PART VIII

.--PHYSIOGRAPHICAL GEOLOGY

This department of geological inquiry investigates the origin and history of the present topographical features of the land. As these features must obviously be related to those of earlier time which are recorded in the rocks of the earth's crust, they cannot be satisfactorily studied until at least the main outlines of the history of these rocks have been traced. Hence physiographical research comes appropriately after the other branches of the science have been considered.

From the stratigraphy of the terrestrial crust we learn that by far the largest part of the area of dry land is built up of marine formations; and therefore that the present land is not an aboriginal portion of the earth's surface, but has been overspread by the sea in which its rocks were mainly accumulated. We further discover that this submergence of the land did not happen once only, but again and again in past ages and in all parts of the world. Yet although the terrestrial areas varied much from age to age in their extent and in their distribution, being at one time more continental, at another more insular, there is reason to believe that these successive diminutions and expansions have on the whole been effected within, or not far outside, the limits of the existing continents. There is no evidence that any portion of the present land ever lay under the deeper parts of the ocean. The abysmal deposits of the ocean-floor have no true representatives among the sedimentary formations anywhere visible on the land. Nor, on the other hand, can it be shown that any part of the existing ocean abysses ever rose above sea-level into dry land. Hence geologists have drawn the inference that the ocean basins have probably been always where they now are; and that although the continental areas have often been narrowed by submergence and by denudation, there has probably seldom or never been a complete disappearance of land. The fact that the sedimentary formations of each successive geological period consist to so large an extent of mechanically formed terrigenous detritus, affords good evidence of the coexistence of tracts of land as well as of extensive denudation.

_The Geological Record or Order of Succession of the Stratified Formations of the Earth's Crust._

+---+---+-------------------------------------------+----------------------------------+ | | | Europe. | North America. | +---+---+-------------------------------------------+----------------------------------+ | Q | \ Historic, up to the present time. | Similar to the European | | u | \ Prehistoric, comprising deposits of | development, but with scantier | | a | \ the Iron, Bronze, and later | traces of the presence of man. | | t | \ Stone Ages. | | | e | \ Neolithic--alluvium, peat, lake- | | | r | Recent, \ dwellings, loess, &c. | | | n | Post- | Palaeolithic--river-gravels, cave- | | | a | glacial | deposits, &c. | | | r | or | | | | y | Human. | | | | | | | | | o | | | | | r +---------+-------------------------------------+----------------------------------+ | | Pleist- | Older Loess and valley-gravels; | As in Europe, it is hardly | | P | ocene | cave-deposits. | possible to assign a definite | | o | or | Strand-lines or raised beaches; | chronological place to each of | | s | Glacial.| youngest moraines. | the various deposits of this | | t | | Upper Boulder-clays; eskers; marine | period, terrestrial and marine.| | | | | sands and clays. | They generally resemble the | | T | | Interglacial deposits. | European series. The | | e | | Lower boulder-clay or Till, with | characteristic marine, | | r | | striated rock-surfaces below. | fluviatile and lacustrine | | t | | | terraces, which overlie the | | i | / | older drifts, have been | | a | / | classed as the Champlain Group.| | r | / | | | y | / | | | . | / | | +---+---+-------------------------------------------+----------------------------------+ | | P | Newer:--English Forest-Bed Group; Red and | On the Atlantic border | | | l | Norwich Crag; Amstelian and Scaldesian | represented by the marine | | | i | groups of Belgium and Holland; Sicilian | Floridian series; in the | | | o | and Astian of France and Italy. | interior by a subaerial and | | | c | Older:--English Coralline Crag; Diestian | lacustrine series; and on the | | | e | of Belgium; Plaisancian of southern | Pacific border by the thick | | | n | France and Italy. | marine series of San Francisco.| | | e | | | | | . | | | | +---+-------------------------------------------+----------------------------------+ | | M | Wanting in Britain; well developed in | Represented in the Eastern States| | | i | France, S. E. Europe and Italy; | by a marine series (Yorktown or| | C | o | divisible into the following groups in | Chesapeake, Chipola and | | a | c | descending order: (1) Pontian; (2) | Chattahoochee groups), and in | | i | e | Sarmatian; (3) Tortonian; (4) Helvetian;| the interior by the lacustrine | | n | n | (5) Langhian (Burdigalian). | Loup Fork (Nebraska), Deep | | o | e | | River, and John Day groups. | | z | . | | | | o +---+-------------------------------------------+----------------------------------+ | i | | In Britain the "fluvio-marine series" of | On the Atlantic border no | | c | O | the Isle of Wight; also the volcanic | equivalents have been | | | l | plateaux of Antrim and Inner Hebrides | satisfactorily recognised, but | | o | i | and those of the Faeroe Isles and | on the Pacific side there are | | r | g | Iceland. In continental Europe the | marine deposits in N. W. | | | o | following subdivisions have been | Oregon, which may represent | | T | c | established in descending order: (1) | this division. In the interior | | e | e | Aquitanian, (2) Stampian (Rupelian), | the equivalent is believed to | | r | n | (3) Tongrain (Sannoisian). | be the fresh-water White River | | t | e | | series, including (1) | | i | . | | _Protoceras_ beds, (2) | | a | | | _Oreodon_ beds, and (3) | | r | | | _Titanothervum_ beds. | | y +---+-------------------------------------------+----------------------------------+ | . | | Barton sands and clays; Ludian series of | Woodstock and Aquia Creek groups | | | | France. | of Potomac River; Vicksburg, | | | | Bracklesham Beds; Lutetian (Calcaire | Jackson, Claiborne, Buhrstone, | | | E | grossier and Caillasses) of Paris | and Lignitic groups of | | | o | basin. | Mississippi. | | | c | London clay, Woolwich and Reading Beds; | In the interior a thick series of| | | e | Thanet sands; Ypresian or Londinian of | fresh-water formations, | | | n | N. France and Belgium; Sparnacian and | comprising, in descending | | | e | Thanetian groups. | order, the Uinta, Bridger, | | | . | | Wind River, Wasatch, Torrejon, | | | | | and Puerco groups. | | | | | On the Pacific side the marine | | | | | Tejon series of Oregon and | | | | | California. | |---+---+-------------------------------------------+----------------------------------| | | | Upper | On the Atlantic border both | | | | ===== | marine strata and others | | | | Danian--wanting in Britain; uppermost | containing a terrestrial flora | | | | limestone of Denmark. | represent the Cretaceous series| | | | Senonian--Upper Chalk with Flints of | of formations. | | | | England; Aturian and Emscherian stages | In the interior there is also a | | | | on the European continent. | commingling of marine with | | | | Turonian--Middle Chalk with few flints, | lacustrine deposits. At the top| | | | and comprising the Angoumian and stages.| lies the Laramie or Lignitic | | | C | Cenomanian--Lower Chalk and Chalk Marl. | series with an abundant | | | r | | terrestrial flora, passing down| | | e | Lower | into the lacustrine and | | | t | ===== | brackish-water Montana series. | | | a | Albian--Upper Greensand and Gault. | Of older date, the Colorado | | | c | Aptian--Lower Greensand; Marls and | series contains an abundant | | | e | limestones of Provence, &c. | marine fauna, yet includes also| | | o | Urgonian (Barremian)--Atherfield clay; | some Niobrara marls and | | | u | massive Hippurite limestones of | limestones are likewise of | | | s | southern France. | marine origin, but the lower | | | . | Neocomian--Weald clay and Hastings sand; | members of the series (Benton | | | | Hauterivian and Valanginian sub-stages | and Dakota) show another great | | | | of Switzerland and France. | representation of fresh-water | | M | | | sedimentation with lignites and| | e | | | coals. | | s | | | In California a vast succession | | o | | | of marine deposits (Shasta- | | z | | | Chico) represents the | | o | | | Cretaceous system; and in | | i | | | western British N. America | | c | | | coal-seams also occur. | | +---+-------------------------------------------+----------------------------------+ | o | | Purbeckian--Purbeck beds; Munder Mergel; | Representatives of the Middle and| | r | | largely present in Westphalia. | lower Jurassic formations have | | | | Portlandian--Portland group of England, | been found in California and | | S | | represented in S. France by the thick | Oregon, and farther north among| | e | | Tithonian limestones. | the Arctic islands. | | c | | Kimmeridgian--Kimmeridge Clay of England; | Strata containing Lower Jurassic | | o | | Virgulian and Pterocerian groups of N. | marine fossils appear in | | n | J | France; represented by thick limestones | Wyoming and Dakota; and above | | d | u | in the Mediterranean basin. | them come the _Atlantosaurus_ | | a | r | Corallian--Coral Rag, Coralline Oolite; | and _Baptanodon_ beds, which | | r | a | Sequanian stages of the Continent, | have yielded so large a | | y | s | comprising the sub-stages of Astartian | variety of deinosaurs and other| | . | s | and Rauracian. | vertebrates, and especially the| | | i | Oxfordian--Oxford Clay; Axgovian and | remains of a number of genera | | | c | Neuvizyan stages. | of small mammals. | | | . | Callovian--Kellaways Rock, Divesian | | | | | sub-stage of N. France. | | | | | Bathonian--series of English strata from | | | | | Cornbrash down to Fuller's Earth. | | | | | Bajocian--Inferior Oolite of England. | | | | | Lassic--divisible into (1) Upper Lias | | | | | or Toarcian, (2) Middle Lias, Marlstone | | | | | or Charmouthian, (3) Lower Lias of | | | | | Sinemurian and Hettangian. | | | +---+-------------------------------------------+----------------------------------+ | | | In Germany and western Europe this | In New York, Connecticut, New | | | T | division represents the deposits of | Brunswick, and Nova Scotia | | | r | inland seas or lagoons, and is divisible| a series of red sandstone | | | i | into the following stages in descending | (Newark series) contains land- | | | a | order: (1) Rhaetic, (2) Keuper, (3) | plants and labyrinthodonts | | | s | Muschelkalk, (4) Bunter. In the | like the lagoon type of central| | | s | eastern Alps and the Mediterranean | and western Europe. On the | | | i | basin the contemporaneous sedimentary | Pacific slope, however, marine | | | c | formations are those of open clear | equivalents occur, representing| | | . | sea, in which a thickness of many | the pelagic type of south- | | | | thousand feet of strata was accumulated.| eastern Europe. | +---+---+-------------------------------------------+----------------------------------+ | | P | Thuringian--Zechstein, Magnesian | To this division of the geologi- | | | e | Limestone; named from its development | cal record the Upper Barren | | | r | in Thuringia; well represented | Measures of the coal-fields of | | | m | also in Saxony, Bavaria and Bohemia. | Pennsylvania, Prince Edward | | | i | Saxonian--Rothliegendes Group; Red | Island, Nova Scotia and | | | a | Sandstones, &c. | New Brunswick have been | | | n | Autunian--where the strata present the | assigned. | | | . | lagoon facies, well displayed at Autun | Farther south in Kansas, Texas, | | | | in France; where the marine type is | and Nebraska the representa- | | | | predominant, as in Russia, the group | tives of the division have an | | | | has been termed Artinskian. | abundant marine fauna. | | +---+-------------------------------------------+----------------------------------+ | | C | Stephanian or Uralian--represented in | Upper productive Coal-measures. | | | a | Russia by marine formations, and in | Lower Barren measures. | | | r | central and western Europe by numerous | Lower productive Coal-measures. | | | b | small basins containing a peculiar | Pottsville conglomerate. | | | o | flora and in some places a great variety| Mauch Chunk shales; limestones | | | n | of insects. | of Chester, St Louis, &c. | | | i | Westphalian or Moscovian--Coal-measures, | Pocono series; Kinderhook | | | f | Millstone Grit. | limestone. | | | e | Culm or Dinantian--Carboniferous Limestone| | | | r | and Calciferous Sandstone series. | | | | o | | | | | u | | | | | s | | | | | . | | | | +---+-------------------------------------------+----------------------------------+ | | Devonian and Old Red Sandstone. | | P +----------------------+------------------------+----------------------------------+ | a | Devonian type. | Old Red Sandstone | | | l | | type. | | | a +----------------------+------------------------+ / Catskill red sandstone; Old | | e | / Famennian. | Yellow and red | | Red Sandstone type: the | | o | Upper < | sandstone with |< strata below show the | | z | \ Frasnian. | _Holoptychius_, | | Devonian type. | | o | | _Bothriolepis_,&c. | | Chemung Group. | | i | | | \ Genesee " | | c | | | | | | / Givetian. | Caithness Flagstones | | | o | Middle < | with _Osteolepus_, | / Hamilton Group. | | r | \ Eifelian. | _Dipterus_, | \ Marcellus " | | | | _Homosteus_, &c. | | | P | | | | | r | | Red and purple | / Corniferous Limestone. / Upper | | i | /Coblentizian.| sandstones and | | | Held- | | m | Lower < | conglomerates with |< Onondaga Limestone. < erberg| | a | \Gedinnian. | _Cephalaspis_, | | \ Group.| | r | | _Pteraspis_, &c. | \ Oriskany Sandstone. | | y +---+------------------+------------------------+----------------------------------+ | . | | | / Lower Helderberg Group. | | | S | / Ludlow Group. | | Water-Lime. | | | i | Upper < Wenlock " | < Niagara Shale and Limestone. | | | l | \ Llandovery" | | Clinton Group. | | | u | | \ Medina " | | | r | | | | | i | | / Cincinnati Group. | | | a | Lower / Caradoc or Bala Group. | | Utica " | | | n | (Ordovician) < Llandeilo " | < Trenton " | | | . | \ Arenig " | | Chazy " | | | | | \ Calciferous " | | +---+-------------------------------------------+----------------------------------+ | | C | Upper or _Olenus_ series--Tremadoc | Upper or Potsdam series with | | | a | slates and _Lingula_ Flags. | _Olenus_ and _Dicelocephalus_ | | | m | Middle or _Pardoxides_ series--Menevian | fauna. | | | b | Group. | Middle or Acadian series with | | | r | Lower or _Olenellus_ series--Llanberis | _Paradoxides_ fauna. | | | i | and Harlech Group, and _Olenellus_- | Lower or Georgian series with | | | a | zone. | _Olenellus_ fauna. | | | n | | | | | . | | | +---+---+-------------------------------------------+----------------------------------+ | | | Archean, Pre-Cambrian, Eozoic. | +---+---+-------------------------------------------+----------------------------------+ | | | In Scotland, underneath the Cambrian | In Canada and the Lake Superior | | | | Olenellus group, lies unconformably | region of the United States | | | | a mass of red sandstone and con- | a vast succession of rocks of | | | | glomerate (Torridonian) 8000 or 10,000 | Pre-Cambrian age has been | | | | ft. thick, which rests with a strong | grouped into the following | | | | gneisses and schists (Lewisian). A | subdivisions in descending | | | | thick series of slates and phyllites | order: (1) Keweenwan, lying | | | | lies below the oldest Palaeozoic rocks | unconformably on (2) Animikie, | | | | in central Europe, with coarse | separated by a strong | | | | gneisses below. | unconformability from (3) Upper| | | | | Huronian, (4) Lower Huronian | | | | | with an unconformable base, (5)| | | | | Goutchiching, (6) Laurentian. | | | | | In the eastern part of Canada, | | | | | Newfoundland, &c., and also in | | | | | Montana, sedimentary formations| | | | | of great thickness below the | | | | | lowest Cambrian zone have been | | | | | found to contain some obscure | | | | | organisms. | +---+---+-------------------------------------------+----------------------------------+

From these general considerations we proceed to inquire how the existing topographical features of the land arose. Obviously the co-operation of the two great geological agencies of hypogene and epigene energy, which have been at work from the beginning of our globe's decipherable history, must have been the cause to which these features are to be assigned; and the task of the geologist is to ascertain, if possible, the part that has been taken by each. There is a natural tendency to see in a stupendous piece of scenery, such as a deep ravine, a range of hills, a line of precipice or a chain of mountains, evidence only of subterranean convulsion; and before the subject was taken up as a matter of strict scientific induction, an appeal to former cataclysms was considered a sufficient solution of the problems presented by such features of landscape. The rise of the modern Huttonian school, however, led to a more careful examination of these problems. The important share taken by erosion in the determination of the present features of landscape was then recognized, while a fuller appreciation of the relative parts played by the hypogene and epigene causes has gradually been reached.

1. The study of the progress of denudation at the present time has led to the conclusion that even if the rate of waste were not more rapid than it is to-day, it would yet suffice in a comparatively brief geological period to reduce the dry land to below the sea-level. But not only would the area of the land be diminished by denudation, it could hardly fail to be more or less involved in those widespread movements of subsidence, during which the thick sedimentary formations of the crust appear to have been accumulated. It is thus manifest that there must have been from time to time during the history of our globe upward movements of the crust, whereby the balance between land and sea was redressed. Proofs of such movements have been abundantly preserved among the stratified formations. We there learn that the uplifts have usually followed each other at long intervals between which subsidence prevailed, and thus that there has been a prolonged oscillation of the crust over the great continental areas of the earth's surface.

An examination of that surface leads to the recognition of two great types of upheaval. In the one, the sea-floor, with all its thick accumulations of sediment, has been carried upwards, sometimes for several thousand feet, so equably that the strata retain their original flatness with hardly any sensible disturbance for hundreds of square miles. In the other type the solid crust has been plicated, corrugated and dislocated, especially along particular lines, and has attained its most stupendous disruption in lofty chains of mountains. Between these two phases of uplift many intermediate stages have been developed, according to the direction and intensity of the subterranean force and the varying nature and disposition of the rocks Of the crust.

(a) Where the uplift has extended over wide spaces, without appreciable deformation of the crust, the flat strata have given rise to low plains, or if the amount of uprise has been great enough, to high plains, plateaux or tablelands. The plains of Russia, for example, lie for the most part on such tracts of equably uplifted strata. The great plains of the western interior of the United States form a great plateau or tableland, 5000 or 6000 ft. above the sea, and many thousands of square miles in extent, on which the Rocky Mountains have been ridged up.

(b) It is in a great mountain-chain that the complicated structures developed during disturbances of the earth's crust can best be studied (see Parts IV. and V. of this article), and where the influence of these structures on the topography of the surface is most effectively displayed. Such a chain may be the result of one colossal disturbance; but those of high geological antiquity usually furnish proofs of successive uplifts with more or less intervening denudation. Formed along lines of continental displacement in the crust, they have again and again given relief from the strain of compression by fresh crumpling, fracture and uprise. The chief guide in tracing these successive stages of growth is supplied by unconformability. If, for example, a mountain-range consists of upraised Silurian rocks, upon the upturned and denuded edges of which the Carboniferous Limestone lies transgressively, it is clear that its original upheaval must have taken place in the period of geological time represented by the interval between the Silurian and the Carboniferous Limestone formations. If, as the range is followed along its course, the Carboniferous Limestone is found to be also highly inclined and covered unconformably by the Upper Coal-measures, a second uplift of that portion of the ground can be proved to have taken place between the time of the Limestone and that of the Upper Coal-measures. By this simple and obvious kind of evidence the relative ages of different mountain-chains may be compared. In most great chains, however, the rocks have been so intensely crumpled, and even inverted, that much labour may be required before their true relations can be determined.

The Alps furnish an instructive example of the long series of revolutions through which a great mountain-system may have passed before reaching its present development. The first beginnings of the chain may have been upraised before the oldest Palaeozoic formations were laid down. There are at least traces of land and shore-lines in the Carboniferous period. Subsequent submergences and uplifts appear to have occurred during the Mesozoic periods. There is evidence that thereafter the whole region sank deep under the sea, in which the older Tertiary sediments were accumulated, and which seems to have spread right across the heart of the Old World. But after the deposition of the Eocene formations came the gigantic disruptions whereby all the rocks of the Alpine region were folded over each other, crushed, corrugated, fractured and displaced, some of their older portions, including the fundamental gneisses and schists, being squeezed up, torn off, and pushed horizontally for many miles over the younger rocks. But this upheaval, though the most momentous, was not the last which the chain has undergone, for at a later epoch in Tertiary time renewed disturbance gave rise to a further series of ruptures and plications. The chain thus successively upheaved has been continuously exposed to denudation and has consequently lost much of its original height. That it has been left in a state of instability is indicated by the frequent earthquakes of the Alpine region, which doubtless arise from the sudden snapping of rocks under intense strain.

A distinct type of mountain due to direct hypogene action is to be seen in a volcano. It has been already pointed out (