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geological record is not to be measured by mere depth of strata. PART. VII.-STRATIGRAPHICAL GEOLOGY

While it may be reasonably assumed that, in general, a great This branch of the science arranges the rocks of the earth's thickness of stratified rock must mark the passage of a long crust in the order of their appearance, and interprets the sequence period of time, it cannot safely be affirmed that a much less of events of which they form the records. Its province is to thickness elsewhere must represent a correspondingly diminished cull from the other departments of geology the facts which may period. The need for this caution may sometimes be made be needed to show what has been the progress of our planet, evident by an unconformability between two sets of rocks, as and of each continent and country, from the earliest times of has already been explained. The total depth of both groups which the rocks have preserved any memorial. Thus from together may be, say 1000 ft. Elsewhere we may find a single mineralogy and petrography it contains information regarding unbroken formation reaching a depth of 10,000 ft.; but it would the origin and subsequent mutations of minerals and rocks. be unwarrantable to assume that the latter represents ten times From dynamical geology it learns by what agencies the materials the length of time indicated by the former two. So far from of the earth's crust have been formed, altered, broken, upheaved this being the case, it might not be difficult to show that the and melted. From geotectonic geology it understands the minor thickness of rock really denotes by far the longer geological various processes whereby these materials were put together interval. If, for instance, it could be proved that the upper so as to build up the complicated crust of the earth. From part of both the sections lies on one and the same geological palaeontological geology it receives in well-determined fossil platform, but that the lower unconformable series in the one remains a clue by which to discriminate the different stratified locality belongs to a far lower and older system of rocks than the formations, and to trace the grand onward march of organized base of the thick conformable series in the other, then it would existence upon this planet. Stratigraphical geology thus be clear that the gap marked by the unconformability really gathers up the sum of all that is made known by the other indicates a longer period than the massive succession of deposits. departments of the science, and makes it subservient to the 7. Fossil evidence furnishes the chief means of comparing the interpretation of the geological history of the earth.

relative value of formations and groups of rock. A “ break in The leading principles of stratigraphy may be summed up the succession of organic remains," as already explained, marks as follows:

an interval of time often unrepresented by strata at the place 1. In every stratigraphical research the fundamental requisite where the break is found. The relative importance of these is to establish the order of superposition of the strata. Until breaks, and therefore, probably, the comparative intervals this is accomplished it is impossible to arrange the dates, and of time which they mark, may be estimated by the difference make out the sequence of geological history.

of the facies or general character of the fossils on each side. 2. The stratificd portion of the earth's crust, or what has been If, for example, in one case we find every species to be dissimilar called the “ geological record,” can be subdivided into natural above and below a certain horizon, while in another locality only groups, or series of strata, characterized by distinctive organic half of the species on each side are peculiar, we naturally inser, remains and recognizable by these remains, in spite of great if the total number of species seems large enough to warrant changes in lithological character from place to place. A bed, the inference, that the interval marked by the former break or a number of beds, linked together by containing one or more was much longer than that marked by the second. But we may distinctive species or genera of fossils is termed a zone or horizon, go further and compare by means of fossil evidence the relation and usually bears the name of one of its more characteristic between breaks in the succession of organic remains and the fossils, as the Planorbis-zone of the Lower Lias, which is so depth of strata between them. called from the prevalence in it of the ammonite Psiloceras Three formations of fossiliserous strata, A, C, and H, may occur planorbis. Two or more such zones related to each other by the conformably above each other. By a comparison of the fossil possession of a number of the same characteristic species or species are peculiar to its lower, others to its higher portions, yet the genera have been designated beds or an assise. Two or more

majority extend throughout the formation. li now it is found that sets of beds or assises similarly related form a group or stage; a of the total number of species in the upper portion of A only one-third number of groups or stages make a series, formation or section, passes up into C, it may be inferred with some plausibility that the and a succession of formations may be united into a system.

time represented by the break between A and C was really longer

than that required for the accumulation of the whole of the formation 3. Some living species of plants and animals can be traced A. It might even be possible to discover elsewhere a thick inter. downwards through the more recent geological formations; mediate formation B filing up the gap between A and C. In like but the number which can be so followed grows smaller as the manner were it to be discovered that, while the whole of the formation examination is pursued into more ancient deposits. With their Cis characterized by a common suite of fossils, not one of the species disappearance other species or genera present themselves which hardly be resisted that the gap between the two formations marks are no longer living. These in turn may be traced backward into the passage of a far longer interval than

was needed for the deposition earlier formations, till they too cease and their places are taken by of the whole of C. And thus we reach the remarkable conclusion yet older forms. It is thus shown that the stratified rocks contain that, thick though the stratified formations of a country may be, the records of a gradual progression of organic forms. A species as do the gaps in their succession,-in other words, that non-deposi

in some cases they may not represent so long a total period of time which has once died out does not seem ever to have reappeared. tion was more frequent and prolonged than deposition, or thai the

4. When the order of succession of organic remains among the intervals of time which have been recorded by strata have not been stratified rocks has been determined, they become an invaluable so long as those which have not been so recorded. guide in the investigation of the relative age of rocks and the In all speculations of this nature, however, it is necessary structure of the land. Each zone and formation, being character to reason from as wide a basis of observation as possible, seeing ized by its own species or genera, may be recognized by their that so much of the evidence is negative. Especially needful means, and the true succession of strata may thus be confidently is it to bear in mind that the cessation of one or more species established even in a country wherein the rocks have been at a certain line among the rocks of a particular district may shattered by dislocation, folded, inverted or metamorphosed: mean nothing more than that, onward from the time marked

5. Though local differences exist in regard to the precise zone by that line, these species, owing to some change in the conditions in which a given species of organism may make its first appearance, of life, were compelled to migrate or became locally extinct or, the general order of succession of the organic forms found in the from some alteration in the conditions of fossilization, were no rocks is never inverted. The record is nowhere complete in any longer imbedded and preserved as fossils. They may have region, but the portions represented, even though extremely continued to flourish abundantly in neighbouring districts for imperfect, always follow each other in their proper chronological a long period afterward. Many examples of this obvious order, unless where disturbance of the crust has intervened to truth might be cited. Thus in a great succession of mingled destroy the original sequence.

marine, brackish-water and terrestrial strata, like that of the 6. The relative chronological value of the divisions of the I Carboniserous Limestone series of Scotland, corals, crinoids

area.

zones.

and brachiopods abound in the limestones and accompanying | remind us of the debt we owe to William Smith, by whoin so shales, but disappear as the sandstones, ironstones, clays, coals many of them were first used-Lias, Gault, Crag, Cornbrash. and bituminous shales supervene. An observer meeting for the Others of later date recognize an order of superposition ag first time with an instance of this disappearance, and remember- already established among formations-Old Red Sandstone, ing what he had read about breaks in succession, might be New Red Sandstone. By common consent it is admitted that tempted to speculate about the extinction of these organisms, names taken from the region where a formation or group of rocks and their replacement by other and later forms of life, such as is typically developed are best adapted for general use. the ferns, lycopods, estuarine or fresh-water shells, ganoid Cambrian, Silurian, Devonian, Permian, Jurassic are of this Gishes and other fossils so abundant in the overlying strata. class, and have been adopted all over the globe. But further research would show him that high above the plant- But whatever be the name chosen to designate a particular bearing sandstones and coals other limestones and shales might group of strata, it soon comes to be used as a chronological or be observed, once more charged with the same marine fossils homotaxial term, apart altogether from the stratigraphical as before, and still farther overlying groups of sandstones, coals character of the strata to which it is applied. Thus we speak and carbonaceous beds followed by yet higher marine limestones. of the Chalk or Cretaceous system, and embrace under that He would thus learn that the same organisms, after being term formations which may contain no chalk; and we may locally exterminated, returned again and again to the same describe as Silurian a series of strata utterly unlike in lithological

After such a lesson he would probably pause before 100 characters to the formations in the typical Silurian country. confidently asserting that the highest bed in which we can In using these terms we unconsciously allow the idea of relative detect certain fossils marks their final appearance in the history date to arise prominently before us. Hence such a word as of life. Some breaks in the succession may thus be extremely

“ chalk" or

cretaceous ” does not suggest so much to us the local, one set of organisms having been driven to a different part group of strata so called as the interval of geological history of the same region, while another set occupied their place until which these strata represent. We speak of the Cretaceous, the first was enabled to return.

Jurassic, and Cambrian periods, and of the Cretaccous fauna, 8. The geological record is at the best but an imperfect the Jurassic flora, the Cambrian trilobites, as if these adjectives chronicle of the geological history of the earth. It abounds denoted simply epochs of gcological time. in gaps, some of which have been caused by the destruction of The stratified formations of the earth's crust, or geological strata oring to metamorphism, denudation or otherwise, others record, are classificd into five main divisions, which in their by original non-deposition, as above explained. Nevertheless order of antiquity are as follows: (1) Archean or Pre-Cambrian, from this record alone can the progress of the earth be traced. called also sometimes Azoic (lifeless) or Eozoic (dawn of life); It contains the registers of the appearance and disappearance (2) Palaeozoic (ancient life) or Primary; (3) Mesozoic (middle of tribes of plants and animals which have from time to time life) or Secondary; (4) Cainozoic (recent lise) or Tertiary; flourished on the earth. Only a small proportion of the total (5) Quaternary or Post-Tertiary. These divisions are further number of species which have lived in past time have been thus ranged into systems, formations, groups or stages, assises and chronicled, yet by collecting the broken fragments of the record Accounts of the various subdivisions named are given an outline at least of the history of life upon the earth can be in separate articles under their own headings. In order, however, deciphered.

that the sequence of the formations and their parallelism, in It cannot be too frequently stated, nor too prominently kept | Europe and North America may be presented together a stratiin view, that, although gaps occur in the succession of organic graphical table is given on next page. remains as recorded in the rocks, they do not warrant the conclusion that any such blank intervals ever interrupted the progress

PART VIII.-PHYSIOGRAPHICAL GEOLOGY of plant and animal lise upon the globe. There is every reason to believe that the march of life has been unbroken, onward and This department of geological inquiry investigates the origin upward. Geological history, therefore, if its records in the and history of the present topographical features of the land, stratified formations were perfect, ought to show a blending As these features must obviously be related to those of earlier and gradation of epoch with cpoch. But the progress has been time which are recorded in the rocks of the earth's crust, they constantly interrupted, now by upheaval, now by volcanic cannot be satisfactorily studied until at least the main outlines outbursts, now by depression. These interruptions serve as of the history of these rocks have been traced. Hence physionatural divisions in the chronicle, and enable the geologist to graphical research comes appropriately after the other branches arrange his history into periods. As the order of succession of the science have been considered. among stratified rocks was first made out in Europe, and as many From the stratigraphy of the terrestrial crust we learn that of the gaps in that succession were found to be widespread over by far the largest part of the area of dry land is built up of marine the European area, the divisions which experience established formations; and therefore that the present land is not an for that portion of the globe came to be regarded as typical, aboriginal portion of the earth's surface, but has been overspread and the names adopted for them were applied to the rocks of by the sea in which its rocks were mainly accumulated. We other and far distant regions. This application has brought out further discover that this submergence of the land did not the fact that some of the most marked breaks in the European happen once only, but again and again in past ages and in all series do not exist elsewhere, and, on the other hand, that some parts of the world. Yet although the terrestrial areas varied portions of that series are much more complete than the corre- much from age to age in their extent and in their distribution, sponding sections in other regions. Hence, while the general being at one time more continental, at another more insular, similarity of succession may remain, different subdivisions and there is reason to believe that these successive diminutions and nomenclature are required as we pass from continent to continent. expansions have on the whole been effected within, or not far

The nomenclature adopted for the subdivisions of the geological outside, the limits of the existing continents. There is no record bears witness to the rapid growth of geology. It is a evidence that any portion of the present land ever lay under the patch-work in which no system nor language has been adhered deeper parts of the ocean. The abysmal deposits of the ocean. to, but where the influences by which the progress of the science floor have no true representatives among the sedimentary has been moulded may be distinctly traced. Some of the earliest formations anywhere visible on the land. Nor, on the other names are lithological, and remind us of the fact that mineralogy hand, can it be shown that any part. of the existing ocean and petrography preceded geology in the order of birth-Chalk, abysses ever rose above sea-level into dry land. Hence geologists Oolite, Greensand, Millstone Grit. Others are topographical, have drawn the inference that the ocean basins have probably and often recall the labours of the early gcologists of England - been always where they now are; and that although the conLondon Clay, Oxford Clay, Purbeck, Portland, Kimmeridge beds. tinental areas have often been narrowed by submergence and by Others are taken from local English provincial names, and I denudation, there has probably seldom or never been a complete

Jurassic -- continued.

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glacial or

Human Quaternary or Post-Tertiary. Pleistorene or Clacial.

Mesozoic or Secondary-continued

Triassic.

Pliocene.

Permian.

Miocene.

are

Cainozoic or Tertiary.

Oligocene.

Carboniferous.

The Geological Record or Order of Succession of the Stratified

Europe

North America.
Formations of the Earth's Crust.

Corallian-Coral Rag, Coralline Colite; which have yielded so large 3
Europe.
North America.

Sequanian stages of the Continent, variety of deinosaurs and other

comprising the sub-stages of Astartian vertebrates, and especially the Historic, up to the present time Similar to the European de

and Rauracian.

remains of a number of genera Prehistoric, comprising deposits of the velopment, but with scantier

Oxfordian-Oxford Clay: Argovian and of small mammals.
Iron, Bronze, and later Stone Ages traces of the presence of man.

Neuvizyan stiges
Neolithic-alluvium, peat, lake-dwell-

Callovian-Kellaways Rock, Divesian
ings, loess, &c.

sub-stage of N France
Palacolithic-river-gravels,
care-de-

Bathonian-series of English strata from
posits, &c.

Cornbrash down to Fuller's Earth.

Bajocian-Interior Oolite of England.
Older Loess and valley-gravels; cave As in Europe, it is bardly pos-

Liassic-divisible into (1) Ypper Lias
deposits.

sible to assign
debate

or Toarcian, (r) Middle Lias, Marl. Strand-lines or raised beaches; youngest chronological place to each

stone or Charmouthian, (3) Lower
moraines
of the various deposits of tnis

Lias of Sipemurian and Hettangian,
Upper Boulder-clays; eskers; marine period, terrestrial and marine
sinds and class.
They generally resemble the

In Germany and western Europe this la New York, Connecticut, Ner
Interglacial deposits.
European series. The charac

division represents the deposits of Brunswick, and Nova Scotia Lower boulder clay or Tal, with striated teristic marine, fluviatile and

inland seas or lagoons, and is divisible

series of red sandstone rock-surfaces bclow. lacustrine terraces, which

into the following stages in descending (Newark series) contains tod. Overlie the older drifts, hare

order (1) khaetic, (a) Keuper, (3) plants and labyrinthodonts been classed as the Champ

Muschelkalk, (4) Buntcr In the like the lagoon type of central lain Group

castern Alps and the Mediterranean and western Europe. On the

basin the contemporaneous sediment- Pacific slope, however, marine Newer :-English Forest-Bed Group: On the Atlantic border repre

ary formations are those of open dear equivalents occur. represcatRed and Norwich Crag: Amstclian sented by the marine Flondian

sea, in which a thickness of many ing the pelagic type of southand Scaldesian groups of Belgium series, in the interior by a

thousand feel of strata was accumu- castern Europe. and Holland; Sicilian and Astian of subacrial and lacustrine series;

lated. France and Italy.

and on the Pacific border by Older :-English Coralline Crag; Dies the thick marine series of San

Thuringian-Zechstein, Magnesian To this division of the geologitian of Belgium; Plaisancian of south- Francisco.

Limestone, named from its develop cal record the Upper Bartea ern France and Italy

ment in Thuringia; well represented Measures of the coul-belds of

also in Saxony, Bavaria and Bobemia. Pennsylvania, Prince Edward Wanting in Britain, well developed in Represented in the

Eastern

Saxonian-Rothliegendes Group; Red Island, Nova Scotia 20d France, S. E Europe and Italy; divis. States by a marine series

Sandstones, &c.

New Brunswick .have been ible into the following groups in (Yorktown or Chesapeake,

Autunun--where the strata present the assigned. descending order. (1) Pontian, (2) Chipola and Chattahoochee

lagoon facies, well displayed at Autun Farther south in Kansas, Texas, Sarmatian; (3) Tortonian; (4) Hel.

in France; where the marine type is and Nebraska the representavetian; (s) Langhian (Burdigalian). the lacustrine Loup Fork

fredominant.com in Russia, the group

lives of the division bart as (Nebraska), Deep River, and

has Artinskian.

abundant marine launa. Joho Day groups.

Stephanian or Uralian -- represented in Upper productive Coal. In Britain the "fluvio-marine series of On the Atlantic border no

Russia by marine formations, and in measures the Isle of Wight; also the volcanic equivalents bave been satis

central and western Europe by bumer Lower Barre measures. plateaux of Antrim and Inner Hebrides factorily recognised, but on

ous small basins containing a peculiar Lower productive' Coal. and those of the Facroc Isles and Ice- the Pacitic side ihere

flora and in some places a great variety measures. land. lo continental Europe the marine deposits in N. W

ol insects.

Poltsville conglomerate. following subdivisions bave becn Oregon, which may represent

Westphalian or Muscovian-Coal- Mauch Chunk shales; limeestablished in descending order (1) this division. In the interior

measures, Millstone Grit

stones of Chester, St Louis, &c. Aquitanian, (2) Stampian (Rupelian), the equivalent is believed to

Culm or Dinantian-Carboniferous Lime Pocono serics; Kinderbook
(3) Tongraín (Sannoisian).
be the fresh-water White River

stone and Calciferous Sandstone series. limestone
series, including (1) Proto-
Ceras beds, (2) Orcodon beds,

Devonian typc.

Old Red Sandand (3) Tilanotherium beds.

stone type.

Catskill red sandstone; Old Barton sands and clays; Ludian series Woodstock and Aquia Creek

Ked Sandstone type: the of France. BTOUPS of Potornac River;

Yelow and red

strata below Bracklesham Beds; Lutetian (Calcaire Vicksburg.

sbor Jackson, Clai

Famennian. sandstone with Devonian type.
grossier and Caillasses) of Pari
borne, Buhrstone, and Lig-

Holotyckius, Chemung Group.
London clay, Woolwich and Reading nitic groups of Mississippi

Botkriolepis,&c. Genesce
Beds; Thanct sands; Ypresian or in the interior a thick series of
Londinian of N. France and Belgium; fresh-water formations, cum-

Caithness Flag-
Spernacian and Thanetian groups. prising, in descending order,

stones with
ihe Vinta, Bridger, Wind

Givetian. Osteolepus. Dip

{ Hamilton Group River, Wasatch, Torrejon,

less, Home

Marcellus and Puerco groups.

sleus, &c.
On the Pacific side the marine

Red and purple
Tejon series of Oregon and

sandstones and

Corniferous Lime (Upper California.

Heder-
Coblentzian.

stone.
conglomerates
with Cephal

Onondaga Lime berg
Danian-wanting in Britain; upper most on the Atlantic border both

stone.

aspis, Pler.
limestone of Denmark.
marine strata and others con-

dspis, &c. Oriskany Sandstone.
Senonian - Upper Chalk with Flints of

taining a terrestrial flora re-
England; "Aturian and Emscherian
present the Cretaceous series

Lower Helderberg Group. stages on the European continent. of formations.

Water-Lime.

Ludlow Group
Turonian-Middle Chalk with few in the interior there is also a

Upper Wenlock

Niagara Shale and Limestone Aints, and comprising the Angoumian commingling of marine with and Ligerian stages.

Clinton Group
lacustrine deposits,

Llandovery"
At the

Medina
Cenomanian-Lower Chalk and Chalk top lies the Laramie or Lig.

Cincinnati Group
Marl.
nitic series with an abundant

Ulica

Caradoc or Bala Group
Albian - Upper Greensand and Gault.

Lower
terrestrial flora, passing down

Llandeilo

Trenton
Aptian - Lower Greensand: Marls and into the lacustrine and

Chazy
limestones of Provence, &c.
brackish-water Montana

Calcilerous
Urgonian (Barremian)-Atherfield clay: series or older date, the
massive Hippurite limestones ol Colorado series contains

Upper or Olonus serics-Tremadoc slates Upper or Potsdam scries with
southern France.
abundant marine fauna, yet

and Lingida 17.gs.

Olentis

and Diceloce phalus Neocomian-- Weald day and Hastings includes also some coal-scams.

Middle of Paradoxides series- Mene

fauna. sand; Hauterivian and Valanginian The Niobrara maris aod limpe

vian Group

Middle or Acadian series with sub-stages of Switzerland and France. stones are likewise of marine

Lower Olanellus series-Lanheris

Paradoxides fauna. origin, but the lower members

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Upper { Frasnian

Eocene.

Palacozoic or Primary.

Devonian and Old Red Sandstone.

Middle { Birelian

Lower {Gedindian

Group

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and Harlech Group and Olacius Lower or Georgian series with of the series (Benton and

Olandlus fauna.

zone. Dakota) show another great

In Scotland, underneath the Cambrian In representation of fresh-water

Canada and the Lake sedimentation with lignites

Olenellus group. lies unconformably

Superior region of the United and coals.

mass of red sundstone and con- Sualcs a vast succession of In California a vast succession

glomerate (Torridonian) 8000 or 10,000

rocks of Pre-Cambrian age of marine deposits (Shasta

it. thick, which rests with a strong

has been grouped into the Chico) represents the Cre

uncooformability on a series of coarse

following subdivisions in de taceous system, and in western

gneisses and schists (Lewisian). A scending order. (1) Kewcena. British N. America coul-scams

Thick series of slates and phyllites lies

wan, Tying unconformably on also occur.

below the oldest Palaeozoic rocks in (2) Animikic, separated by a central Europe, with course gocisses

strong unconformability from Purbeckian-Purbeck beds; Münder Representatives of the Middle

below.

(2) Upper Huronian, (4) Lower Mergel; largely present in West- and lower Jurassic forma

Huronian with an unconformphalia. tion: have been found in

able base, (3) Goutchiching. Portlandian--Portland group of Eng- California and Oregon, and

(6) Laurentian. In the eastern land, represented in S. France by the farther north among the Arctic

part of Canada, Newfoundthick Tithonian limestones. islands

land, &c., and also in Mon Kimmeridgian-Kimmeridge Clay of Strata containing Lower Juras

tana, sedimentary formations England: Virgulian and Plcrocrrian sic marine fossils appear in

of great thickness below the group of N. France represented by Wyoming and Dakoa; and

lowest Cumbrian cone have Thick limestones in the Mediterranean above them come the Atlanto

beca found to contain some basin, sourus and Ba Nanodon beds,

obscure organisms.

&

Arcbean, Pre-Cambrian Eozoic.

Jurassic.

disappearance of land. The fact that the sedimentary forma- | relief from the strain of compression by fresh crumpling, fracture tions of each successive geological period consist to so large an and uprise. . The chief guide in tracing these successive stages extent of mechanically formed terrigenous detritus, affords of growth is supplied by unconformability. If, for example, a good evidence of the coexistence of tracts of land as well as of mountain-range consists of upraised Silurian rocks, upon the extensive denudation.

upturned and denuded edges of which the Carboniferous LimeFrom these general considerations we proceed to inquire how stone lies transgressively, it is clear that its original upheaval the existing topographical features of the land arose. Obviously must have taken

place in the period of geological time represented the co-operation of the two great geological agencies of hypogene by the interval between the Silurian and the Carboniferous and epigene energy, which have been at work from the beginning Limestone formations. "If, as the range is followed along its of our globe's decipherable history, must have been the cause course, the Carboniferous Limestone is found to be also highly to which these features are to be assigned; and the task of the inclined and covered unconformably by the Upper Coal-measures, geologist is to ascertain, if possible, the part that has been taken a second uplift of that portion of the ground can be proved to by each. There is a natural tendency to see in a stupendous have taken place between the time of the Limestone and that of piece of scenery, such as a deep ravine, a range of hills, a line of the Upper Coal-measures. By this simple and obvious kind of precipice or a chain of mountains, evidence only of subterranean evidence the relative ages of different mountain-chains may convulsion; and before the subject was taken up as a matter be compared. In most great chains, however, the rocks have of strict scientific induction, an appeal to former cataclysms been so intensely crumpled, and even inverted, that much was considered a sufficient solution of the problems presented labour may be required before their true relations can be deterby such features of landscape. The rise of the modern mined. Huttonian school, however, led to a more careful examination The Alps furnish an instructive example of the long series of of these problems. The important share taken by erosion in the revolutions through which a great mountain-system may have determination of the present features of landscape was then passed before reaching its present development. The first recognized, while a fuller appreciation of the relative parts beginnings of the chain may have been upraised before the played by the hypogene and epigene causes has gradually been oldest Palaeozoic formations were laid down. There are at reached.

least traces of land and shore-lines in the Carboniserous period. 1. The study of the progress of denudation at the present Subsequent submergences and uplifts appear to have occurred time has led to the conclusion that even if the rate of waste during the Mesozoic periods. There is evidence that thereafter were not more rapid than it is to-day, it would yet suffice in a the whole region sank deep under the sea, in which the older comparatively brief geological period to reduce the dry land to Tertiary sediments were accumulated, and which seems to below the sea-level. But not only would the area of the land be have spread right across the heart of the Old World. But after diminished by denudation, it could hardly fail to be more or the deposition of the Eocene formations came the gigantic less involved in those widespread movements of subsidence, disruptions whereby all the rocks of the Alpine region were during which the thick sedimentary formations of the crust folded over each other, crushed, corrugated, fractured and appear to have been accumulated. It is thus manifest that there displaced, some of their older portions, including the fundamental must have been from time to time during the history of our gneisses and schists, being squeezed up, torn ofl, and pushed globe upward movements of the crust, whereby the balance horizontally for many miles over the younger rocks. But this between land and sea was redressed. Proofs of such movements upheaval, though the most momentous, was not the last which have been abundantly preserved among the stratified formations. the chain has undergone, for at a later çpoch in Tertiary time We there learn that the uplifts have usually followed each other renewed disturbance gave rise to a further series of ruptures at long intervals between which subsidence prevailed, and thus and plications. The chain thus successively upheaved has that there has been a prolonged oscillation of the crust over the been continuously exposed to denudation and has consequently great continental areas of the earth's surface.

lost much of its original height. That it has been left in a state An examination of that surface leads to the recognition of two of instability is indicated by the frequent earthquakes of the great types of upheaval. In the one, the sea-floor, with all its Alpine region, which doubtless arise from the sudden snapping thick accumulations of sediment, has been carried upwards, of rocks under intense strain. sometimes for several thousand feet, so equably that the strata A distinct type of mountain duc to direct hypogene action is retain their original flatness with hardly any sensible disturbance to be seen in a volcano. It has been already pointed out (Part IV. for hundreds of square miles. In the other type the solid crust sect. I) that at the vents which maintain a communication has been plicated, corrugated and dislocated, especially along between the molten magma of the earth's interior and the particular lines, and has attained its most stupendous disruption surface, eruptions take place whereby quantities of lava and in losty chains of mountains. Between these two phases of uplift fragmentary materials are heaped round cach orifice of many intermediate stages have been developed, according to discharge. A typical volcanic mountain takes the form of a the direction and intensity of the subterranean force and the perfect cone, but as it grows in size and its main vent is choked, varying nature and disposition of the rocks of the crust.

while the sides of the cone are unable to withstand the force of (a) Where the uplift has extended over wide spaces, without the explosions or the pressure of the ascending column of lava, appreciable deformation of the crust, the flat strata have given eruptions take place laterally, and numerous parasitic cones rise to low plains, or if the amount of uprise has been great arise on the flanks of the parent mountain. Where lava flows enough, to high plains, plateaux or tablclands. The plains of out from long fissures, it may pile up vast shcets of rock, and Russia, for example, lie for the most part on such tracts of bury the surrounding country under several thousand feet of equably uplifted strata. The great plains of the western interior solid stone, covering many hundreds of square miles. In this of the United States form a great platcau or tableland, 5000 or way volcanic tablelands have been formed which, attacked by 6000 ft. above the sea, and many thousands of square miles in the denuding forces, are gradually trenched by valleys and extent, on which the Rocky Mountains have been ridged up. ravines, until the original level surface of the lava-field may be

(b) It is in a great mountain-chain that the complicated almost or wholly lost. As striking examples of this physiostructures developed during disturbances of the earth's crust graphical type reference may be made to the plateau of Abyssinia, can best be studied (see Parts IV. and V of this article), and the Ghats of India, the plateaux of Antrim, the Inner Hebrides where the influence of these structures on the topography of the and Iceland, and the great lava-plains of the western territories surface is most effectively displayed. Such a chain may be the of the United States. result of one colossal disturbance; but those of high geological 2. But while the subterranean movements have upraised antiquity usually furnish proofs of successive uplifts with more portions of the surface of the lithosphere above the level of the or less intervening denudation. Formed along lines of continental ocean, and have thus been instrumental in producing the existing displacement in the crust, they have again and again given I tracts of land, the detailed topographical features of a landscape are not solely, nor in general even chiefly, attributable to these denudation have been ceaselessly engaged since it emerged movements. From the time that any portion of the sea-floor from the sea. They have excavated valleys, sometimes along appears above sea-level, it undergoes erosion by the various depressions provided for them by the subterranean disturbances, epigene agents. Each climate and geological region has its own sometimes down the slopes of the disrupted blocks of ground. development of these agents, which include air, aridity, rapid and so powerful has been this erosion that valleys cut out along frequent alternations of wetness and dryness or of heat and lines of anticline, which were natural ridges, have sometimes cold, rain, springs, frosts, rivers, glaciers, the sea, plant and become more important than those in lines of syncline, which animal life. In a dry climate subject to great extremes of were structurally depressions. The same subaerial forces have temperature the character and rate of decay will differ from eroded lake-basins, dug out corries or cirques, notched the those of a moist or an arctic climate. But it must be remembered ridges, splintered the crests and furrowed the slopes, leav. that, however much they may vary in activity and in the results ing no part of the original surface of the uplifted chain which they effect, the epigene forces work without intermission, unmodified. while the hypogene forces bring about the upheaval of land only It has often been noted with surprise that features of after long intervals. Hence, trifling as the results during a underground structure which, it might have been confidently human lise may appear, if we realize the multiplying influence anticipated, should have exercised a marked influence on the of time we are led to perceive that the apparently seeble super- topography of the surface have not been able to resist the ficial agents can, in the course of ages, achieve stupendous levelling action of the denuding agents, and do not now affect transformations in the aspect of the land. If this efficacy may the surface at all. This result is conspicuously seen in coal-fields be deduced from what can be seen to be in progress now, it where the strata are abundantly traversed by faults. These may not less convincingly be shown, from the nature of the dislocations, having sometimes a displacement of several hundred sedimentary rocks of the earth's crust, to have been in progress feet, might have been expected to break up the surface into from the early beginnings of gcological history. Side by side a network of cliffs and plains; yet in general they do not modify with the various upheavals and subsidences, there has been a the level character of the ground above. One of the most continuous removal of materials írom the land, and an equally remarkable faults in Europe is the great thrust which bounds persistent deposit of these materials under water, with the the southern edge of the Belgian coal-field and brings the consequent growth of new rocks. Denudation has been aplly Devonian rocks above the Coal-measures. It can be traced compared to a process of sculpturing wherein, while each of the

across Belgium into the Boulonnais, and may not improbably implements employed by nature, like a special kind of graving run beneath the Secondary and Tertiary rocks of the south of tool, produces its own characteristic impress on the land, they England. It is crossed by the valleys of the Meuse and other all combine harmoniously towards the achievement of their northerly-flowing streams. Yet so indistinctly is it marked one common task. Hence the present contours of the land in the Meuse valley that no one would suspect its existence from depend partly on the original configuration of the ground, and any peculiarity in the general form of the ground, and even an the influence it may have had in guiding the operations of the experienced geologist, until he had learned the structure of the erosive agents, partly on the vigour with which these agents district, would scarcely detect any fault at all. perform their work, and partly on the varying structure and Where faults have influenced the superficial topography, powers of resistance possessed by the rocks on which the erosion it is usually by giving rise to a hollow along which the subaerial is carried on.

agents and especially running water can act effectively. Such Where a new tract of land has been raised out of the sca a hollow may be eventually widened and deepened into a valley. by such an energetic movement as broke up the crust and On bare crags and crests, lines of fault are apt to be marked by produced the complicated structure and tumultuous external notches or clets, and they thus help to produce the pinnacles forms of a great mountain chain, the influence of the hypogene and serrated outlines of these exposed uplands. forces on the topography attains its highest development. It was cogently enforced by Hutton and Playfair, and inde But even the youngest existing chain has suffered so greatly pendently by Lamarck, that no co-operation of underground from denudation that the aspect which it presented at the time agency is needed to produce such topography as may be seen of its uplift can only be dimly perceived. No more striking in a great part of the world, but that is a traci of sca-floor were illustration of this feature can be found than that supplied by upraised into a wide plain, the fall of rain and the circulation the Alps, nor one where the geotectonic structures have been of water over its surface would in the end carve out such a system so fully studied in detail. On the outer flanks of these mountains of hills and valleys as may be seen on the dry land now. No the longitudinal ridges and valleys of the Jura correspond with such plain would be a dead-level. It would have inequalities lines of anticline and syncline. Yet though the dominant on its surface which would serve as channels to guide the drainage topographical elements of the region have obviously been from the first showers of rain. And these channels would be produced by the 'plication of the stratified formations, each slowly widened and deepened until they would become ravines ridge has suffered so large an amount of crosion that the younger and valleys, while the ground between them would be left projectrocks have been removed from its crest where the older members ing as ridges and hills. Nor would the erosion of such a system of the series are now exposed to view, while on every slope of water-courses require a long series of geological periods for proofs may be seen of extensive denudation. If from these its accomplishment. From measurements and estimates of the long wave-like undulations of the ground, where the relations amount of crosion now taking place in the basin of the Mississippi between the disposition of the rocks below and the forms of river it has been computed that valleys 8oo ft. deep might be the surface are so clearly traceable, the observer proceeds carved out in less than a million years. In the vasi tablelands inwards to the main chain, he finds that the plications and of Colorado and other western regions of the United States an displacements of the various formations assume an increasingly impressive picture is presented of the results of mere subaerial complicated character; and that although proofs of great erosion on undisturbed and nearly level strata. Systems of denudation continue lo abound, it becomes increasingly difficult stream-courses and valleys, river gorges unexampled elsewhere to form any satisfactory conjecture as to the shape of the ground in the world for depth and length, vast winding lines of escarpwhen the upheaval ended or any reliable estimate of the amount ment, like ranges of sea-cliffs, terraced slopes rising from plateau of material which has since then been removed. Along the to plateau, huge buttresses and solitary stacks standing like central heights the mountains lift themselves towards the sky islands out of the plains, great mountain-masses lowering into like the storm-swept crests of vast earth-billows. The whole picturesque peaks and pinnacles cleft by innumerable gullies, aspect of the ground suggests intense commotion, and the yet everywhere marked by the parallel bars of the horizontal impression thus given is often much intensified by the twisted strata out of which they have been carved-these are the orderly and crumpled strata, visible from a long distance, on the crags symmetrical characteristics of a country where the scenery is

On this broken-up surface the various agents of due entirely to the action of subaerial agents on the one hand and

and crests.

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