Page images

together, just as iron-sand may be seen gathered into thin sheets have been much more, than the 100 millions of years which Lord on sandy beaches at the present day. Again, the same series Kelvin was at one time willing to concede. of primeval sediments includes intercalations of fine silt, which has been deposited as regularly and intermittently there as it Part III.-GEOGNOSY. THE INVESTIGATION OF THE NATURE has been among the most recent formations. These bands of AND COMPOSITION OF THE MATERIALS OF WHICH THE shale have been diligently searched for fossils, as yet without

EARTH CONSISTS success; but they may eventually disclose organic remains older

This division of the science is devoted to description of the than any hitherto found in Europe.

parts of the earth—of the atmosphere and ocean that surround We now come to the consideration of the palaeontological the planet, and more especially of the solid materials that underlie evidence as to the value of geological time. Here the conclusions these envelopes and extend downwards to an unknown distance derived from a study of the structure of the sedimentary forma- into the interior. These various constituents of the globe are tions are vastly strengthened and extended. In the first place, here considered as forms of matter capable of being analysed, the organization of the most ancient plants and animals furnishes and arranged according to their composition and the place they no indication that they had to contend with any greater violence take in the general composition of the globe. of storm, flood, wave or ocean-current than is familiar to their

Viewed in the simplest way the earth may be regarded as modern descendants. The oldest trees, shrubs, ferns and made up of three distinct parts, each of which ever since an club-mosses display no special structures that suggest a difference carly period of planetary history has been the theatre of imin the general conditions of their environment. The most portant geological operations. (1) An envelope of air, termed ancient crinoids, sponges, crustaceans, arachnids and molluscs the almosphere, which surrounds the whole globe; (2) A lower were as delicately constructed as those of 10-day, and their and less extensive envelope of water, known as the hydrosphere remains are often found in such perfect preservation as to show (Gr. üdwp, water) which, constituting the oceans and seas, that neither during their lifetime nor after their death were they covers nearly three-fourths of the underlying solid surface of the subject to any greater violence of the elements than their living planet; (3) A globe, called the lithosphere (Gr. Aidos, stone), representatives now experience. Of much more cogency, the external part of which, consisting of solid stone, forms the however, is the evidence supplied by the grand upward succession crust, while underneath, and forming the vast mass of the of organic forms, from the most ancient stratified rocks up to interior, lies the nucleus, regarding the true constitution of the present day. No biologist now doubts for a moment that which we are still ignorant. this marvellous succession is the result of a gradual process of

1. The Almosphere.—The general characters of the atmosphere evolution from lower to higher types of organization. There are described in separate articles (see especially ATMOSPHERE; may be differences of opinion as to the causes which have governed METEOROLOGY). Only its relations to geology have here to be this process and the order of the steps through which it has considered. As this gaseous envelope encircles the whole advanced, but no one who is conversant with the facts will now globe it is the most universally present and active of all the venture to deny that it has taken place, and that, on any possible agents of geological change. Its efficacy in this respect arises explanation of its progress, it must have demanded an enormous partly from its composition, and the chemical reactions which lapse of time. In the Cambrian or oldest fossiliferous formations it effects upon the surface of the land, partly from its great there is already a large and varied fauna, in which the leading variations in temperature and moisture, and partly from its groups of invertebrate life are represented. On no tenable hypothesis can these be regarded as the first organisms that

Many speculations have been made regarding the chemical came into being on our planet. They must have had a long composition of the atmosphere during former geological periods. ancestry, and as Darwin first maintained, the time required for There can indeed be little doubt that it must originally have differed their evolution may have been “as long as, or probably far greatly from its present condition. If the whole mass of the planet longer than, the whole interval from the Silurian (Cambrian] originally existed in a gascous state, there would be practically no age to the present day.” The records of these earliest eras of to be the surviving relic of this condition, after all the other con organic development' have unfortunately not survived the stituents have been incorporated into the hydrosphere and lithogeological revolutions of the past; at least, they have not yet sphere. The oxygen, which now forms fully a half of the outer been recovered. But it cannot be doubted that they once combination, part of the atmosphere. So, too, the vast beds of coal

crust of the earth, was doubtless originally, whether free or in existed and registered their testimony to the prodigious lapse of found all over the world, in geological formations of many different time prior to the deposition of the most ancient fossiliferous ages, represent so much carbonic acid once present in the air. The formations which have escaped destruction.

chlorides and other salts in the sea may likewise partly represent The impressive character of the evidence furnished by the materials carried down out of the atmosphere in the primitive sequence of organic forms throughout the

great series of fossili- tinually increased ever since by contributions from the drainage of ferous strata can hardly be fully

realized without a detailed and the land. It has often been suggested that, during the Carboniferous careful study of the subject. Professor E. B. Poulton, in an period, the atmosphere must have been warmer and more charged address to the zoological section of the British Association at with aqueous vapour and carbon dioxide than at the present day,

to admit of so luxuriant a flora as that from which the coal-seams the Liverpool Meeting in 1896, showed how overwhelming are were formed. There scems, however, to be at present no method the demands which this evidence makes for long periods of time, of arriving at any certainty on this subject. Lastly, the

amount of and how impossible it is of comprehension unless these demands carbonic acid absorbed in the weathering of rocks at the surface, and be conceded. The history of life upon the earth, though it will the consequent production of carbonates, represents an enormous probably always be surrounded with great and even insuperable

As at present constituted, the atmosphere is regarded as a difficulties, becomes broadly comprehensible in its general progress when sufficient time is granted for the evolution

The subject of the age of the earth has also been discussed by which it records; but it remains unintelligible on any other Professor J. Joly and Professor W. J. Sollas. The former geologist,

approaching the question from a novel point of view, has estimated conditions.

the total quantity of sodium in the water of the ocean and the Taken then as a whole, the body of evidence, geological and quantity of that element received annually by the occan from the palaeontological, in favour of the high antiquity of our globe denudation of the land. Dividing the one sum by the other, he is so great, so manifold, and based on such an ever-increasing 90 and 100 millions of years (Trans. Roy. Dublin Soc. ser. ii

. vol. vii., breadth of observation and reflection, that it may be confidently 1899, p. 23: Geol. Mag. 1900, p. 220). Professor Sollas believes appealed to in answer to the physical arguments which would that this limit exceeds what is required for the evolution of geological seek to limit that antiquity to ten or twenty millions of years. history, that the lower limit assigned by Lord Kelvin falls

short of

what the facts demand, and that geological time will probably be In the present state of science it is out of our power to state

found to have been comprised within some indeterminate period positively what must be the lowest limit of the age of the earth. between these limits. (Address to Section C, Bril. Assoc. Report, But we cannot assume it to be much less, and it may possibly 1900; Age of the Earth, London, 1905.)





mechanical mixture of nearly four yolumes of nitrogen and one of I were adduced to show the great rigidity of the earth as a whole, every 10,000 parts of air, and minute quantities of various other the idea of a thin crust enclosing a molten nucleus was reluctantly gases and solid particles of the va pours contained in it by far the abandoned by geologists, who found the problem of the earth's most important is that of water which, although always present, interior to be incapable of solution by any evidence which their varies greatly in amount according to variations in temperature. science could produce. They continued, however, lo use the By condensation the water vapour appears in visible form as dew,


as a convenient word to denote the cool outer mist, cloud, rain, hail, snow and ice, and in these forms includes and carries down some of the other vapours, gases and solid particles layer of the earth's mass, the structure and history of which present in the air. The circulation of water from the atmosphere to form the main subjects of geological investigation. More the land, from the land to the sea, and again from the sca to the recently, however, various lines of rescarch have concurred in condition of the planet is maintained and the surface of the land suggesting that, whatever may be the condition of the interior, is sculptured (Part IV.).

its substance must differ greatly from that of the outer shell,

and that there may be more reason than appcared for the 2. The Hydrosphere.—The water envelope covers nearly

Observations on earthquake three-fourths of the surface of the earth, and forms the various retention of the name of crust. oceans and seas which, though for convenience of reference motion by Dr John Milne and others, show that the rate and

character of the waves transmitted through the interior of the distinguished by separate names, are all linked together in one

carth differ in a marked degree from those propagated along the great body. The physical characters of this vast envelope are

This difference indicates that rocky material, such as discussed in separate articles (see OCEAN AND OCEANOGRAPHY).

we know at the surface, may extend inwards for some 30 m., Viewed from the geological standpoint, the features of the sea that specially deserve attention are first the composition of below which the earth's interior rapidly becomes fairly homoits waters, and secondly its movements.

geneous and possesses a high rigidity. From measurements Sea-water is distinguished from that of ordinary lakes and rivers of the force of gravity in India by Colonel S. G. Burrard, it has by its greater specinc gravity and its saline taste.

Its average

been inferred that the variations in density of the outer parts of density is about 1.026, but it varies even within the same ocean,

the earth do not descend farther than 30 or 40 m., which might being least where large quantities of fresh water are ad from be assumed to be the limit of the thickness of the crust. Recent rain or melting snow and ice, and greatest where evaporation is most active. That sea-water is heavier than fresh arises from the greater in rocks suggest that the crust is not more than 50 m. thick,

researches in regard to the radio-active substances present proportion of salts which it contains in solution. These salts constitute about three and a half parts in every hundred of water. and that the interior differs from it in possessing little or no They consist mainly of chlorides of sodium and magnesium, the radio-active material. sulphates of magnesium, calcium and potassium, with minuter

Though we cannot hope ever to have direct acquaintance with quantities of magnesium bromide and calcium carbonate. Still smaller proportions of other substances have been detected, gold for

more than the mere outside skin of our planet, we may be led example having been found in the proportion of 1 part in 15,180,000. to infer the irregular distribution of materials within That many of the salts have existed in the sea from the time of the crust from the present distribution of land and

interior.' its first condensation out of the primeval atmosphere appears to be probable. It is manifest, however, that, whatever may have water, and the observed differences in the amount of been the original composition of the oceans, they have for a vast

deflection of the plumb-line near the sea and near mountain. section of geological time been constantly receiving mineral matter chains. The fact that the southern hemisphere is almost wholly in solution from the land. Every spring, brook and river removes covered with water appears explicable only on the assumption various salts from the rocks over which it moves, and these şub- of an excess of density in the mass of that portion of the planet, stances, thus dissolved, eventually find their way into the 'sea. Consequently sea-water ought to contain more or less traceable The existence of such a vast sheet of water as that of the Pacific proportions of every substance which the terrestrial waters can Ocean is to be accounted for, as Archdeacon J. H. Pratt pointed remove from the land, in short, of probably every element present out, by the presence of "some excess of matter in the solid in the outer shell of the globe, for there seems to be no constituent parts of the earth between the Pacific Ocean and the earth's of this earth which may not, under certain circumstances, be held in solution in water. Morcover, unless there be some counteracting centre, which retains the water in its place, otherwise the

ocean process to remove these mineral ingredients, the ocean water ought would flow away to the other parts of the earth.” A deflection to be growing, insensibly perhaps, but still assuredly, salter, for the of the plumb-line towards the sea, which has in a number of supply of saline matter from the land is incessant.

cases been observed, indicates that “the density of the crust To the geologist the presence of mineral solutions in sea-water is a fact of much importance, for it explains the

origin of a considerable beneath the mountains must be less than that below the plains, part of the stratified rocks of the earth's crust. By evaporation and still less than that below the ocean-bed.” Apart therefore the water has given rise to deposits of rock-salt, sypsum and other from the depression of the earth's surface in which the oceans materials. The lime contained in solution, whether as sulphate or lie, we must regard the internal density, whether of crust or which have thus built up out of their remains vast masses of solid nucleus, to be somewhat irregularly arranged, there being an limestone, of which many mountain-chains largely consist.

excess of heavy materials in the water hemisphere, and beneath Another important geological feature of the sea is to be seen the ocean-beds, as compared with the continental masses. in the fact that its basins form the great receptacles for the detritus worn away from the land. Besides the limestones, the visible parts nucleus of our planet, an argument has sometimes been based

In our ignorance regarding the chemical constitution of the rocks which were originally laid down on the sea-bottom. More upon the known fact that the specific gravity of the globe over, by its various movements, the sea occupies a prominent place as a whole is about double that of the crust. This has been among the epigene or superficial agents which produce geological held by some writers to prove that the interior must consist of changes on the surface of the globe,

much heavier material and is therefore probably metallic. But 3. The Lithosphere.--Beneath the gaseous and liquid envelopes the effect of pressure ought to make the density of the nucleus lies the solid part of the planet, which is conveniently regarded much higher, even if the interior consisted of matter no heavier as consisting of two parts,-(a) the crust, and (b) the interior | than the crust. That the total density of the planet does not or nucleus.

greatly exceed its observed amount seems only explicable on It was for a long time a prevalent belief that the interior of the the supposition that some antagonistic force counteracts the globe is a molten mass round which an outer shell has gradually effects of pressure. The only force we can suppose capable of so

formed through cooling. Hence the term " crust acting is heat. But comparatively little is yet known regarding

was applied to this external solid envelope, which the compression of gases, liquids and solids under such vast was variously computed to be 10, 20, or more miles in thickness. pressures as must exist within the nucleus. The portion of this crust accessible to human observation was That the interior of the earth possesses a high temperature seen to afford abundant evidence of vast plications and corruga- is inferred from the evidence of various sources. (1). Volcanoes, tions of its substance, which were regarded as only explicable, which are openings that constantly, or intermittently, give out on the supposition of a thin solid collapsible shell floating on a hot vapours and molten lava from reservoirs beneath the crust. denser liquid interior. When, however, physical arguments . Besides active volcanoes, it is known that former eruptive vents

The crust.

have been abundantly and widely distributed over the globe, envelopes of the ocean and the air, we thus perceive that these from the earliest geological periods down to our own day. outer parts of our planet consist of more than three-fourths of non(2) Hot springs are found in many parts of the globe, with The combinations of the elements which are of most importance temperatures varying up to the boiling point of water. (3) in the constitution of the terrestrial crust consist of oxides. From From mines, tunnels and deep borings into the earth it has the mean of a large number of analyses of the rocks of the lower or been ascertained that in all quarters of the globe below the primitive portion of the crust, it has been ascertained that silica superficial zone of invariable temperature, there is a progressive the whole. The other combinations in order of importance

are increase of heat towards the interior. The rate of this increase lime (Ca0) 4.90 %, magnesia (MgO) 4:36, soda (Na2O) 3:55, ferrous varies, being influenced, among other causes, by the varying oxide (FCO) 3:52, potash (Ķ,0) 2.80, lerric oxide (Fe20) 2-63, water conductivity of the rocks. But the average appears to be (1:0) 1:52, titanium oxide (Tio.) 0.60, phosphoric acid” (P:0.) about ro Fahr. for every 50 or 60 ft. of descent, as far down as

0:22; the other combinations of elements thus form less than 1%

of the crust. observations have extended. Though the increase may not These different combinations of the elements enter into further advance in the same proportion at great depths, the inference combinations with each other so as to produce the wide assortment has been confidently drawn that the temperature of the nucleus of simple minerals (see MINERALOGY). Thus, silica and alumina are

combined to form the aluminous silicates, which enter so largely must be exceedingly high.

into the composition of the crust of the carth. The silicates of The probable condition of the earth's interior has been a fruit- magnesia, potash and soda constitute other important families of ful source of speculation ever since geology came into existence; minerals. A mass of material composed of one, but more usually but no general agreement has been arrived at on the subject of more than one mineral, is known as a rock. Under this term Three chief hypotheses have been propounded: (1) that the geologists are accustomed to class not only plid stone, such as nucleus is a molten mass enclosed within a solid shell; (2) that, peat and even loose sand. The accessible portion of the earth's save in local vesicular spaces which may be filled with molten crust consists of various kinds of rocks, which differ from each other or gaseous material, the globe is solid and rigid to the centre; of diverse classifications according to the point of view from which (3) that the great body of the nucleus consists of incandescent they are considered. The details of this subject will be found in vapours and gases, especially vaporous iron, which under the the article PETROLOGY. gigantic pressure within the earth are so compressed as to confer Classification of Rocks.-Various systems of classification of rocks practical rigidity on the globe as a whole, and that outside this have been proposed, but none of them is wholly satisfactory. The main part of the nucleus the gases pass into a shell of molten most useful arrangement for most purposes of the geologist is one

ased on the broad differences between them in regard to their mode magma, which, in turn, shades off outwards into the compara- of origin. From this point of view they may be ranged in three tively thin, cool solidificd crust. Recent seismological observa- divisions: tions have led to the inference that the outer crust, some 30 to

1. In the first place, a large number of rocks may be described 45 m. thick, must rapidly merge into a fairly homogeneous as original or underived, for it is not possible to trace them back to nucleus which, whatever be its constitution, transmits undulatory planet, and, as they have all come up from below through the crust, movements through its substance with uniform velocity and is they serve to show the nature of the material which lies immediately believed to possess a high rigidity.

below the outer parts of that crust. They include the numerous The origin of the earth's high internal temperature has been varieties of lava, which have been poured out in a molten state from

volcanic vents, also a great series of other rocks which, though they variously accounted for. Most usually it has been assumed to may never have been erupted to the surface, have been forced be the residue of the original “tracts of fluent heat " out of upward in a melted condition into the other rocks of the crust and which the planet shaped itself into a globe. According to another have solidified there. From their mode of origin this great class of supposition the effects of the gradual gravitational compression show no definite internal structure save such as may result from of the earth's mass have been the main source of the high joints, they have been termed massive" unstratificd," to temperature. Recent researches in radio-activity, to which distinguish them from those of the second division which are reference has already been made, have indicated another possible strongly marked out by the presence of a stratified structure. The source of the internal heat in the presence of radium in the igneous rocks present

a considerable range of composition. For

the most part they consist mainly, of aluminous silicates, some of rocks of the crust. This substance has been detected in all them being highly acid compounds with 75% or more of silica. igneous rocks, especially among the granites, in quantity But they also include highly basic varieties wherein the proportion sufficient, according to the Hon. R. J. Strutt, to account for the of silica sinks to 40%, and where magnesia greatly, predominates observed temperature-gradient in the crust, and to indicate over alumina. The textures of igneous rocks likewise comprise a

On the one hand, some are completely that this crust cannot be more than 45 m. thick, otherwise the vitreous, like obsidian, which is a natural

glass. From this extreme outflow of heat would be greater than the amount actually every gradation may be traced through gradual increase of the ascertained. Inside this external crust containing radio-active products of devitrification, until the mass may become completely substances, it is supposed, as already stated, that the nucleus crystallinc. Again, some crystalline igneous rocks are so fine in consists of some totally different matter containing little or no grain as not to show their component crystals save under the micro

scope, while in others the texture is so coarse as to present the radium.

component minerals in separate crystals an inch or more in length. Constitulion of the Earth's Crust.-As the crust of the earth contains These differences indicate that, at first, the materials of the rock the "geological record," or stony chronicle from which geology may have been as completely molten as artificial glass, and that interprets the history

of our globe, it forms the main subject of study the crystalline condition has been subsequently developed by cooling, to the geologist. The materials of which this crust consists are and the separation of the chemical constituents into definite crystalknown as minerals and rocks. From many chemical analyses, line minerals. Many of the characters of igneous rocks have been which have been made of these materials, the general chemical reproduced experimentally by fusing together their minerals, or the constitution of, at least, the accessible portion of the crust has been constituents of their minerals, in the proper proportion. But it has satisfactorily ascertained. This information becomes of much not yet been found possible to imitate the structure of such rocks importance in speculations regarding the early history of the globe. as granite. Doubtless these rocks consolidated with extreme or the elements known to the chemist the great majority form but a slowness at great depths below the surface, under vast pressures small proportion of the composition of the crust, which is mainly and probably in the presence of water or water-vapour-conditions built up of about twenty of them. Of these by far the most important which cannot be adequately imitated in a laboratory. are the non-metallic clements oxygen and silicon. The former Though theigneous rocks occupy extensive areas in some countries, forms about 47% and the latter rather more than 28% of the they nevertheless cover a much smaller part of the whole surface of original crust, so that these two elements make up about three the land than is taken up by the second division or stratified rocks. fourths of the whole. Next after them come the metals aluminium But they increase in quantity downwards and probably extend (8.16%), iron (4.64). calcium (3:50), magnesium (2.62), sodiuin continuously round the globe below the other rocks.' This important (2.63), and potassium (2:35). The other twelve elements included series brings before us the relations of the molten magma within the in the twenty vary in amount from a proportion of 0.41% in the earth to the overlying crust and to the outer surface. On the one case of titanium, to not more than 0.01% of chlorine, fluorine, hand, it includes the oldest and most deep-seated extravasations chromium, nickel and lithium. The other fisty or more elements of that magma, which have been brought to light by ruptures and exist in such minute proportions in the crust that, probably, not upheavals of the crust and prolonged denudation. On the other, one of them amounts to as much as 0.01%, though they include it presents to our study the varied outpourings of molten and the useful metals, except iron. Taking the crust, and the external fragmentary materials in the discharges of modern and ancient

[ocr errors]


volcanoes.. Between these two extremes of position and age, we i writers to be part of the original or primitive surface of our globe find that the crust has been, as it were, riddled with injections of that first consolidated on the moiten nucleus. But the progress of the magma from below. These features will be further noticed in investigation all over the world has shown that this supposition Part V. of this article.

cannot be sustained. The oldest known rocks present none of the 2. The " sedimentary " or "stratified rocks." form by much the characters of molten material that has cooled and hardened in the larger part of the dry land of the globe, and they are prolonged to air, like the various forms of recent lava. On the contrary, thcy an unknown distance from the shores under the bed of the sea. possess many of the features characteristic of bodies of eruptive They include those masses of mineral matter which, unlike the material that have been injected into the crust at some depth under igneous rocks, can be traced back to a definite origin on the surface ground, and are now visible at the surface, owing to the removal of the earth. Three distinct types may be recognized among them: by denudation of the rocks under which they consolidated. In their (@) By far the largest proportion of them consists of different kinds less foliated portions they can be recognized as true eruptive rocks. of sediment derived from the disintegration of pre-existing rocks. In many places gneisses that possess å thoroughly typical foliation In this " fragmental", group are placed all the varieties of shingle, have been found to pierce ancient sedimentary formations as intrusive gravel, sand, clay and mud, whether these materials remain in a bosses and veins. loose incoherent condition, or have been compacted into solid stone. (b) Another group consists of materials that have been deposited by

Part IV.-DYNAMICAL GEOLOGY chemical precipitation from solution in water. The white sinter laid down by calcareous springs is a familiar example on a small

This section of the science includes the investigation of those scale. Beds of rock-salt, gypsum and dolomite have, in some processes of change which are at present in progress upon the regions, been accumulated to a thickness of many thousand feet, carth, whereby modifications are made on the structure and by successive precipitations of the salt contained in the water of composition of the crust, on the relations between the interior mentary formations has been formed from the remains of plants and and the surface, as shown by volcanoes, earthquakes and other animals. Such accumulations may arise either from the transport terrestrial disturbances, on the distribution of oceans and and deposit of these remains, as in the case of sheets of drift-wood, continents, on the outlines of the land, on the form and depth and banks of drifted sea-shells, or from the growth and decay of of the sea-bottom, on climate, and on the races of plants and the organisms on the spot, as happens in peat bogs and in coral animals by which the earth is tenanted. It brings before us, reeís.

As the sedimentary rocks have for the most pait been laid down in short, the whole range of activities which it is the province of under er, and more especially on the sea-floor, they are often geology to study, and leads us to precise notions regarding their spoken of as “ aqueous," in contradistinction to the igneous rocks. relations to each other and the results which they achieve. A wind upon loose materials, and are known as aeolian" formations. knowledge of this branch of the subject is thus the essential Familiar instances of such wind-formed deposits are the sand-dunes groundwork of a true and fruitful acquaintance with the principles along many parts of the sca coast. Much more extensive in area are of geology, seeing that it necessitates a study of the present order the sands of the great deserts in the arid regions of the globe. of nature, and thus provides a key for the interpretation of the

It is from the sedimentary rocks that the main portion of geological history is derived. They have been deposited one over another past: in successive strata from a remote period in the development of

The whole range of operations included within the scope of the globe down to the present

time." From this arrangement they inquiry in this branch of the science may be regarded as a vast have been termed "stratified," in contrast to the unstratified or cycle of change, into which we may break at any point, and igneous series. They have preserved memorials of the geographical round which we may travel, only to find ourselves brought above all, in the abundant fossils which they have enclosed,'they back to our starting-point. It is a matter of comparatively furnish a momentous record of the various tribes of plants and small moment at what part of the cycle we begin our inquiries. animals which have successively flourished and sea. Their | We shall always find that the changes we see in action have investigation is thus the most important task which devolves upon resulted from some that preceded, and give place to others the geologist. 3. In the third place comes a series of rocks which are not now

which follow them. in their original condition, but have undergone such alteration as At an early time in the earth's history, anterior to any of the to have acquired new characters that more or less conceal their periods of which a record remains in the visible rocks, the chief while of many it is difficult to decide what may have been their If, as is generally supposed, the planet still retained a great igneous masses; others are as manifestly of sedimentary origin; sources of geological action probably lay within the earth itself. pristine character. To this series the term " metamorphic" has store of its initial heat, it was doubtless the theatre of great been applied. Its members are specially distinguished by a prevailing chemical changes, giving rise, perhaps, to manifestations of fissile, or schistose, structure which they did not at first possess, and volcanic energy somewhat like those which have so marvellously rocks... This fissility is combined with a more or less pronounced roughened the surface of the moon. As the outer layers of the crystalline structure. These changes are believed to be the result globe cooled, and the disturbances due to internal heat and of movements within the crust of the earth, whereby the most solid chemical action became less marked, the conditions would influence of a high temperature and the presence of water, they arise in which the materials for geological history were accumuunderwent internal chemical reactions, which led to a rearrangement lated. The influence of the sun, which must always have and recomposition of their mineral constituents and the production operated, would then stand out more clearly, giving rise to that of a crystalline structure (see METAMORPHISM). Among the less altered metamorphic rocks of sedimentary origin, ture and the circulation of air and water over the surface of the

wide circle of superficial changes wherein variations of temperathe successive laminae of deposit of the original sediment can be easily, observed; but they are also traversed by a'new set of divi- earth come into play. sional planes, along which they split across the original bedding: In the pursuit of his inquiries into the past history and into Together with this superinduced cleavage there have been developed the present régime of the earth, the geologist must needs keep in them minute hairs, scales and rudimentary crystals. Further his mind ever open to the reception of evidence for kinds stages of alteration are marked by the increase of micaceous scales, garnets and other minerals, especially along the planes of cleavage, and especially for degrees of action which he had not before until the whole rock becomes crystalline, and displays its chief | imagined. Human experience has been too short to allow him component minerals in successive discontinuous folia which merge to assume that all the causes and modes of geological change into each other, and are often crumpled and puckered. Massive have been definitively ascertained. On the earth itself there may and cleavage, and to have ultimately assumed a crystalline foliated remain for future discovery evidence of former operations by character. Rocks which present this aspect are known as schists heat, magnetism, chemical change or otherwise, which may (q.v.). They range from the finest silky slates, or phyllites, up to the explain many of the phenomena with which geology has to deal. coarsest gneisses, which in hand-specimens can hardly be distinguished from granites. There is indeed every reason to believe

Of the influences, so many and profound, which the sun exerts that such gneisses were probably originally true granites, and that upon our planet, we can as yet only perceive a little. Nor can their foliation and recrystallization have been the result of meta we tell what other cosmical influences may have lent their aid in morphism.

the evolution of geological changes. The schists are more especially to be found in the heart of

Much useful information regarding many geological processes mountain-chains, and in regions where the lowest and oldest of the earth's crust have, in the course of geological revolutions, has been obtained from experimental research in laboratories been exposed to the light of day. They have been claimed by some and elsewhere, and much more may be confidently looked for

from future extensions of this method of inquiry. The early subterranean phases of volcanic action are thereby revealed which experiments of Sir James Hall, already noticed, formed the are wholly concealed in an active volcano. Hence to obtain as starting point for numerous subsequent researches, which have complete a conception as possible of the nature and history of

volcanic action, regard must be had, not merely to modern volcanoes, elucidated many points in the origin and history of rocks. It but to the records of ancient eruptions which have been preserved is true that we cannot hope to imitate those operations of nature

within the crust. which demand enormous pressures and excessively high tempera. Gases and vapours: which, dissolved in the molten magma of the

The substances discharged from volcanic vents consist of -(1) lures combined with a long lapse of time. But experience interior, take the chief 'share in volcanic activity. They include has shown that in regard to a large number of processes, it is in greatest abundance water-gas, which condenses into the clouds possible to imitate nature's working with sufficient accuracy of steam so conspicuous in volcanic eruptions. Hydrochloric acid to enable us to understand them, and so to modify and control and sulphuretted hydrogen are likewise plentiful, together with the results as to obtain a satisfactory solution of some geological perature, take a solid form on cooling at the surface. (2) Molten

many other substances which, sublimed by the high internal temproblems.

rock or lava: which ranges from the extremely acid type of the In the present state of our knowledge, all the geological obsidians and rhyolites with 70% or more of silica, to the more basic energy upon and within the earth must ultimately be traced and heavy varieties such as basalts and leucite-lavas with much iron, back to the primeval energy of the parent nebula or sun. There of Javas varies between 2-37 and 3:22, and the texture ranges from is, however, a certain propriety and convenience in distinguishing nearly pure glass, like obsidian, to a coarse granitoid compound, bet ween that part of it which is due to the survival of some of asin some rhyolites. (3) Fragmentary materials, which are sometimes the original energy of the planet and that part which arises discharged in enormous quantity and dispersed over a wide extent from the present supply of energy received day by day from the for hundreds of miles. These materials arise either from the explosion sun. In the former case we have to deal with the interior of of lava by the sudden expansion of the dissolved vapours and gases, the earth, and its reaction upon the surface; in the latter, we as the molten rock rises to the surface, or from the breaking up and deal with the surface of the earth and to some extent with its expulsion of portions of the walls of the vent, or of the lava, which reaction on the interior. This distinction allows of a broad finest impalpable dust and ashes, through increasing stages of

happens to have solidified within these walls. They vary from the treatment of the subject under two divisions:

coarseness up to huge "bombs" torn from the upper surface of the I. Hypogene or Plutonic Action: The changes within the molten rock in the vent, and large blocks of already solidified lava, earth caused by internal heat, mechanical movement and

or of non-volcanic rock detached from the sides of the pipe up which

the eruptions take place. chemical rearrangements.

Nothing is yet known as to the determining cause of any particular II.-Epigene or Surface Action: The changes produced on the volcanic eruption. Some vents, like that of Stromboli, in the superficial parts of the earth, chiefly by the circulation of air Mediterrancan, are continually active, and have been so ever since and water set in motion by the sun's heat.

man has observed them. Others again have been only intermittently

in eruption, with intervals of centuries between their periods of DIVISION 1.-HYPOGENE OR PLUTONIC ACTION

activity. We are equally in the dark as to what has determined

the sites on which volcanic action has manifested itself. There is In the discussion of this branch of the subject we must carry reason, indeed, to believe that extensive fractures of the terrestrial in our minds the conception of a globe still possessing a high crust have often provided passages up which the vapours, imprisoned internal temperature, radiating heat into space and consequently in the internal magma, have been able to make their way, accomcontracting in bulk.' Portions of molten rocks from inside are panied by other products. Where chains of volcanoes rise along

dehnite lines, like those of Sumatra, Java, and many other tracts from time to time poured out at the surface. Sudden shocks both in the Old and the New World. there appears to be little doubt are generated by which destructive earthquakes are propagated that their linear distributio should be attributed to this cause. through the diameter of the globe as well as to and along But where a volcano has appeared by itself, in a region previously its surface. Wide geographical areas are pushed up or sink cannot be so confidently presumed. The study of certain ancient down. In the midst of these movements remarkable changes volcanoes, the roots of which have been exposed by long denudation, are produced upon the rocks of the crust; they are plicated, has shown an absence of any visible trace of their having availed fractured, crushed, rendered crystalline and even fused.

themselves of fractures in the crust. The inference has been drawn that volcanic energy is capable of itself drilling an orifice through

the (A) Volcanoes and Volcanic Action.

crust, probably at some weaker part, and ejecting its products at the surface. The source of this

energy is to be sought in the enormous This subject is discussed in the article Volcano, and only a expansive force of the vapours and gases dissolved in the magma. general view of its main features will be given here. Under the term They are kept in solution by the enormous pressure within the earth; volcanic action (vulcanism, vulcanicity) are embraced all the but as the lava approaches the surface and this pressure is relieved phenomena connected with the expulsion of heated materials from these dissolved vapours and gases rush out with explosive violence, the interior of the earth to the surface. A volcano may be defined blowing the upper part of the lava column into dust, and allowing as a conical hill or mountain, built up wholly or mainly of materials portions of the liquid mass below to rise and escape, either from the which have been ejected from below, and which have accumulated crater or from some fissure which the vigour of explosion has opened around the central vent of eruption. As a rule its truncated summit on the side of the cone. So gigantic is the energy of these pent-up presents a cup-shaped cavity, termed the crater, at the bottom of vapours, that, after a long period of volcanic quiescence, they which is the opening of the main funnel or pipe' whereby com sometimes burst forth with such violence as to blow off the whole of munication is maintained with the heated interior. From time to the upper part or even one side of a large cone. The history of time, however, in large volcanoes rents are formed on the sides of Vesuvius, and the great eruptions of Krakatoa in 1883 and of the cone, whence steam and other hot vapours and also streams of Bandaizan in 1888 furnish memorable examples of great volcanic molten lava are poured forth. On such rents smaller or parasitic convulsions. It has been observed that such stupendous discharges cones are often formed, which imitate the operations of the parent of acriform and fragmentary matter may be attended with the cone and, after repeated eruptions, may rise to hills hundreds of emission of little or no lava. On the other hand, some of the largest feet in height. In course of centuries the result of the constant outflows of lava have been accompanied by comparatively little outpouring of volcanic materials may be to build up a large mountain fragmentary material. Thus, the great lava-floods of Iceland in like Etna, which towers above the sea to a height of 10,840 feet, and 1783 spread for 40 m. away from their parent fissure, which was has some 200 minor cones along its flanks.

marked only by a line of little cones of slag. But all volcanic eruptions do not proceed from central orifices. The temperature of lava as it issues from underground has been In Iceland it has been observed that, from fissures opened in the measured more or less satisfactorily, and affords an indication of ground and extending for long distances, molten material has issued that existing within the earth. At Vesuvius it has been ascertained in such abundance as to be spread over the surrounding country to be more than 2000° Fahr. At first the molten rock glows with a for many miles, while along the lines of fissure small cones or hillocks white light, which rapidly reddens, and disappears under the rugged of fragmentary material have accumulated round more active parts brown and black crust that forms on the surface. Underneath this of the rent. There is reason to believe that in the geological past badly conducting crust, the lava cools so slowly that columns of this fissure type of eruption has repeatedly been developed, as well steam have been noticed rising from its surface more than 80 years as the more common form of central cones like Vesuvius or Etna. after its eruption.

In the operations of existing volcanoes only the superficial mani. Considerable alteration in the topography of volcanic regions festations of volcanic action are observable. But when the rocks of may be produced by successive eruptions. The fragmentary the earth's crust are studied, they are found to enclose the relics materials are sometimes discharged in such abundance as to cover of former volcanic eruptions. The roots of ancient volcanoes have the ground for many miles around with a deposit of loost -ahes. thus been laid bare by geological revolutions; and some of the Icinders and slag. Such a deposit accumulating to a depth on many

« ՆախորդըՇարունակել »