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other acids, which, together with decaying organic matter taken up | affected the meteorological conditions of different countries: (1) by passing moisture, become potent in effecting the chemical decom- By removing forests, and laying bare to the sun and winds areas position of rocks and in promoting the disintegration of soils. (3) which were previously kept cool and damp under trees, or which, By inserting their roots or branches between joints of rock, which lying on the lee side, were protected from tempests. It is supposed are thereby loosened, so that large slices may be eventually wedged that the wholesale destruction of the woodlands formerly existing off. (4) By attracting rain, as thick woods, forests and peat-mosses in countries bordering the Mediterranean has been in part the cause do, and thus accelerating
the general waste of a country by running of the present desiccation of these districts. (2) By drainage, whereby water. (5) By promoting the decay of diseased and dead plants and the discharged rainfall is rapidly removed, and the evaporation is animals, as when fungi overspread a damp rotting tree or the carcase lessened, with a consequent diminution of rainfall and some increase of a dead animal.
in the general temperature of a country. . (3) By the other processes That plants also exert a conservative influence on the surface of of agriculture, such as the transformation of moor and bog into the land is shown in various ways. (1) The formation of a stratum cultivated land, and the clothing of bare hillsides with green crops of turf protects the soil and rocks underneath from being rapidly or plantations of coniferous and hardwood trees. disintegrated and washed away by atmospheric action. (2) Many Still more obvious are the results of human interference with the plants, even without forming a layer of turf, serve by their roots or flow of water: (1) By increasing or diminishing the rainfall man branches to protect the loose sand or soil on which they grow from directly affects the volume of rivers. (3) By his drainage
operations being removed by wind. The common sand-carex and other arenace- he makes the rain to run off more rapidly than before, and thereby ous plants bind the loose sand-dunes of our coasts, and give them a increases the magnitude of foods and of the destruction caused by permanence, which would at once be destroyed were the sand laid them. (3) By wells, bores, mines, or other subterranean works he bare again to storms. The growth of shrubs and brushwood along interferes with the underground waters, and consequently with the the course of a stream not only keeps the alluvial banks from being discharge of springs. (4) By embanking rivers he confines them to so easily undermined and removed as would otherwise be the case, narrow channels, sometimes increasing their scour, and enabling but serves to arrest the sediment in foods, filtering the water and them to carry their sediment further seaward, sometimes causing thereby adding to the height of the flood plain. (3) Some marine them to deposit it over the plains and raise their level. (5) By his plants, like the calcareous nullipores, afford protection to shore engineering operations for water-supply he abstracts water from its rocks by covering them with a hard incrustation. The tangles and natural basins and depletes the streams. smaller Fuci which grow abundantly on the littoral zone break the In many ways man alters the aspect of a country: (1) By changing force of the waves or diminish the effects of ground swell. (4) forest into bare mountain, or clothing bare mountains with forest. Forests and brushwood protect the soil, especially on slopes, from 3) By promoting the growth or causing the removal of peat-mosses.
Plants contribute by the aggregation of their remains to the motion a process of destruction which may convert hundreds of formation of stratified deposits. Some marine algae which secrete acres of fertile land into waste sand, or by prudently planting the carbonate of lime not only encrust rocks but give rise to shcets of dunes with sand-loving vegetation and thus arresting their landward submarine limestone. An analogous part is played in fresh-water progress. (4) By so guiding the course of rivers as to make them lakes by various lime-secreting plants, such as Charo. Long-con- laid him in reclaiming waste land, and bringing it under cultivation. tinued growth of vegetation has, in some regions, produced thick (5) By piers and bulwarks, whereby the ravages of the sea are accumulations of a dark loam, as in the black cotton soil (regur) of stayed, or by the thoughtless removal from the beach of stones India, and the black earth (ichernocom) of Russia. Peat-mosses which the waves had themselves thrown up, and which would have are formed in temperate and arctic climates by the growth of marsh- server for a time to protect the land. (6) By forming new deposits regions the mangrove swamps on low moist shores form a dense railways, tunnels, villages and towns with which man has covered
ftIn . : , : jungle, sometimes 20 m. broad, which protects these shores from the the sursace of the land will in many cases form a permanent record sea until, by the arrest of sediment and the constarit contribution of of his presence. Under his hand the whole surface of civilized decayed vegetation, the spongy ground is at last turned into firm countries is very slowly covered with a stratum, either formed soil. Some plants (diatoms) can abstract silica and build it into wholly by him or due in great measure to his operations and con. their framework, so that their remains form a siliceous deposit or taining many relics of his presence. The soil of ancient towns has ooze which covers spaces of the deep sea-floor estimated at more been increased to a depth of many feet by their successive destructhan ten millions of square miles in extent.
tions and renovations. (6) Animals - These exert a destructive influence in the following Perhaps the most subtle of human influences are to be seen in the ways: (1) By, seriously affecting the composition and arrangement distribution of plant and animal life upon the globe. Some of man's of the vegetable soil. Worms bring up the lower portions of the doings in this domain are indeed plain enough, such as the extirpation soil to the surface, and while thus promoting its fertility increase of wild animals, the diminution or destruction of some forms of its liability to be washed away by rain. Burrowing animals, by vegetation, the introduction of plants and animals useful to himself. throwing up the soil and subsoil, expose these to be dried and blown and especially the enormous predominance given by him to the away by the wind.
At the same time their subterranean passages cereals and to the spread of sheep and cattle. But no such extensive servé tó drain off the superficial water and to injure the stability disturbance of the normal conditions of the distribution of life can of the surface of the ground above them. In Britain the mole and take place without carrying with it many secondary effects, and rabbit are familiar examples. (2) By interfering with or even divert, setting in motion a wide cycle of change and of reaction in the ing the flow of streams. Thus beaver-dams check the current of animal and vegetable kindgoms. For example, the incessant water-courses, intercept floating materials, and sometimes turn warsare waged by man against birds and beasts of prey in districts strcams into new channels. The embankments of the Mississippi given up to the chase leads sometimes to unforeseen results. The are sometimes weakened to such an extent by the burrowings of the weak game is allowed to live, which would otherwise be killed off cray-fish as to give way and allow the river to inundate the sur- and give more room for the healthy remainder. Other animals rounding country Similar results have happened in Europe from which seed perhaps on the same materials as the game are by the subterranean operations of rats. (3) Some mollusca bore into stone sa me cause permitted to live unchecked, and thereby to act as a or wood and by the number of contiguous perforations greatly further hindrance to the spread of the protected species. But the weaken the material. (4) Many animals exercise a ruinously indirect results of man's interference with the régime of plants and destructive influence upon vegetation. Of the numerous plagues animals still require much prolonged observation. of this kind the locust, phylloxera and Colorado beetle may be cited.
The most important gcological function performed by animals is the formation of new deposits out of their remains. It is chicfly by
Part V-GEOTECTONIC OR STRUCTURAL GEOLOGY the lower grades of the animal kingdom that this work is accom. plished, especially by molluscs, corals and foraminisera. Shell-banks
From a study of the nature and composition of minerals and are formed abundantly in such comparatively shallow and enclosed rocks, and an investigation of the difierent agencies by which basins as that of the North Sea, and on a much more extensive scale they are formed and modified, the geologist proceeds to inquire on the floor of the West Indian seas. By the coral polyps thick how these materials have been put together so as to build up the masses of limestones have been built up in the warmer seas of the visible part of the earth's crust. globe (see CORAL REEFs). The floor of the Atlantic and other occans
He soon ascertains that they is covered with a fine calcareous ooze derived mainly from the have not been thrown together wholly at random, but that they remains of foraminisera, while in other regions the bottom shows a show a recognizable order of arrangement. Some of them, siliceous ooze formed almost entirely of adiolaria animals give rise to phosphatic deposits formed sometimes of their especially those of most recent growth, remain in their original excrement, as in guano and coprolites, sometimes of an accumulation condition and position, but, in proportion 10 their antiquity, of their bones.
they generally present increasing alteration, until it may no (c) Man.-No survey of the geological workings of plant and longer be possible to tell what was their pristine state. As by animal life upon the surface of the globe can be complete which does far the largest accessible portion of the terrestrial crust consists of not take account of the influence of man-an influence of enormous stratified rocks, and as these furnish clear evidence of most of the and increasing consequence in physical geography, for man has introduced, as it were, an clement of antagonism to nature, His modifications to which they have been subjected in the long interference shows itself in his relations to climate, where he has course of geological history, it is convenient to take them into consideration first. They possess a number of structures which remains, as shown in Part VI., where the grouping of the stratified belong to the original conditions in which they were accumulated. rocks into formations and systems is described. They present in addition other structures which have been super
(6) Igneous Rocks.--As part of the earth's crust these rocks induced upon them, and which they share with the unstratified present characters by which they are strongly differentiated or igneous rocks.
from the stratified series. While the broad petrographical
distinctions of their several varieties remain persistent, they 1. ORIGINAL STRUCTURES
present sufficient local variations of type to point to the existence (c) Stratified Rocks. This extensive and important series is of what have been called petrographic provinces, in each of above all distinguished by possessing a prevailing stratified which the eruptive masses are connected by a general family arrangement. Their materials have been laid down in laminae, relationship, differing more or less from that of a neighbouring layers and strata, or beds, pointing generally to the intermittent province. In each region presenting a long chronological series deposition of the sediments of which they consist. As this of eruptive rocks a petrographical sequence can be traced, which stratification was, as a rule, originally nearly or quite horizontal , is observed to be not absolutely the same everywhere, though
its it serves as a base from which to measure any subsequent dis- general features may be persistent. The earliest manifestations turbance which the rocks have undergone. The occurrence of of eruptive material in any district appear to have been most false-bedding, i.e. bands of inclined layers between the normal frequently of an intermediate type between acid and basic, planes of stratification, does not form any real exception; but passing thence into a thoroughly acid series and concluding indicates the action of shifting currents whereby the sediment with an effusion of basic material. was transported and thrown down. Other important records of
Considered as part of the architecture of the crust of the earth, the original conditions of deposit are supplied by ripple-marks, igncous rocks are conveniently divisible into two great series: sun-cracks, rain-prints and concretions.
(1) those bodies of material which have been injected into the From the nature of the material further light is cast on the geo the surface and have been ejected there, either in a molten state
crust and have solidified there, and (2) those which have reached graphical conditions in which the strata were accumulated. Thus, conglomerates indicate the proximity of old shore-lines, sandstones as lava or in a fragmental form as dust, ashes and scoriae. The mark deposits in comparatively shallow water, clays and shales first of these divisions represents the plutonic, intrusive of point to the tranquil accumulation of fine silt at a greater depth subsequent phase of eruptivity; the second marks the volcanic, and further from land, while fossiliferous limestones bear witness to interstratified or contemporaneous phase. clearer water in which organisms flourished at some distance from deposits of sand and mud. Again, the alternation of different kinds 1. The plutonic or intrusive rocks, which have been forced into of sediment suggests a variability in the conditions of deposition, the crust and have consolidated there, present a wide range of texture such as a shifting of the sediment-bearing currents and of the areas from the most coarse-grained granites to the most perfect natural of muddy and clear water. A thick group of conformable strata, glass. Seeing that they have usually cooled with extreme slowness that is, a series of deposits which show no discordance in their underground, they are as a general rule more largely crystalline stratification, may usually be regarded as having been laid down on than the volcanic series. The form assumed by cach individual a sea-floor that was gently sinking. Here and there evidence is body of intrusive material has depended upon the shape of the space obtainable of the limits or of the progress of the subsidence by what into which it has been injected, and where it has cooled and become is called "overlap." of the absolute length of time represented by solid. This shape has been determined by the local structure of any strata or groups of strata no satisfactory estimates can yet be the carth's crust on the one hand and by the energy of the eruptive formed. Certain general conclusions may indeed be drawn, and force on the other. It offers a convenient basis for the classification comparisons may be made between different series of rocks. Sand of the intrusive rocks, which,
as part of the framework of the crust, stones full of false-bedding were probably accumulated more rapidly may thus be grouped according to the shape of the cavity which than finely-laminated shales or clays. lí is not uncommon in certain received them, as bosses, sills, dikes and necks. Carboniferous formations to find coniferous and other trunks em Bosses, or stocks, are the largest and most shapeless extravasations bedded in sandstone. Some of these trees seem to have been carried of erupted material. They include the great bodies of granite which, along and to have sunk, their heavier or root end touching the in most countries of the world, have risen for many miles through bottom and their upper end slanting upward in the direction of the the stratified formations and have altered the rocks around them current, exactly as in the case of the snags of the Mississippi. In by contact-metamorphism. Sills, or intrusive sheets, are bed-like other cases the trees have been submerged while still in their positions masses which have been thrust between the plants of sedimentary of growth. The continuous deposit of sand at last rose above the or even of igneous rocks. The term laccolite has been applied to level of the trunks and buried them. It is clear then that the rate sills which are connected with bosses. Intrusive sheets are disof deposit must have been sometimes sufficiently rapid to allow tinguishable from true contemporaneously
intercalated lavas by not sand to accumulate to a depth of 30 ft. or more before the decay keeping always to the same platform, but breaking across and of the wood. Modern instances are known where, under certain altering the contiguous strata, and by the closeness of their texture circumstances, submerged trees may last for some centuries, but where they come in contact with the contiguous rocks, which, being even the most durable must decay in what, after
all, is a brief space cold, chilled the molten material and caused it to consolidate on its of geological time. Since continuous layers of the same kind of outer margins more rapidly than in its interior. Dikes or veins deposit suggest a persistence of geological conditions, while numerous are vertical walls or ramifying branches of intrusive material which alternations of different kinds of sedimentary matter point to has consolidated in hissures or irregular clefts of the crust. Necks vicissitudes or alternations of conditions, it may be supposed that are volcanic chimneys which have been filled up with erupted the time represented by a given thickness of similar strata was less material, and have now been exposed at the surface after prolonged than that shown by the same thickness of dissimilar strata, because denudation has removed not only the superficial volcanic masses the changes needed to bring new varieties of sediment into the area originally associated with them, but also more or less of the upper of deposit would usually require the lapse of some time for their part of the
vents. Plutonic rocks do not present evidence of their completion. But this conclusion may often be erroneous. It will precise geological age. All that can be certainly affirmed from be best supported when, from the very nature of the rocks, wide ihem is that they must be younger than the rocks into which they variations in the character of the water-bottom can be established. have been intruded. From their internal structure, however, and Thus a group of shales followed by a fossiliferous limestone would from the evidence of the rocks associated with them, some more or almost always mark the lapse of a much longer period than an equal less definite conjectures may be made as to the limits of time within depth of sandy strata. A thick mass of limestone, made
of which they were probably injected.
2. The interstratified or volcanic series is of special importance remains are crowded together generation above generation, must in geology, inasmuch as it contains the records of volcanic action have demanded many years or centuries for its formation.
during the past history of the globe. It was pointed out in Part 1 But in all speculations of this kind we must bear in mind that the that while towards the end of the 18th and in the beginning of the length of time represented by a given depth of strata is not to be 19th century much attention was paid by Hutton and his followers estimated merely from their thickness or lithological character to the proofs of intrusion afforded by what they called the The interval between the deposit of two successive
laminae of shale erupted lavas" within the earth's crust, these observers lost sight may have been as long as, or even longer than, that required for of the possibility that some of these rocks might have been erupted the formation of one of the laminac. In like manner the interval at the surface, and might thus be chronicles of volcanic action in needed for the transition from one stratum or kind of strata to former geological periods. It is not always possible to satisfactorily another may often have been more than equal to the time required discriminate between the two types of contemporaneously inter: for the formation of the strata on either side. But the relative calated and subsequently injected material. But rocks of the chronological importance of the bars or lines in the geological former type have not broken into or involved the overlying strata, record can seldom be satisfactorily discussed merely on lithological and they are usually, marked by the characteristic structures of grounds. This must mainly be decided on the evidence of organic 1 superficial lavas and by their association with volcanic tufts. By
Where one serie
means of the evidence which they supply, it has been ascertained strata (see Alps). Further proof of the colossal compression to that volcanic action has been manifested in the globe since the which the rocks have been subjected is afforded by their intense earliest geological periods. In the British Isles, for example, the crumpling and corrugation, and by the abundantly faulted and volcanic record is remarkably full for the long series of ages from crushed condition to which they have been reduced. Similar Cambrian to Permian time, and again for the older Tertiary period. evidence as to stresses in the terrestrial crust and the important
changes which they produce among the rocks may also be obtained 2. SUBSEQUENTLY INDUCED STRUCTURES
on a smaller scale in many non-mountainous countries. After their accumulation, whether as stratified or eruptive has been induced in some rocks by the production of the fissile
Another marked result of the compression of the terrestrial crust masses, all kinds of rocks have been subject to various changes, structure which is typically shown in roofing-slate (cleavage). and have acquired in consequence a variety of superinduced Closely connected with this internal rearrangement has been the
It has been pointed out in the part of this article development of microscopic microlites or crystals (rutile, mica, &c.) dealing with dynamical geology that one of the most important mud and silt. From this incipient form of metamorphism successive forms of energy in the evolution of geological processes is to be stages may be traced through the various kinds of argillite and found in the movements that take place within the crust of the phyllite into mica-schist, and thence into more crystalline gneissoid earth. Some of these movements are so slight as to be only varieties (foliation, slate, mica-schist, gneiss). "The Alps afford recognizable by means of delicate instruments; but from this excellent illustrations of these transformations. inferior limit they range up to gigantic convulsions by which defined divisional planes, like cracks across a pane of glass. Much
The fissures produced in the crust are sometimes clean, sharply mountain-chains are upheaved. The crust must be regarded as more usually, however, the rocks on either side have becn broken up in a perpetual state of strain, and its component materials are by the friction of movement, and the fault is marked by a variable therefore subject to all the effects which flow from that condition. breadth of this broken material. Sometimes the walls have separated
and molten rock has risen from below and solidified between them It is the one great object of the geotectonic division of geology to
as a dike. Occasionally the fissures have opened to the surface. study the structures which have been developed in consequence and have been filled in from above with detritus, as in the sandstoneof earth-movements, and to discover from this investigation the dikes of Colorado and California. In mineral districts the fissures nature of the processes whereby the rocks of the crust have been have been filled with various spars and ores, forming what are known
as mineral veins. brought into the condition and the positions in which we now
of rocks is covered by another without any find them. The details of this subject will be found in separate break or discordance in the stratification they are said to be conarticles descriptive of each of the technical terms applied to the formable. But where the older series has been tilted up or visibly several kinds of superinduced structures. All that need be denuded before being overlain by the younger, the latter is termed offered here is a general outline connecting the several portions structural geology, for it marks a gap in the geological record, which
unconformable. This relation is one of the greatest value in of the subject together.
may represent a vast lapse of time not there recorded by strata. One of the most universal of these later structures is to be seen
Part VI.--PALAEONTOLOGICAL GEOLOGY in the divisional planes, usually vertical or highly inclined, by which rocks are split into quadrangular or irregularly shaped blocks, This division of the science deals with fossils, or the traces To these planes the name of joints has been given. They are of of plants and animals preserved in the rocks of the carth's crust, prime importance from an industrial point of view, seeing that the art of quarrying consists mainly in detecting and making proper,
and endeavours to gather from them information as to the history use of them.
Their abundance in all kinds of rocks, from those of of the globe and its inhabitants. The term “ fossil ” (Lat. recent date up to those of the highest antiquity, affords a remarkable fossilis, Irom fodere, to dig up), meaning literally anything testimony to the strains which the terrestrial crust has suffered.
" dug up,” was formerly applied indiscriminately to any mineral They have arisen sometimes from tension, such as that caused by contraction from the drying and consolidation of an aqueous sedi substance taken out of the earth's crust, whether organized or ment or from the cooling of a molten mass; sometimes from torsion
Since the time of Lamarck, however, the meaning of the during movements of the crust.
word has been restricted, so as to include only the remains or Although the stratified rocks were originally deposited in a more
traces of plants and animals preserved in any natural formation or less nearly horizontal position on the floor of the sea, where now visible on the dry land they are seldom found to have retained their
whether hard rock or superficial deposit. It includes not merely Aatness. On the contrary, they are seen to have been generally the petrified structures of organisms, but whatever was directly tilted up at various angles, sometimes even placed on end (crop connected with or produced by these organisms. Thus the dip, strike). When a sufficiently large area of ground is examined, resin which was exuded from trees of long-perished forests the inclination into which the strata have been thrown may be is as much a fossil as any portion of the stem, leaves, flowers observed not to continue far in the same direction, but to turn over to the opposite or another quarter. It can then be seen that in or fruit, and in some respects is even more valuable to the reality the rocks have been thrown into undulations. From the geologist than more determinable remains of its parent trees, lowest and Aattest arches where the departure from horizontality because it has often preserved in admirable perfection the insects may be only trifling, every step may be followed up to intense which fitted about in the woodlands. The burrows and trails curvature, where the strata have been compressed and plicated as if they had been piles of soft carpets (anticline, syncline, monocline, of a worm preserved in sandstone and shale claim recognition as geo-anticline, geo-syncline, isoclinal, plication, curvature, qua-qua: fossils, and indeed are commonly the only indications to be met versal). It has further happened abundantly all over the surface of with of the existence of annelid life among old geological forma. the globe that relief from internalstrain in the crust has been obtained
tions. The droppings of fishes and reptiles, called coprolites, by fracture, and the consequent subsidence or elevation of one or both sides of the fissure. The differential movement between the
are excellent fossils, and tell their tale as to the presence and wo sides may be scarcely, perceptible in the feeblest dislocation, food of vertebrate life in ancient waters. The little agglutinated but in the extreme cases it may amount to many thousand feet cases of the caddis-worm remain as fossils in formations from (fault, fissure, dislocation, hade, slickensides). The great faults in a country are among its most important structural features, and as
which, perchance, most other traces of life may have passed they not infrequently continue to be lines of weakness in the crust
away. Nay, the very handiwork of man, when preserved in along which sudden slipping may from time to time take place, they any natural manner, is entitled to rank among fossils; as become the lines of origin of earthquakes. The San Francisco where his fint-implements have been dropped into the preearthquake of 1906, already cited, affords a memorable illustration historic gravels of river-valleys or where his canoes have been of this connexion.
It is in a great mountain-chain that the extraordinary complica. buried in the silt of lake-bottoms. tion of plicated and faulted structures in the crust of the earth can A study of the land-surfaces and sea-floors of the present time be most impressively beheld. The combination of overturned folds shows that there are so many chances against the conservation with rupture has been already referred to as a characteristic feature of the remains of cither terrestrial or marine animals and plants in the Alps (Part IV.). The gigantic folds have in many places been that if, as is probable, the same conditions existed in former geological pushed over each other
so as to lie almost flat, while the upper limb periods, we should regard the occurrence of organic remains among has not infrequently been driven for many miles beyond the lower the stratified formations of the earth's crust as generally the result by a rupture along the axis. In this way successive slices of a thick of various fortunate accidents. series of formations have been carried northwards on the northern Let us consider, in the first place, the chances for the preservation slope of the Alps, and have been piled so abnormally above each of remains of the present fauna and flora of a country. The surface other that some of their oldest members recur several times on of the land may be densely clothed with forest and abundantly different thrust-planes, the whole being underlain by Tertiary I peopled with animal life. But the trees die and moulder into soil.
The animals, too, disappear, generation after generation, and leave times. In the marine Mollusca, therefore, we have a common ground lew or no perceptible traces of their existence. If we were not aware of comparison between the stratified formations of different periods from authentic records that central and northern Europe were They have been styled the alphabet of palaeontological inquiry. covered with vast forests at the beginning of our era, how could we
There are two main purposes to which fossils may be put in know this fact? What has become of the herds of wild oxen, the bears, wolves and other denizens of primeval Europe? How could geological research. (1) to throw light upon former conditions we prove from the examination of the surface soil of any country of physical geography, such as the presence of land, rivers, that those creatures had once abounded there? The conditions for lakes and seas, in places where they do not now exist, changes the preservation of any relics of the plant and animal life of a ter.
of climate, and the former distribution of plants and animals; restrial surface must obviously be always exceptional. They are supplied only where the organic remains can be protected from the and (2) to furnish a guide in geological chronology whereby air and superficial decay Hence they may be observed in (1) the rocks may be classified according to relative date, and the facts deposits on the floors of lakes; (2) in peat-mosses; (3) in deltas at of geological history may be arranged and interpreted as a river-mouths; and (4) under the stalagmite of caverns in limestone connected record of the earth's progress. districts. But in these and other favourable places a mere infinitesimal fraction of the fauna or flora of a land-surface is likely to be 1. As examples of the first of these two directions of inquiry entombed or preserved.
reference may be made to (a) former land-surfaces revealed by the In the second place, although in the sea the conditions for the occurrence of layers of soil with tree-stumps and roots still in the preservation of organic remains are in many respects more favourable position of growth (see PURBECKIAN); (6) ancient lakes proved by than on land, they are apt to be frustrated by many adverse circum- beds of marl or limestone full of lacustrine shells; (c) old sea-bottoms stances. While the level of the land remains stationary, there can marked by the occurrence of marine organisms; (d) variations in be but little effective entombment of marine organisms in littoral the quality of the water, such as freshness or saltness, indicated by deposits; for only a limited accumulation of sediment will be formed changes in the size and shape of the fossils; (c) proximity to former until subsidence of the sea-floor takes place. In the trifling beds of land, suggested by the occurrence of abundant drist-wood in the sand or gravel thrown up on a stationary shore, only the harder and strata : () former conditions of climate, different from the present, more durable forms of life, such as gastropods and lamellibranchs, as evidenced by such organisms as tropical types of plants and which can withstand the triturating effects of the beach waves, are animals intercalated among the strata of temperate or northern likely to remain uneffaced.
countries. Below tide-marks, along the margin of the land where sediment 2. In applying fossils to the determination of geological chronology is gradually deposited, the conditions are more favourable for the it is first necessary to ascertain the order of superposition of the preservation of marine organisms. In the sheets of sand and mud rocks. Obviously, in a continuous series of undisturbed sedimentary there laid down the harder parts of many forms of life may be deposits the lowest must necessarily be the oldest, and the plants or entombed and protected from decay. But only a small proportion animals which they contain must have lived and died before any of of the total
marine fauna may be expected to appear in such deposits. the organisms that occur in the overlying strata. This order of At the best, merely littoral and shallow-water forms will occur, and, superposition having been settled in a series of formations, it is even under the most favourable conditions, they will represent but found that the fossils at the bottom are not quite the same as those a fraction of the whole assemblage of life in these juxta-terrestrial at the top of the series. Tracing the beds upward, we discover that parts of the ocean. As we recede from the land the rate of deposition species alter species of the lowest platforms disappears, until perhaps of sediment on the sea-floor must become feebler, until, in the remote not one of them is found. With the cessation of these older spccics central abysses, it reaches a hardly appreciable minimum. Except, others make their entrance. These, in turn, are found to die out, therefore, where some kind of ooze or other deposit is accumulating and to be replaced by newer forms. After patient examination of in these more pelagic regions, the conditions must be on the whole the rocks, it has been ascertained that every well-marked " forma. unfavourable for the preservation of any adequate representation tion," or group of strata, is characterized by its own species or of the deep-sea fauna. Hard durable objects, such as teeth and genera, or by a general assemblage, or facies, of organic forms. bones, may slowly accumulate, and be protected by a coating of Such a generalization can only, of course, be determined by actual peroxide of manga nese, or of some of the silicates now forming here practical experience over an arca of some size. When the typical and there over the deep-sea bottom; or the rate of growth of the lossils of a formation are known, they serve to identify that formation abysmal deposit may be so tardy that most of the remains of at in its progress across a country. Thus, in tracts where the true least the larger animals will disappear, owing to decay, before they order of superposition cannot be determined, owing to the want of can be covered and preserved. Any such deep-sea formation, sections or to the disturbed condition of the rocks, Fossils serve as a if raised into land, would supply but a meagre picture of the whole means of identification and furnish a guide to the succession of the life of the sea.
rocks. They even demonstrate that in some mountainous ground It would thus appear that the portion of the sea-floor best suited the beds have been turned completely upside down, where it for receiving and preserving the most varied assemblage of marine can be shown that the fossils in what are now the uppermost organic remains is the area in front of the land, to which rivers and strata ought properly to lie underneath those in the beds below currents bring continual supplies of sediment. The most favourable them. conditions for the accumulation of a thick mass of marine fossiliserous It is by their characteristic fossils that the stratified rocks of the strata will arise when the area of deposit is undergoing a gradual earth's crust can be most satisfactorily subdivided into convenient subsidence. If the rate of depression and that of deposit were equal, groups of strata and classed in chronological order. Each " forma. or nearly so, the movement might proceed for a vast period without tion is distinguished by its own peculiar assemblage of organic producing any great apparent change in marine geography, and even remains, by means of which it can be followed and recognized, even without seriously affecting the distribution of life over the sea-floor amid the crumplings and dislocations of a disturbed region. The within the area of subsidence.. Hundreds or thousands of feet of same general succession of organic types can be observed over a sedimentary strata might in this way be heaped up round the con- large part of the world, though, of course, with important modificatinents, containing a fragmentary series of organic rem ns belonging tions in different countries. This similarity of succession has been to those forms of comparatively shallow-water life which had hard termed homolaxis, a term which expresses the fact that the order parts capable of preservation. There can be little doubt that such in which the leading types of organized existence have appeared has, in fact, been the history of the main mass of stratified formations upon the earth has been similar even in widely separated regions. in the earth's crust. By far the largest proportion of these piles It is evident that, in this way, a reliable method of comparison of marine strata has unquestionably been laid down in water of no is furnished, whereby the stratified formations of different parts of great depth within the area of deposit of terrestrial sediment. the earth's crust can be brought into relation with each other, The enormous thickness to which they attain seems only explicable Had the geologist continued to remain, as in the days of Werner, by prolonged and repeated movements of subsidence, interrupted, hampered by the limitations imposed by a reliance on mere lithohowever, as we know, by other movements of a contrary kind. logical characters, he would have made little or no progress in
Since the conditions for the preservation of organic remains exist deciphering the record of the successive phases of the history of more favourably under the sea than on land, marine organisms must the globe chronicled in the crust. Just as, at the present time, be far more abundantly conserved than those of the land. This is sheets of gravel in one place are contemporaneous with sheets of true to-day, and has, as far as known, been true in all past geological mud at another, so in the past all kinds of sedimentation have been time. Hence for the purposes of the geologist the fossil remains of in progress simultaneously, and those of one period may not be marine forms of life far surpass all others in value. Among them distinguishable in themselves from those of another. Little or no there will necessarily be a gradation of importance, regulated chiefly reliance can be placed upon lithological resemblances or differences
in comparing the sedimentary formations of different countries. live within the juxta-terrestrial belt of sedimentation, unquestionably In making use of fossil evidence for the purpose of subdividing the Mollusca stand in the place of pre-eminence as regards their the stratified rocks of the earth's crust, it is found to be applicable aptitude for becoming fossils. They almost all possess a hard, durable to the smaller details of stratigraphy as well as to the definition o! shell, capable of resisting considerable abrasion and readily passing large groups of strata. Thus a particular stratum may be marked into a mineralized condition. They are extremely abundant both as by the occurrence in it of various fossils, one or more of which may to individuals and genera. They occur on the shore within tide be distinctive, either from occurring in no other bed above and mark, and range thence down into the abysses. Moreover, they below or from special abundance in that stratum. One or more of appear to have possessed these qualifications from early geological these species is therefore used as a guide to the occurrence of the bed
in question, which is called by the name of the most abundant temporaneous disturbance or denudation, but succeed each other, species. In this way what is called a ".geological horizon," or as if they had been accumulated by one continuous process of zone,” is marked off, and its exact position in the series of formations deposit. It must be admitted that the problem of life-zones in is fixed.
stratigraphical geology has not yet been solved. Perhaps the most distinctive feature in the progress of palaeonto- As Darwin first cogently showed, the history of life has been very logical geology during the last half century has been the recognition imperfectly registered in the stratified parts of the earth's crust. and wide application of this method of zonal stratigraphy, which, Apart from the fact that, even under the most favourable conditions, in itself, was only a further development of William Smith's famous only a small proportion of the total flora and fauna of any period idea, “Strata identified by Organized Fossils.” It was first
carried would be preserved in the fossil state, enormous gaps occur where out in detail by various palaeontologists in reference to the Jurassic no record has survived at all. It is as if whole chapters and books formations, notably by F. A. von Quenstedt and C. A. Oppel in were missing from a historical work. Some of these lacunae are Germany and A. D. d'Orbigny in France. The publication of sufficiently obvious. Thus, in some cases, powerful dislocations have Oppel's classic work Die Juraformation Englands, Frankreichs und thrown considerable portions of the rocks out of sight. Sometimes des südwestlichen Deutschlands (1856-1858) marked an epoch in the extensive metamorphism has so affected them that their original development of stratigraphical geology. Combining what had been characters, including their organic contents, have been destroyed. done by various observers with his own laborious researches in Ostenest of all, denudation has come into play, and vast masses of France, England, Württemberg and Bavaria, he drew up a classifica fossiliserous rock have been entirely worn away, as is demonstrated tion of the Jurassic system, grouping its several formations into zones, by the abundant unconformabilities in the structure of the earth's cach characterized by some distinctly predominant fossil after which it was named (see Lins). The same method of classification was While the mere fact that one series of rocks lies unconformably afterwards extended to the Cretaceous series by A. D. d'Orbigny, on another proves the lapse of a considerable interval between their E. Hébert and others, until the whole Mesozoic rocks from the respective datęs, the relative length of this interval may sometimes Trias to the top of the Chalk has now been partitioned into zones, be proved by means of fossil evidence, and by this alone. Let us each named after some characteristic species or genus of fossils. suppose, for example, that a certain group of formations has been More recently the principle has been extended to the Palaeozoic disturbed, upraised, denuded and covered unconformably by, a formations, though as yet less fully than to the younger parts of the second group. In lithological characters the two may closely resemble geological record. It has been successfully applied by Professor C. each other, and there may be nothing to show that the gap repreLapworth to the investigation of the Silurian series (see SILURIAN; sented by their unconformability is of an important character. In ORDOVICIAN SYSTEM). He found that the species of graptolites many cases, indeed, it would be quite impossible to pronounce any have each a comparatively narrow vertical range, and they may well-grounded judgment as to the amount of interval, even measured consequently be used for stratigraphical purposes. Applying the by the vague relative standards of geological chronology. But if method, in the first instance, to the highly plicated Silurian rocks of each group contains a well-preserved suite of organic remains, it the south of Scotland, he found that by means of graptolites he was may not only be possible, but easy, to say exactly how much of the able to work out the structure of the ground. Each great group of geological record has been left out between the two sets of formations. strata was seen to possess its own graptolitic zones, and by their By comparing the fossils with those obtained from regions where the means could be identified not only in the original complex Scottish geological record is more complete, it may be ascertained, perhaps, area, but in England and Wales and in Ireland. It was eventually that the lower rocks belong to a certain platform or stage in geological ascertained that the succession of zones in Great Britain could be history which for our present purpose we may call D, and that the recognized on the Continent, in North America and even in Australia. upper rocks can in like manner be paralleled with stage H. It would The brachiopods and trilobites have likewise been made use of for be then apparent that at this locality the chronicles of three great zonal purposes among the oldest sedimentary, formations. The geological periods E, F, and G were wanting, which are elsewhere most ancient of the Palaeozoic systems has as its fitting base the found to be intercalated between D and H. The lapse of time repreOlencllus zone.
sented by this unconformability would thus be equivalent to that Within undefined and no doubt variable geographical limits required for the accumulation of the three missing formations in palaeontological zones have been found to be remarkably persistent. those regions where sedimentation was more continuous. They follow each other in the same general order, but not always Fossil evidence may be made to prove the existence of gaps which with equal definiteness. The type fossil may appear in some districts are not otherwise apparent. As has been already remarked, changes on a higher or a lower platform than it does in others. Only to a in organic forms must, on the whole, have been extremely slow in limited degree is there any coincidence between lithological variations the geological past. The whole species of a sea-floor could not pass in the strata and the sequence of the zones. In the surassic forma entirely away, and be replaced by other forms, without the lapse tions, indeed, where frequent alternations of different sedimentary of long periods of time. 'll then among the conformable stratised materials are to be met with, it is in some cases possible to trace a formations of former ages we encounter sudden and abrupt changes definite upward or downward limit for a zone by some abrupt in the facies of the fossils, we may be certain that these must mark change in the sedimentation, such as from limestone to shale. But omissions in the record, which we may hope to fill in from a more such a precise demarcation is impossible where no distinct bands of perfect series elsewhere. The complete biological contrasts between different sediments are to be seen. The zones can then only be the fossil contents of unconformable strata are sufficiently explicable. vaguely determined by finding their characteristic fossils, and noting It is not so easy to give a satisfactory, account of those which occur where these begin to appear in the strata and where they cease. where the beds are strictly conformable, and where no evidence can It would seem, therefore, that the sequence of palaeontological be observed of any considerable change of physical conditions at the zones, or lise-horizons, has not depended merely upon changes in time of deposit. A group of strata having the same general litho. the nature of the conditions under which the organisms lived. We logical characters throughout may be marked by a great discrepance should naturally expect that these changes would have had a marked between the fossils above and below a certain line. A few species influence; that, for instance, a diffcrence should be perceptible may pass from the one into the other, or perhaps every species may between the character of the fossils in a limestone and that of those be different. In cases of this kind, when proved to be not merely in a shale or a sandstone. The environment, when a limestone was local but persistent over wide areas, we must admit, notwithstanding in course of deposition, would generally be one of clear water, the apparently undisturbed and continuous character of the original favourable for a more vigorous and more varied fauna than where deposition of the strata, that the abrupt transition from the one facies a shale series was accumulating, when the water would be dis- of fossils to the other represents a long interval of time which has not coloured, and only such animals would continue to live in it, or on been recorded by the deposit of strata. A. C. Ramsay, who called the bottom, as could maintain themselves in the midst of mud. attention to these gaps, termed them " breaks in the succession of But no such lithological reason, betokening geographical changes organic remains. He showed that they occur abundantly among that would affect living creatures, can be adduced as a universally the Palacozoic and Secondary rocks of England. It is obvious, of applicable explanation of the occurrence and limitation of palaeon course, that such breaks, even though traceable over wide regions, tological zones. One of these zones may be only a few inches, or were not general over the whole globe. There have never been any feet or yards in vertical extent, and no obvious lithological or other universal interruptions in the continuity of the chain of being, cause can be seen why its specially characteristic fossils should so far as geological evidence can show. But the physical changes not be found just as frequently in the similar strata above and which caused the breaks may have been general over a zoological below. There is often little or no evidence of any serious change district or minor region. They no doubt often caused the complete in the conditions of sedimentation, still less of any widespread extinction of gencra and species which had a small geographical physical disturbance, such as the catastrophes by which the range. older geologists explained the extinction of successive types of From all these facts it is clear that the geological record, as it now
exists, is at the best but an imperfect chronicle of geological history. It has been suggested that, where the lifc-zones are well defined, In no country is it complete. The lacunac of one region must be sedimentation has been extremely slow, and that though these zones supplied from another, Yet in proportion to the geographical follow each other with no break in the sedimentation, they were distance between the localities where the gaps occur and those really separated by prolonged intervals of time during which organic whence the missing intervals are supplied, the element of uncertainty evolution could come effectively into play. But it is not easy to in our reading of the record is increased, The most desirable explain how, for example in the Lower Lias, there could have
been method of research is to exhaust the evidence for each area or a succession of prodigious intervals, when practically no sediment province, and to compare the general order of its succession as a was laid down, and yet that the strata should show no sign of con- wbole with that which can be established for other provinces.