularly of forms of the genus Globigerina; hence this deposit has been termed foraminiferal or globigerina coze. Sometimes it is mainly siliceous, consisting of the remains of Radiolaria (Radiolarian ooze) or of diatoms (Diatom ooze). These deposits are further referred to in the section of this article which treats of the geological aspects of the ocean. Shell-sand is a sand composed in great measure or wholly of comminuted shells, found commonly on a low shelving coast exposed to prevalent on-shore winds. This deposit when thrown above the reach of the waves and often wetted by rain, or by trickling runnels of water, is apt to become consolidated into a mass, owing to the solution and redeposit of lime round the grains of shell.

Flint is a

Chert

Flint and Chert are siliceous rocks (which, though not strictly fragmental, may be conveniently placed here) found in nodules and layers in limestones of many different geological ages. dark horny substance, breaking with a splintery to conchoidal fracture. It is particularly abundant in the chalk formation. is an impure flint, containing more clay or lime with the silica. These substances seem in some cases to have had a directly organic origin, having been secreted from sea-water by the living organisms; in other cases, where for example we find a calcareous shell, or echinus, or coral, converted into silica, it would seem that the substitution of silica for lime has been effected by a process of chemical pseudomorphism either after or during the formation of the lime. stone,1

(e.). Volcanic Fragmental Rocks form an interesting group composed of the loose materials ejected from volcanic vents. In their typical condition they consist merely of consolidated volcanic debris, including bombs, scoriæ, ejected blocks, sand, lapilli, and dust. It is evident, however, that, when these materials were deposited in water, there would necessarily be a limit beyond which they would not extend, and where they would be mingled with and would insensibly pass into ordinary non-volcanic sediment. Hence we may expect to find transitional varieties between rocks formed directly from the results of volcanic explosion and ordinary sedimentary deposits. Moreover, as these fragmental vol. canic masses usually consist almost wholly of the detritus of different lavas, which have been blown into fragments in the volcanic chimneys, we may expect to find, on the other hand, a passage from them into rocks derived from consolidated lava-beds by ordinary aqueous erosion. (See part iv.)

Volcanic Conglomerate is a rock composed mainly or wholly of rounded or subangular fragments of any volcanic rocks in a paste derived chiefly or wholly from the same materials, usually exhibiting a stratified arrangement, and often found intercalated between successive sheets of lava. In most cases conglomerates of this kind have been formed by the accumulation of materials ejected from volcanic vents; occasionally, as just remarked, they may have resulted from the aqueous erosion of previously solidified lavas, or from a combination of both these processes. There does not appear at present to be any satisfactory method of always determining the exact mode of formation, except that well-rounded and smoothed stones will almost certainly indicate long-continued water-action

rather than trituration in a volcanic vent.

The volcanic conglomerates may receive different names according to the nature of the component fragments: thus we have basalt-conglomerates, where these fragments are wholly or mainly of basalt, trachyte-conglomerates, porphyrite-conglomerates, phonolite-congloerates, &c.

Volcanic Breccia resembles volcanic conglomerates, except that the stones are angular. This angularity indicates an absence of aqueous erosion, and, under the circumstances in which it is found, usually points to volcanic explosions. There is a great variety of breccias, as basalt-breccia, diabase-breccia, &c.

Volcanic Agglomerate is the name given to a tumultuous assemblage of blocks of all sizes up to masses several yards in diameter. It is met with in the "necks" or pipes of old volcanic orifices. The stones and paste are commonly of one or more volcanic locks, such as basalt, or porphyrite, but they include also fragments of the surrounding rocks, whatever these may be, through which the volcanic orifice has been drilled. As a rule agglomerate is devoid of stratification; but sometimes it includes portions which have a more or less distinct arrangement in beds of coarser and finer detritus, often placed on end or inclined in different directions at high angles.

Hull and Hardman on Chert, Trans. Roy. Dub. Soc., new ser., vol. i. 71, 1878.

Volcanic Tuff.-This general term may be made to include all the finer kinds of volcanic detritus, ranging on the one hand through coarse gravelly deposits into conglomerates, and on the other into exceedingly compact fine-grained rocks formed of the finest and most impalpable kind of volcanic dust. Some tuffs are full of microlites or imperfect forms of crystallization derived from the lava which has blown into dust. Others are formed of small rounded or angular grains of different lavas with fragments of various rocks through which the volcanic funnels have been drilled. Minutely cellular grains, as if derived from the ebullition of very fluid glassy lava like palagonite, constitute much of the tuff in some of the volcanic necks of Carboniferous age in central Scotland. Some tuffs have consolidated under water, others on dry land. As a rule they are distinctly stratified. Near the original vents of eruption they commonly present rapid alternations of finer and coarser detritus indicative of successive phases of volcanic activity. The tuffs may be subdivided according to the nature of the lava from the disintegration of which they have been formed. Thus we have felsite-tuffs, trachyte-tuffs, basalt-tuffs, pumice-tuffs, porphyritetuffs, palagonite-tuffs. Some varieties have received special names. Trass (Duckstein, Tuffstein) is a compact yellow pumiceous tuff which has filled up some of the valleys of the Eifel region and is largely quarried as an hydraulic mortar. Peperino is an Italian tuff of late geological date, full of separate crystals of augite an other mineral

PART III-DYNAMICAL GEOLOGY.

Under this section is included the investigation of those processes of change which are at present in progress upon the earth, whereby modifications are made on the structure and composition of the crust, on the relations between the interior and the surface, as shown by volcanoes, earthquakes, and other terrestrial disturbances, on the distribution of oceans and continents, on the outlines of the land, on the form and depth of the sea-bottom, on climate, and on the races of plants and animals by which the earth is tenanted. It brings before us, in short, the whole range of activities which it is the province of geology to study, and leads us to precise notions regarding their relations to each other, and the results which they achieve. A knowledge of this branch of the subject is thus the essential groundwork of a true and fruitful acquaintance with the principles of geology, seeing that it necessitates a study of the present order of nature, and thus provides a key for the interpretation of the past.

The whole range of operations included within the scope of inquiry in this branch of the science may be regarded as a vast cycle of change, into which we may break at any point, and round which we may travel, only to find ourselves brought back to our starting-point. It is a matter of comparatively small moment at what part of the cycle we begin our inquiries. We shall always find that the changes we see in action have resulted from some that preceded, and give place to others which follow them.

At an early time in the earth's history, anterior to any of the periods of which a record remains in the visible rocks, the chief sources of geological action probably lay within the earth itself. The planet still retained a great store of its initial heat, and in all likelihood was the theatre of great chemical changes, giving rise. perhaps, to manifestations of volcanic energy somewhat like those which have so marvellously roughened the surface of the moon. As the outer layers of the globe cooled, and the disturbances due to internal heat and chemical action became less marked, the influence of the sun, which must always have operated, would then stand out more clearly, giving rise to that wide circle of superficial changes wherein variations of temperature and the circulation of air and water over the surface of the earth come into play.

In the pursuit of his inquiries into the past history and into the present régime of the earth, the geologist must needs keep his mind ever open to the reception of evidence for kinds and especially for degrees of action which he had not before imagined. Human experience has been too short

to allow him to assume that all the causes and modes of geological change have been definitively ascertained. On the earth itself there may remain for future discovery evidence of former operations by heat, magnetism, chemical change, or otherwise, which may explain many of the phenomena with which geology has to deal. Of the influences, so many and profound, which the sun exerts upon our planet, we can as yet only dimly perceive a little. Nor can we tell what other cosmical influences may have lent their aid in the evolution of geological changes.

In the present state of our knowledge, all the geological energy upon and within the earth must ultimately be traced back to our parent sun. There is, however, a certain propriety and convenience in distinguishing between that part of it which is due to the survival of some of the original energy of the planet, and that part which arises from the present supply of energy received day by day from the sun. In the former case we have to deal with the interior of the earth and its reaction upon the surface; in the latter we deal with the surface of the earth, and to some extent with its reaction on the interior. This distinction allows of a broad treatment of the subject under two divisions :-

I. Hypogene or Plutonic Action-the changes within the earth caused by original internal heat and by chemical action.

II. Epigene or Surface Action--the changes produced on the superficial parts of the earth, chiefly by the circulation of air and water set in motion by the sun's heat.

DIVISION I.-HYPOGENE ACTION.

An Inquiry into the Geological Changes in Progress
beneath the Surface of the Earth.

In the discussion of this branch of the subject we must carry in our minds the conception of a globe still intensely hot in its interior, radiating heat into space, and consequently contracting in bulk. Portions of molten rocks from inside are from time to time poured out at the surface. Sudden shocks are generated by which destructive earthquakes are propagated to and along the surface. Wide geographical areas are pushed up or allowed to sink down. In the midst of these movements very remarkable changes are produced upon the rocks of the crust; they are shattered, fractured, squeezed, crumpled, rendered crystalline, and even fused.

Section I.-Volcanoes and Volcanic Action.

The term volcanic action (vulcanism or vulcanicity) embraces all the phenomena connected with the expulsion of heated materials from the interior of the earth to the surface. Among these phenomena there are some of an evanescent character, while others leave permanent proofs of their existence. It is naturally to the latter that the geologist gives the chief attention, for it is by their means that he can trace the former phases of volcanic activity in regions where, for many ages, there have been no volcanic eruptions. In the operations of existing volcanoes he can observe only the superficial manifestations of volcanic action. But, examining the rocks of the earth's crust, he discovers that in the lapse of ages, amid the many terrestrial revolutions which geology reveals, the very roots of former volcanoes have been laid bare, displaying subterranean phases of vulcanism which could not be studied in any modern volcano. Hence an acquaintance only with active volcanoes will not give us a complete knowledge of volcanic action. It must be supplemented and enlarged by an investigation of the traces of former volcanoes preserved in the crust of the earth.

The openings by which the heated materials from the interior reach the surface include volcanoes (with their

accompanying orifices), hot-springs, and gas-springs. A volcano may be defined as a 'conical eminence, composed wholly or mainly of materials which have been ejected from below, and which have accumulated at the surface round the vent of eruption. As a rule it presents at its summit a cup-shaped cavity termed the crater, at the bottom of which is the top of the main funnel or pipe whereby the communication is maintained with the heated interior. A volcano, when of small size, may consist merely of one diminutive cone; when of the largest dimensions, it forms a huge mountain, with many subsidiary cones and many lateral fissures or pipes, from which the heated volcanic products are given out.

Volcanoes may break through any kind of geological formation. In Auvergne, in the Miocene period, they burst through the granitic and gneissose plateau of central France. In Lower Old Red Sandstone times they pierced contorted Silurian rocks in central Scotland. În late Tertiary and post-Tertiary ages they found their way through soft marine strata, and formed the huge piles of Etna, Somma, and Vesuvius. On the banks of the Rhine, at Bonn and elsewhere, they have penetrated some of the older alluvia of that river. In many instances, also, newer volcanoes have appeared on the sites of older ones. In Scotland the Carboniferous volcanoes have risen on the sites of those of the Old Red Sandstone, those of the Permian period have broken out among the earlier Carboniferous eruptions, while the Miocene lavas have been injected into all these older volcanic masses. Again, the newer pays of Auvergne were sometimes erupted through much older and already greatly denuded basalt-streams. Somma and Vesuvius have arisen out of the great Neapolitan plain of marine tuff. In central Italy, also, newer cones have been thrown up upon the great Roman plain of more ancient volcanic debris.

It is usual to class volcanoes as active, dormant, and extinct. This arrangement, however, often presents considerable difficulty in its application. An active volcano cannot of course be mistaken, for even when not in actual eruption it shows, by its abundant evolution of steam and hot vapours, that it might break out into activity at any moment. But it is in many cases impossible to decide whether a volcano should be called extinct or only dormant. The volcanoes of Silurian age in Wales, of Carboniferous age in Ireland, of Permian age in the Hartz, of Miocene age in the Hebrides, are certainly all extinct. But the Miocene volcances of Iceland are still represented there by SkaptarJökull, Hecla, and their neighbours, Somma, in the first century of the Christian era, would have been naturally regarded as an extinct volcano. Its fires had never been known to have been kindled within human tradition; its vast crater was a wilderness of wild vines and brushwood, haunted, no doubt, by wolf and wild-boar. Yet in a few days, in the autumn of the year 79 the half of the crater walls was blown out by a terrific series of explosions, the present Vesuvius was then formed within the limits of the earlier crater, and since that time volcanic action has been intermittently exhibited up to the present day. Some of the intervals of quietude, however, have been so considerable that the mountain might then again have been claimed as an extinct volcano. Thus, in the 131 years between 1500 and 1631, so completely had eruptions ceased that the crater had once more become choked with copsewood. A few pools and springs of very salt and hot water remained as memorials of the former condition of the mountain. But this period of quiescence closed with the eruption of 1631, the most powerful of all the known explosions of Vesuvius, except the great one of 79.

In the island of Ischia, Mont Epomeo was last in eruption in the year 1302, its previous outburst having taken place,

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