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In Hopkins's hypothesis he assumed the crust to be | infinitely rigid and unyielding, which is not true of any material substance. Sir William Thomson has recently returned to the problem, in the light of his own researches in vortex-motion. He now finds that, while the argument against a thin crust and vast liquid interior is still invincible, the phenomena of precession and nutation do not decisively settle the question of internal fluidity, though the solar semi-annual and lunar fortnightly nutations absolutely disprove the existence of a thin rigid shell full of liquid. If the inner surface of the crust or shell were rigorously spherical, the interior mass of supposed liquid could experience no precessional or nutational influence, except in so far as, if heterogeneous in composition, it might suffer from external attraction due to non-sphericity of its surfaces of equal density. But "a very slight deviation of the inner surface of the shell from perfect sphericity would suffice, in virtue of the quasi-rigidity due to vortexmotion, to hold back the shell from taking sensibly more precession than it would give to the liquid, and to cause the liquid (homogeneous or heterogeneous) and the shell to have sensibly the same precessional motion as if the whole constituted one rigid body."1

The assumption of a comparatively thin crust requires that the crust shall have such perfect rigidity as is possessed by no known substance. The tide-producing force of the moon and sun exerts such a strain upon the substance of the globe that it seems in the highest degree improbable that the planet could maintain its shape as it does unless the supposed crust were at least 2000 or 2500 miles in thickness. That the solid mass of the earth must yield to this strain is certain, though the amount of deformation is so slight as to have hitherto escaped all attempts to detect it. Had the rigidity been even that of glass or of steel, the deformation would probably have been by this time detected, and the actual phenomena of precession and nutation, as well as of the tides, would then have been very sensibly diminished. The conclusion is thus reached that the mass of the earth" is on the whole more rigid certainly than a continuous solid globe of glass of the same diameter."4

(b.) Argument from the tides.-The phenomena of the oceanic tides are only explicable on the theory that the earth is either solid to the centre, or possesses so thick a crust (2500 miles or more) as to give to the planet practical solidity. Sir William Thomson remarks that, "were the crust of continuous steel, and 500 kilometres thick, it would yield very nearly as much as if it were india-rubber to the deforming influences of centrifugal force, and of the sun's and moon's attractions." It would yield, indeed, so freely to these attractions "that it would simply carry the waters of the ocean up and down with it, and there would be no sensible tidal rise and fall of water relatively to land."5 Mr George H. Darwin has recently investigated mathematically the bodily tides of viscous and semi-elastic spheroids, and the character of the ocean tides on a yielding nucleus. His results tend to increase the force of Sir William Thomson's argument, since they show that "no very considerable portion of the interior of the earth can even distantly approach the fluid condition," the effective rigidity of the whole globe being very great.

(c.) Argument from relative densities of melted and solid rock. The two preceding arguments must be considered decisive against the hypothesis of a thin shell or crust covering a nucleus of molten matter. It has been further urged, however, as an objection to this hypothesis, that cold

1 Sir W. Thomson, Brit. Assoc. Rep., 1876, Sections, p. 5.
Thomson, Proc. Roy. Soc., April, 1862. 3 Thomson, loc. cit.
Thomson, Trans. Roy. Soc. Edin., xxiii. 157.
Thomson, Brit. Assoc. Rep., 1876, Sections. p. 7.
Proc. Roy. Soc., No. 188, 1878.

solid rock is necessarily more dense than hot melted rock, and that even if a thin crust were formed over the central molten globe it would immediately break up and the frag ments would sink towards the centre. Undoubtedly this would happen were the material of the earth's mass of the same density throughout. But, as has been already pointed out, the specific gravity of the interior is at least twice as much as that of the visible parts of the crust. If this difference be due, not merely to the effect of pressure, but to the presence in the interior of intensely heated metallic substances, we cannot suppose that solidified portions of such rocks as granite and the various lavas could ever have sunk into the centre of the earth, so as to build up there the honey-combed cavernous mass which might have served as a nucleus in the ultimate solidification of the whole planet. From the considerations above advanced we have seen that the earth's central mass may be plausibly conjectured to be metallic. Into this dense central mass the comparatively light crust could not sink, though its earliest formed portions would no doubt descend until they reached a stratum with specific gravity agreeing with their own, or until they were again melted,

3. The ingenious suggestion of Mr Fisher, already cited (ante, p. 217), in favour of the existence of a possible fluid or viscous substratum between the flexible outer shell and an inner rigid nucleus, is made with the view of reconciling the requirements of physics with those facts in geology which seem to demand the existence of a mobile mass of intensely hot matter at no great depth beneath the surface. Whether it does so must be left for physicists to decide. But, on geological grounds, it may be questioned whether such a fluid substratum is needed. We must bear in mind that the land of the globe, regarding the geological structure of which alone we know anything, covers but a small part of the whole surface of the planet; that the existing continents seem from earliest times to have specially suffered from the reaction between the heated interior and the cooled exterior, forming, as it were, lines of relief from the strain of compression; and that along such lines, if the substance of the interior be everywhere just about the melting point, relief from pressure by corrugation would cause liquefaction of the matter so relieved, and its ascent towards the surface; so that evidences of volcanic action on the terrestrial ridges might be expected to occur, and to be referable to all ages. Mr Fisher assumes the contraction of rock in cooling to be 000007 linear for one degree Fahr.; and he argues that, as this amount would not account for the observed contraction in the crust, we must have recourse to some additional explanation, such as the escape of steam and vapours from volcanic orifices. The validity of the assertion that the amount of horizontal compression of the superficial strata is greater than the cooling of a solid earth can account for may be questioned. The violently contorted rocks indicative of great horizontal compression occur chiefly along the crests of the great terrestrial ridges where the maximum effects of corrugation were to be looked for. To the argument from climate it may be replied on the other hand, with great plausibility, that secular changes may be accounted for by the effect of the variations in the eccentricity of the earth's orbit combined with the precession of the equinoxes, as already described.

(6.) Age of the Earth and Measures of Geological Time.The age of our planet is a problem which may be attacked either from the geological or physical side.

1. The geological argument rests chiefly upon the observed rates at which geological changes are being effected at the

7 This objection has been repeatedly urged by Sir William Thomson. Bee Trans. Roy. Soc. Edin., xxiii. 157; and Brit. Assoc. Rep., 1870, Sections, p. 7.

See D. Forbes, Geol. Mag., vol. iv. p. 435.

present time, and is open to the obvious preliminary objection that it assumes the existing rate of change as the measure of past revolutions, an assumption which may be entirely erroneous, for the present may be a period when all geological events march forward more slowly than they used to do. The argument proceeds on data partly of a physical and partly of au organic kind. (a) The physical evidence is derived from such facts as the observed rates at which the surface of a country is being lowered by rain and streams, and new sedimentary deposits are formed. These facts will be more particularly dwelt upon in later portions of this article. If we assume that the land has been worn away, and that stratified deposits have been laid down nearly at the same rate as at present, then we must admit that the stratified portion of the crust of the earth must represent a very vast period of time. Dr Croll puts this period at not less, but possibly much more, than 60 million years. (b) On the other hand, human experience, so far as it goes, warrants the belief that changes in the organic world proceed with extreme slowness. Yet in the stratified rocks of the earth's crust we have abundant proof that the whole fauna and flora of the earth's surface have passed through numerous cycles of revolution,-species, genera, families, appearing and disappearing many times in succession. On any supposition it must be admitted that these vicissitudes in the organic world can only have been effected with the lapse of vast periods of time, though no reliable standard seems to be available whereby these periods are to be measured. The argument from geological evidence is strongly in favour of an interval of probably not much less than 100 million years since the earliest form of life' appeared upon the earth, and the oldest stratified rocks began to be laid down.

2. The argument from physics as to the age of our planet is based by Sir William Thomson upon three kinds of evidence: (1) the internal heat and rate of cooling of the earth; (2) the tidal retardation of the earth's rotation; and (3) the origin and age of the sun's heat.

(1.) Sir William Thomson, applying Fourier's theory of thermal conductivity, pointed out some years ago (1862) that in the known rate of increase of temperature downward and beneath the surface, and the rate of loss of heat from the earth, we have a limit to the antiquity of the planet. He showed, from the data available at the time, that the superficial consolidation of the globe could not have occurred less than 20 million years ago, or the underground heat would have been greater than it is; nor more than 400 million years ago, otherwise the underground temperature would have shown no sensible increase downwards. He admitted that very wide limits were necessary. In more recently discussing the subject, he inclines rather towards the lower than the higher antiquity, but concludes that the limit, from a consideration of all the evidence, must be placed within some such period of past time as 100 millions of years.1 (2.) The argument from tidal retardation proceeds on the admitted fact that, owing to the friction of the tidewave, the rotation of the earth is retarded, and is therefore much slower now than it must have been at one time. Sir William Thomson contends that had the globe become solid some ten thousand million years ago, or indeed any high antiquity beyond 100 million years, the centrifugal force due to the more rapid rotation must have given the planet a very much greater polar flattening than it actually possesses. He admits, however, that, though 100 million years ago that force must have been about 3 per cent. greater than now, yet "nothing we know regarding the figure of the earth and the disposition of land and water would justify us in saying that a body consolidated when there was more

1 Trans. Roy. Soc. Edin., xxiii. 157; Trans. Geol. Soc. Glasgow, iii. 25.

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centrifugal force by 3 per cent. than now might not now be in all respects like the earth, so far as we know it at present."2 Professor Tait, in repeating this argument, concludes that, taken in connexion with the previous one, "it probably reduces the possible period which can be allowed. to geologists to something less than ten millions of years. He does not state, however, on what grounds he so reduces the available period, nor does he notice the objection urged by Dr Croll that, granting the gradual submergence of the polar lands owing to the slackened speed of rotation, the subaerial denudation of the rising equatorial land might well keep pace with the effects of the oceanic subsidence, so that we cannot infer from the present form of the earth what may have been its precise amount of polar compression at the time of solidification.4

(3.) The third argument, based upon the age of the sun's heat, is confessedly less reliable than the two previous ones. It proceeds upon calculations as to the amount of heat which would be available by the falling together of masses from space, which gave rise by their impact to our sun. The vagueness of the data on which this argument rests may be inferred from the fact that in one passage Professor Tait places the limit of time during which the sun has been illuminating the earth as, " on the very highest computation, not more than about 15 or 20 millions of years," while, in another sentence of the same volume, he admits that, " by calculations in which there is no possibility of large error, this hypothesis [of the origin of the sun's heat by the falling together of masses of matter] is thoroughly competent to explain 100 millions of years solar radiation at the present rate, perhaps more."5 One hundred millions of years is probably amply sufficient for all the requirements of geology.

III. COMPOSITIon of the Earth's Crust.

MINERALS AND ROCKS.

The visible and accessible portion of the earth is formed of minerals and rocks. A mineral may be classified as an inorganic body distinguished by a more or less definite chemical composition, and usually a characteristic geometrical form. A rock is an aggregate mass, sometimes of one, more commonly of two or more minerals Upwards of 800 species of minerals and a vast number of varieties have been described. A very large proportion of these occur but rarely, and, though interesting and important to the mineralogist, do not demand the special attention of the geologist. While almost every mineral may be made to yield data of more or less geological significance, only those which enter into the composition of rock masses, or which are of frequent occurrence as accessories there, require to be familiarly known by the student of geology.

1. Rock-Forming Minerals.

The following are the more important minerals which enter into the composition of rocks :

Quartz (SiO2) occurs either crystallized as rock-crystal, or noncrystalline as calcedony. In the former condition it is an essential constituent of granite, felsite, and many other igneous rocks, as The non-crys well as of sandstone and numerous aqueous rocks. tallized or colloid quartz is chiefly met with in cavities and fissures of rock where it has been slowly deposited from aqueous solution. Numerous varieties of calcedony occur, as agate, carnelian. jasper, flint, chert, Lydian-stone, &c.

Felspars (silicates of alumina, with potash, soda, or lime) constitute the most abundant group of rock-forming minerals. For the purposes of the petrographer they are conveniently divided into two series-(1) the Monoclinic or Orthoclase felspars (with cleavago angles of 90°), containing from 4 to 16 per cent. of potash and

2 Trans. Geol. Soc. Glasgow, iii. 16.

3 Recent Advances in Physical Science, p. 174. • Quart. Jour. Science, July 1877. Op. cit., pp. 153, 175.

usually more or less soda, sometimes as much as 10 per cent., and (2) the Triclinic or Plagioclase felspars (with oblique cleavage angles, or less than 90°), including a soda group with 8 to 12 per cent. of soda, and a lime group with 6 to 20 per cent. of lime. The felspars form a large part of most igneous rocks. By their decay they form clay, and in that condition enter largely into the composition of the argillaceous stratified rocks, such as shale, mudstone, slate, &c. Hornblende is a meta-silicate of magnesium, with lime, iron, or manganese, and frequently alumina. The white non-aluminous varieties (tremolite, actinolite, anthophyllite, asbestos) chiefly occur as constituents of such metamorphic rocks as crystalline limestone, gneiss, &c. The black or dark green aluminous varieties enter as essential constituents into the composition of many rocks, as diorite and hornblende slate.

Augite (resembling hornblende in composition) is divisible into two groups. The pale non-aluminous varieties (diopside, sahlite, coccolite, &c.) occur under conditions like those of the pale hornblendes. The dark aluminous or common augite is abundant as an ingredient of some igneous rocks, as basalt. Allied to augite are diallage (important as a constituent of diallage-rock or gabbro), hypersthene, and bronzite. Uralite is the name of a mineral of frequent occurrence among Palæozoic rocks, having the external form of augite and the cleavage of hornblende.

Olivine (an ortho-silicate of magnesium, with part of the magnesium replaced by iron or manganese) is a conspicuous ingredient among the basalt rocks. It appears also to have been the original magnesian constituent of many rocks now altered into serpentine. Nepheline (a silicate of alumina and soda with a little potash) takes the place of felspar in some lavas. It likewise occurs among the ejected blocks of Somma, and, in the form of elæolite, among the ancient crystalline rocks of Norway.

Leucite (K,Al,Si,O,,) is a characteristic ingredient of many Tertiary and recent lavas. It has not been met with among any of the Paleozoic or Secondary igneous rocks, nor ever in association with quartz.

Hayne and Nosean are two minerals allied to garnet, found in some Tertiary lavas.

Mica. Under this general term are included several species of minerals distinguished by their basal cleavage into thin lamine and by their splendent of silvery lustre. The non-magnesian micas include muscovite or potash-mica, the most abundant of all, and lepidolite or lithia-mica; of the magnesian micas the most important is biotite. Muscovite enters into the composition of granite, gneiss, mica-schist, micaceous sandstone, and many other rocks. Biotite is likewise abundantly distributed among the older crystalline rocks. Lepidomelane is a black mica often found in fine-grained granites. Other species are margarodite-an abundant constituent of many unctuous schists formerly called talc-schists, and haughtonitewhich, according to Heddle, is the common mica of the granites in the Scottish Highlands.

Garnet (an aluminous ortho-silicate with lime, magnesia, iron, or manganese) occurs in rhombic dodecahedrons or allied forms, and also massive in many metamorphic rocks, as mica-schist, eclogite, &c. Epidote (a variable silicate of lime, alumina, iron, or manganese) occurs in yellow or greenish translucent crystals or crystalline masses in many of the older crystalline rocks, though seldom as an abundant constituent. It is probably always an alteration-product. Tourmaline, in its common black variety, schorl, forms with quartz the rock known as schorl-rock, and occurs in some granites, gueisses, schists, and other crystalline rocks.

Zircon (silicate of zirconiuni) is found as a constituent of zirconsyenite, and more sparingly in other crystalline rocks. The hydrous silicates have resulted from the alteration of the anhydrous forms. As constituents of rocks they may be grouped into two series: (1) the aluminous, including the zeolites, and (2) the magnesian, embracing tale, chlorite, serpentine, and their allies.

Zeolites form a numerous genus of minerals distinguished usually by their boiling up before the blowpipe, owing to the escape of their water of crystallization, by their frequent pearly lustre, inferior hardness, and their occurrence in cavities and veins where they have been deposited from solution. They are found as abundant secondary products in many amygdaloids, also in altered limestones and other metamorphic rocks.

Serpentine (SiO,.44 14; MgO, 42-97; H2O,12-89) is a dull impure, usually green, granular to compact, more rarely foliated, mineral, with a hardness of 3 to 4 or even sometimes 5. Like the other hydrous magnesian silicates it has a soapy or greasy feel. It occurs abundantly in many altered rocks as a pseudomorph after some of the anhydrous magnesian silicates, also as a massive rock forming huge beds often associated with metamorphosed limestones.

Chlorite is a general term including several minerals which agree in possessing a greenish colour, soapy feel, hardness of only 2 to 2.5, and speettie gravity of 265 to 2-85. It occurs in chlorite slate and in many rocks as an alteration-product.

Tale SiO.,59 to 63: Mg0.30 to 33: 11.0, from a trace up to 7 per rent.) occurs in hexagonal plates or scales, cleaving readily into flexible uon-elastic lamina, but most commonly granular and

massive, white to pale leek or apple-green, with marked pearly lustre on cleavage-planes. It is met with in tale-slate, also frequently in crystalline rocks as a result of the alteration of hornblende, augite, or other anhydrous magnesian silicate.

Delessite and Saponite are soft green hydrous magnesian silicates found as products of alteration in basalt-rocks.

Carbon occurs chiefly as beds in the form of coal, lignite, peat, &c. Graphite, however, is often met with in black or steel-grey splendent scales and granular masses in metamorphosed rocks. Anthracite also takes sometimes the form of black glancing grains or of a diffused fine black dust through certain paleozoic formations, Carbonates play an important part both as individual minerals and as rock-masses. The three most important are calcite, dolo mite, and siderite.

Calcite (carbonate of lime) is one of the most abundant minerals. It occurs crystallized as a secondary product in most rocks which have undergone decomposition, especially where they contain sili cates into the composition of which lime enters. It is also found massive as limestone, forming. beds having sometimes an aggregate thickness of many hundred feet and an extent of thousands of square miles.

Dolomite (carbonate of lime and magnesia) is likewise both s product of alteration and an original formation. In the former condition it is met with crystallized as bitter-spar in many metamorphic rocks as well as in veins and cavities of unaltered formations. It occurs also as an amorphous granular substance, sometimes replacing calcite, and sometimes in vast beds or masses of original deposit.

Siderite, Chalybite, or Spathic Iron (carbonate of iron) occurs both crystallized and massive. In the crystallized form it is comparatively unimportant as a constituent of rocks, being then found chiefly in veins and cavities where other alteration-products have been deposited. But in its massive condition it is found mixed with clay and other impurities, and forming beds and nodules which are among the most important ores of iron.

Sulphur, though seldom occurring in large masses, is widely diffused as an accessory ingredient of rocks. It occurs crystallized or finely granular in mineral veins, in nodules of limestone, and other concretions, and in beds of limestone and marl. It also takes the form of a crust in the sublimations of volcanic vents. Its frequent association in Tertiary strata with the remains of lacustrine shells, insects, and plants shows that it has in these cases been formed at ordinary temperatures from aqueous solutions.

Sulphides, combinations of sulphur with the metals, iron, copper, lead, zinc, and a few others, have a wide distribution among rocks. Where aggregated into masses they form mineral veins. It is the iron sulphides which deserve chiefly the attention of the petrographer. They occur in two varieties pyrite, crystallizing in isometric forms, and marcasite, in rhombic forms. The former has a remarkably extensive diffusion throughout rocks of all ages, usually as minute crystals and thin streaks, but often in concretions and more massive veins. Marcasite also is abundantly distributed though less so than pyrite. From its greater liability to oxidation the strata through which it is diffused are apt to yield rapidly to the action of the weather. sulphuric acid and different alum com pounds being produced.

Sulphates.-The most generally occurring sulphates in rocks are gypsum and barytes. Gypsum (hydrous sulphate of lime) in minute monoclinic prisms and macles may be obtained by the evaporation of sea-water, and in larger crystals of the same form it is found in many stratified formations. It likewise occurs as a secondary product in laminar or fibrous veins through rocks of igneous origin. Beds of gypsum, resulting froin aqueous deposition, frequently appear interstratified with rock-salt and the associated products of evaporation. The anhydrous sulphate, anhydrite, likewise occurs among rock-salt deposits, but has a much more limited diffusion than gypsum. Barytes (sulphate of baryta) almost always occurs in veins or threads running through rocks. It is a common vein-stone in association with metallic ores.

Halite or Rock-salt (chloride of sodium) is more widely diffused than was formerly supposed. Microscopic research has shown its presence in the form of cubes in the minute cavities in the quartz of granite and other rocks. It occurs as scattered crystals, generally replaced by clay or some other substance, in many stratified formations. Its chief habitat, however, is in the various saliferous deposits where it takes the form of solid beds of salt.

Fluorite or Fluor-spar (fluoride of lime) is essentially a vein-stone, associated with metallic ores, especially with sulphides of lead and zinc. It occurs also in scattered cubes through various crystalline rocks, such as granite, gneiss, porphyrite.

Apatite (phosphate of lime, with fluorine and often chlorine) has been shown by microscopic investigation to have a very wide dis tribution among crystalline rocks. It occurs in fine needles or stouter hexagonal prisms in a large number of crystalline rocks, as granite, quartz-trachyte. syenite, diorite, basalt, and many others. It also oc curs massive as beds among the more ancient geological formations. Iron oxides.-These are abundantly distributed through rocks of

all ages. Hæmatite (peroxide of iron) occurs crystallized in veins through crystalline rocks, also massive and earthy in beds, and sometimes in minute scales (rubia-glimmer) disseminated through the minerals of many crystalline rocks. Magnetite (Fe,O) has an extensive diffusion in the form of minute octohedra or grains through crystalline rocks. In some of these rocks indeed, as in basalt, it plays the part of a chief constituent. It also occurs in many metamorphic rocks both scattered in detached crystals and segregated into veins or beds. Titanoferrite or titaniferous iron is likewise found as a plentiful ingredient in many crystalline rocks, particularly among the older basalts and dolerites. Hydrous iron oxide or limonite is diffused through almost all rocks. It is the usual brown or yellow colouring substance of minerals, and may be looked for wherever rocks containing iron have been exposed to the weather. It occurs also mixed with clay and other impurities in beds, as in the bog-iron-ore of lakes and marshes.

[blocks in formation]

1. Structure, or the manner in which the component particles have been built up into the mineral masses called rocks, is the fundamental character. Viewed broadly, there are two leading types of structure among rocks-crystalline or massive, and fragmental.

(a.) Crystalline-consisting of a network of interlaced crystals and crystalline particles. Sometimes those crystals are large (half an inch or more in length), as in many granites, when the texture is called coarse or macrocrystalline; in other cases they are so minute as not to be discernible with the naked eye, when the texture is microcrystalline or compact. While the crystalline structure is particularly characteristic of rocks which have crystallized from igneous fusion, it is not altogether peculiar to them. It may be produced by chemical deposit from aqueous solutions, or it may be developed in rocks previously granular by chemical infiltration and metamorphism.

Under the head of crystalline it is usual to include the glassy or vitreous structure. Rocks possessing this character are natural glasses produced by igneous fusion, such as obsidian and pitchstone. In most of these rocks, however, the process of devitrification may be observed; the glass has evidently become more and more stony as it cooled, by the appearance in it of small spherules, or hairs, or crystals, until in some cases it has become entirely lithoid. These stages are best studied with the microscope, and belong to the internal rather than the external characters.

When larger crystals than those of the compact base are scattered through a rock, the texture is said to be porphyritic. Many rocks, when in a melted condition, have had a cellular texture given to them by their imprisoned steam, like the open, cavernous texture of ill-baked bread. Several varieties of this texture are distinguished,—as vesicular, when there are comparatively few and small holes; scoriaceous, when the cavities occupy about as much space as the solid part, and are of very unequal sizes and forms; pumiceous, when the cells are much more numerous than the solid portion, and when, consequently, a piece of the rock may even float in water; amygdaloidal, when by subsequent infiltration the cells have been filled up with concretions of calcite, calcedony, zeolite, &c., which, from the elongated flattened form of the cells, are frequently almond-shaped.

Foliated rocks have their crystalline ingredients arranged Schistose rocks are those where the foliated arrangement has in more or less defined layers, which usually inosculate. been so produced that the rock splits into rude rough lamina or plates.

Most of the crystalline rocks have resulted from igneous fusion. Some, like limestone, have been formed as deposits in water. The foliated rocks are generally believed to have acquired their peculiar character from the re-crystallization of their ingredients along original divisional planes, such as the lines of deposit.

(b.) Fragmental or Clastic.-These are all derivative from previously formed masses. They vary in texture from coarse masses consisting of accumulated blocks, several feet or even yards in length, to such fine aggregates as only show their secondary origin by microscopic investigation. beds of rounded water-worn pebbles like compacted gravel; They are said to be conglomeratic when they consist of agglomeratic, when the blocks are large, rounded, or subangular, and tumultuously thrown together; brecciated, when the fragments are angular and not water-worn. Most Each bed may consist of many thin layers or lamine, which, clastic rocks are bedded, that is, arranged in beds or layers. when they enable the rock to split up into thin leaves, give what is called a shaly or fissile structure. Many fragmental rocks show a concretionary structure. When the concretions are like the roe of a fish, and of a calcareous nature, they form the oolitic structure; when of larger size, like peas, they give the pisolitic structure. There is often also a crystalline structure developed in rocks originally quite fragmental; many limestones, for example, made up originally of water-worn fragments of shells, corals, &c.,. slowly acquire a crystalline character from the action of percolating and slightly acidulous water. The action of rain on the exposed parts of a recent coral reef produces this change in the dead coral.

2. Colour. This character varies so much even in the same rock, according to the freshness of the surface examined, that it possesses but a subordinate value as a means of discriminating rocks. Nevertheless, when cautiously used, it may be made to afford valuable indications as to the probable nature and composition of rocks. It is in this respect always desirable to compare a freshly-broken with a weathered piece of the rock. White indicates usually the absence or comparatively small amount of the metallic oxides, especially iron. It may either be the original colour of the rock, as in chalk and calc-sinter, or may be developed by weathering, as the white crust on flints and on many porphyries. Black seldom occurs on a weathered surface of rock. Its existence may be due either to the presence of carbon, when weathering will not change it much, or to some iron-oxide (magnetite chiefly), or some silicate rich in iron (as hornblende and augite). Many rocks (basalts and dolerites particularly) which look quite black on a fresh surface, become red, brown, or yellow on exposure. Yellow, as a dull earthy colouring matter, almost always indicates the presence of hydrated peroxide of iron. Bright, metallic, gold-like yellow is usually that of iron-sulphide. Brown occurs as the original colour in some carbonaceous rocks (lignite), and ferruginous beds (bog-iron-ore, clay-ironstone, &c.). It very generally, on weathered surfaces, points to the oxida tion and hydration of minerals containing iron. Red, in the vast majority of cases, is due to the presence of granular peroxide of iron. This mineral gives dark blood-red to pale flesh-red tints. As it is liable, however, to hydration, these hues are often mixed with brown and yellow. Green, as the prevailing tint of rocks, occurs among metamorphic schists, when its presence is usually due to some of the hydrous magnesian silicates (chlorite, tale, serpentine). It occurs also among the igneous rocks, especially those of older

geological formations, where some of the hornblende, olivine, or other similar silicates have been altered. Among the sedimentary rocks it is principally due to the proto-silicate of iron in glauconite. Carbonate of copper colours some rocks a bright emerald or verdigris green. The mottled character so common among many stratified rocks is frequently traceable to unequal weathering, some portions being more oxidized than others; while some, on the other hand, become deoxidized from the reducing action of decaying organic matter. To the latter cause may be attributed the circular green spots so often found among red strata.

3. Lustre, as an external character of rocks, does not possess the value which it has among minerals. In most rocks the granular texture prevents the appearance of any distinct lustre. Where a rock shows a completely vitreous lustre it will usually be found to consist of a volcanic glass. A splendent semi-metallic lustre may often be observed upon the foliation planes of schistose rocks and upon the laminae of micaceous sandstones. As this silvery lustre is almost invariably due to the presence of mica, it is commonly called distinctively micaceous. A metallic lustre is met with sometimes in beds of anthracite ; more usually its occurrence among rocks indicates the presence of metallic oxides or sulphides.

4. Hardness and Frangibility.-A rock which can easily bo scratched with the nail is almost always much decomposed, though some chloritic and talcose schists are soft enough to be thus affected. Compact rocks which can easily be scratched with the knife, and are apparently not decomposed; may be limestones, or other fragmental masses. Crystalline rocks, as a rule, cannot be scratched with the knife unless considerable force be used. The ease with which a rock may be broken is the measure of its frangibility. Most rocks break most easily in one direction; attention to this point will sometimes throw light upon their internal structure. 5. Fracture is the surface produced when a rock is split or broken, and depends for its character upon the texture of the mass. Finely granular compact rocks are apt to break with a splintery fracture where wedge-shaped plates adhere by their thicker ends to, and lie parallel with, the general surface. When the rock breaks off into concave and convex rounded shell-like surfaces, the fracture is said to be conchoidal, as may be seen in obsidian and other vitreous rocks, and in exceedingly compact limestones. The fracture may also be foliated, slaty, or shaly, according to the structure of the rock. Many black, opaque, compact rocks are translucent on the thin edges of fracture, and afford there, with the aid of a lens, a glimpse of their internal composition.

6. Feel.-Practice enables a geologist to discriminate some rocks by the feel of their weathered or fresh surfaces. The hydrous magnesian silicates, as already mentioned, have a marked soapy or greasy feeling under the fingers. Some micaceous schists, with margarodite or an allied mica, likewise exhibit the same character.

7. Smell. Many rocks when freshly broken emit distinctive odours. Those containing volatile hydrocarbons give sometimes an appreciable bituminous odour, as is the case with some of the dolerites, which in central Scotland have been intruded through coal-seams and carbonaceous shales. Limestones have often a fetid odour; rocks full of decomposing sulphides are apt to give a sulphurous odour; those which are highly siliceous yield, on being struck, an empyreumatic odour. It is very characteristic of argillaceous rocks to emit a strong earthy smell when breathed upon. 8. Specific gravity is an important character among rocks as among minerals. It varies from 06 among the hydrocarbon compounds to 31 among the basalts. As already stated, the average specific gravity of the rocks of the earth's crust niay be taken to be about 25, or from that to 3.0.

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9. Magnetism is a distinguishing feature of many igneous and some metamorphic rocks. In some cases it exists in such development as powerfully to affect the magnetic needle, so that observations with that instrument among rocks of this character are deceptive. But even when much more sparingly present, the existence of magnetic iron in a rock may be shown by reducing the rock to powder in an agate mortar, washing carefully the triturated powder, and drying the heavy residue, from which grains of magnetite may be extracted with a magnet. This may be done with any basalt

ii. Internal Characters.

These are revealed chiefly by the microscope and chemical analysis. By the former we learn what are the component minerals of a rock, how they are built up into its mass, and what changes they have undergone. By the latter we are taught the chemical constitution of rocks, and are enabled to bring into close relations rocks which have externally no resemblance to each other, or, on the other hand, to show that rocks externally similar are chemically very distinct..

1. Microscopic Examination.-This method of inquiry has made great advances during the last 20 years, especially from the labours of German petrographers. Slices are cut from the rocks to be examined, and after being polished on one side with great care, are cemented by that side with Canada balsam to glass, and are then ground down until they attain the requisite transparency. In this way the minutest features in the structure of a rock can be leisurely studied. By the application of polarized light to these thin slices a marvellously delicate method of petrographical analysis is afforded.

Among the igneous rocks three leading types of microscopic structure have been established, chiefly through the researches of Professor Zirkel of Leipsic :-(1.) Purely-crystalline.-Granite (fig. 1) is a good example, consisting, as it does, entirely of crystals interlaced with each other. (2.) Halfcrystalline. In this division, which embraces most of the eruptive masses, the rocks consist of a non-crys talline amorphous matrix with crystals scattered through it. This matrix may be either (a) entirely glassy (figs. 2 and 3); (b) partly devitrified through separation of peculiar little granules and needles which are not "microlites' of the component parts of the rock; (c) an aggregation of such little granules, needles, and hairs, between which no glass, or almost none, appears (microcrystallitic); or (d) microfelsitic, nearly related to the two previous groups, and consisting of an amorphous maɛs marked usually with indefinite or FIG. half-effaced granules and filanients. (3.) Non-crystalline.-Rocks of this class are much less common than those of the other two. In their most typical condition they consist entirely of a non

FIG. 1.-Microscopic Structure of RocksPurely crystalline-Section of Granite (X 18 diameter). The white mineral is quartz; that with shading, orthoclase. Some flakes of mica are shown as striated forms.

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2-Microscopic Structure of OBSIdian. A volcanic glass, with numerous microlites, which have been drawn out a general direction during the flow

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of the melted rock (fiula-structure). (X 18 diameter.)

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