Page images
PDF
EPUB

less than 1,700,000 square kilometres of the present lowlands of Europe. Its influence on the solid rocks over which it passed has not been everywhere equal. Over much of the north German plain, indeed, the rocks are concealed under drift. But in the more undulating hilly ground, particularly in the north and north-west, the ice has effected the most extraordinary abrasion. It is hardly possible, indeed, to describe adequately in words these regions of most intense glaciation. The old gneiss of Norway and Sutherlandshire, for example, has been so eroded, smoothed. and polished, that it stands up in endless rounded hun mocks, many of them still smooth and flowing like dolphin. backs, with little pools, tarns, and larger lakes lying between. them. Seen from a height the ground appears like a billowy sea of cold grey stone. The lakes, every one of them lying in a hollow of erosion, seem scattered broadcast over the landscape. So enduring is the rock, that even after the lapse of so long an interval, it retains its ice-worn aspect almost as unimpaired as if the work of the glacier had been done only a few generations since.

The effect of the movement of the ice was necessarily to remove the soils and superficial deposits of the land surface. Hence in the areas of country so affected, the ground having been scraped and smoothed, the glacial accumulations laid down upon it rest abruptly, and without any connexion, on whatever underlies them. Considerable local differences may be observed in the nature and succession of the different deposits of the glacial period, as they are traced from district to district. It is hardly possible to determine, in some cases, whether certain portions of the series are coeval or belong to different epochs. But the following are the leading facts which have been established for the NorthEuropean area. First, there was a gradual increase of the cold, though with warm intervals, until the conditions of modern North Greenland extended as far south as Middlesex, Wales, the south-west of Ireland, and 50° N. lat. in central Europe. This was the culmination of the Glacial period. Then followed a considerable depression of the land and the spread of cold arctic water over the submerged tracts, with abundant floating ice; next came a re-elevation, with renewed augmentation of the snow-fields and glaciers. Very gradually, and after intervals of increase and diminution, the ice retired towards the north, and with it the arctic flora and fauna that had peopled the European plains. The existing snow-fields and glaciers of the Pyrenees, Switzerland, and Norway are remnants of the great ice-navia, the high grounds of Britain were important enough sheets of the glacial period, while the arctic plants of the mountains are relics of the northern vegetation which was universal from Norway to Spain.

Some idea of the massiveness of the ice-sheet is obtainable from a consideration of the way in which the striæ run across important hill ranges, and athwart what might secr to be their natural direction. Whilst there was a general southward movement from the great snow-fields of Scandi

to have their own independent ice, which, as the striæ show, radiated outward, some of it passing westwards into the Atlantic and some of it eastward into the North Sea. So thick must it have been as it moved off the Scottish Highlands that it went across the broad plains of Perthshire, filling them up to a depth of at least 2000 feet, and passing across the range of the Ochil Hills, which at a distance of

Ice-worn Rocks.-Beginning at the base of the deposits from which this interesting history is compiled, we find the solid rocks over the whole of northern Europe to present the characteristic smoothed flowing outlines which can be produced only by the grinding action of land-ice (ante, p. 282)..12 miles runs parallel with the Highland mountains, and Where they have been long exposed, this peculiar surface is apt to be effaced by the disintegrating action of the weather, though it retains its hold with extraordinary pertinacity. Along the fjords of Norway and the sea-lochs of the west of Scotland, it may be seen slipping into the water, smooth, bare, polished, and grooved as if the ice had only recently retreated. But where the protecting cover of clay or other superficial deposits has been newly removed, the peculiar iceworn surface is as fresh as that by the side of a modern glacier. Observations of the directions of the striæ have shown that on the whole these markings diverge from the main masses of high ground. In Scandinavia they run westwards and south-westwards on the Norwegian coasts, and eastwards or south-eastwards across the lower grounds of Sweden. When the ice descended into the basin of the Baltic and the plains of northern Germany, it moved southwards and south-westwards, but seems to have slightly changed its direction in different areas and at different times. Its movements can be made out partly from the stria on the solid rock, but more generally from the glacial drift which it has left behind. Thus it can be shown to have moved down the Baltic into the North Sea. At Berlin its movement must have been from east to west. But at Leipsic, as recently ascertained by Credner, it came from N.N.W. to S.S.E., being doubtless shed off in that direction by the high grounds of the Harz mountains. Its southern limit can be traced with tolerable clearness from Jevennaar in Holland eastwards across the Rhine valley, along the base of the Westphalian hills, round the projecting promontory of the Harz, and then southwards through Saxony to the roots of the Erzgebirge. Passing next south-eastwards along the flanks of the Riesen and Sudeten chain, it sweeps across Poland into Russia, circling round by Kieff, and northwards by Nijni Novgorod towards the Urals.

It has been estimated that, excluding Finland, Scandinavia, and the British Isles, the ice must have covered not

reaches a height of 2352 feet. In such cases it has been observed that the striæ along the lower slopes of the hill barrier run either parallel with the trend of the ground or slant up obliquely, while those on the summits may cross the ridge at right angles to its course. This shows that there must have been a differential movement in the great icesheet, the lower parts, as in a river, becoming embayed, and being forced to move in a direction sometimes even at a right angle to that of the general advance. On the lower grounds, also, the striæ, converging from different sides, unite at last in one general trend as the various ice-sheets must have done, as they descended from the high grounds on either side and coalesced into one common mass. This is well seen in the great central valley of Scotland. Still more marked is the deflexion of the striæ in Caithness and the Orkney and Shetland Islands. In these districts the general direction of the striation is from S.S. E., which, in Caithness, is nearly at right angles to what might have been anticipated. This deflexion has been attributed to the coalescence of the ice from Norway and from the northern Highlands in the basin of the North Sea, and its subsequent progress along the resultant line into the Atlantic. But it may have been due to the fanshaped spreading out of the vast mass of ice descending into the Moray Firth; for the striæ on the south side of that inlet run E. by S., and at last S.E., on the north-east of Aberdeenshire, showing that the ice, on the one hand, turned southwards into the North Sea, until it met the N. E. stream from Kincardineshire and the valleys of the Dee and Don, while, on the other, it moved northward so as no doubt to join the Scandinavian sheet, and march with it into the Atlantic. The basin of the North Sea must have been choked up with ice in its northern, parts, if not entirely. At that time England and the north-west of France were united, so that any portion of the North Sea basin not invaded by land-ice must have formed a lake,

with its outlet by the hollow through which the Strait of Dover has since been opened. It has been suggested that during such a condition of things the widespread freshwater deposit termed Loess was formed, which covers so large a space in the lower plains of the Rhine and the north of Belgium, and appears in the valleys of the south-caused the ice to move outward from it for a certain discast of England.

The ice is computed to have been at least between 6000 and 7000 feet thick in Norway, measured from the present sea-level. From the height at which its transported debris has been observed on the Harz, it is believed to have been at least 1470 feet thick there, and to have gradually risen in elevation as one vast plateau, like that which at the present time covers the interior of Greenland. Among the Alps it attained almost incredible dimensions. The present snowfields and glaciers of these mountains, large though they are, form no more than the mere shrunken remnants of the great mantle of snow and ice which then overspread Switzerland. In the Bernese Oberland, for example, the valleys were filled to the brim with ice, which, moving northwards, crossed the great plain, and actually overrode a part of the Jura mountains. Huge fragments of granite and other rocks from the central chain of the Alps are found high on the slopes of that range of heights.

Boulder-clay or Till.-Under this name is included the debris which accumulated and was pushed onward under the ice-sheet, the "grund-moräne" or "moraine profonde' (ante, p. 282). All over the low grounds of North Germany, Denmark, and Holland lies a stony or earthy clay passing into sand or gravel, in which, together with locally derived debris, there is a greater or less proportion of fragments from the north. Some of the rocks of Scandinavia, Finland, and the Upper Baltic are of so distinctive a kind that they can be recognized in small pieces in the boulder-clay. Thus the peculiar syenite of Lanrwig in the south of Norway has been recognized abundantly in the drift of Denmark; it occurs in that of Hamburg, and in the boulder-clay of the Holderness cliffs in Yorkshire. The well-known Rhombenporphyr of southern Norway has likewise been recognized at Holderness. Fragments of the Silurian rocks from Gothland, or from the Russian islands Dago or Oesel, have been met with as far as the north of Holland. These transported fragments, so abundant within the line of demarcation just traced, are an impressive testimony to the movements of the northern ice-sheet and floating ice.

The boulder clay is not spread as a uniform sheet over the ground, but varies greatly in thickness and in irregularity of surface. Round the mountainous centres of dispersion it is apt to occur in long ridges or "drums" which run in the general direction of the rock-striation.

In those areas which served as independent centres of dispersion for the ice-sheet, the boulder-clay partakes largely of the local character of the rocks of each district where it occurs. Thus in Scotland the clay varies in colour and composition as it is traced from district to district. Over the Carboniferous rocks it is dark, over the red sandstones it is red, over the Silurian rocks it is fawn-coloured. The great majority of the stones also are of local origin, not always from the immediately adjacent rocks, but from points within a distance of a few miles. Evidence of transport, however, can be gathered from the stones, for they are found in almost every case to include a proportion of fragments which have come from beyond the district. The direction indicated by the percentage of travelled stones is always the same for each region. Thus, in the lower part of the valley of the Firth. of Forth, while most of the fragments are from the surrounding Carboniferous formations, from 5 to 20 per cent. is found to have come eastward from the Old Red Sandstone range of the Ochil Hills-a distance of 25 or 30 miles, while 2 to 5 per cent. are

pieces of the Highland rocks, which must have come from the high grounds at least 50 miles to the north-west. The evidence of dispersion revealed by the stones in the boulderclay harmonizes with that of the striae on the rocks. Every main mass of elevated ground in Britain seems to have tance, until the stream coalesced with that descending from some other height. In Scotland the ice was massive enough to move out into the basin of the North Sea (then doubtless in great. part usurped by the glaciers) until it met that which was streaming down from Scandinavia. Hence no Scandinavian blocks have ever been found in Scotland. But the Norwegian ice which crept southwards across Denmark, may once have extended across the North Sea to the Yorkshire coast, if the Scandinavian stones of Holderness were not carried on floating ice.

The stones in the boulder-clay have a characteristic form and surface. They are usually oblong, have one or more flat sides or "soles," are smoothed or polished, and have their edges worn round. Where they consist of a fine-grained enduring rock, they are almost invariably found to be striated, the striae running with the long axis of the stone. These markings are precisely similar to those on the solid rocks underneath the boulder clay, aud like them have manifestly been produced by the friction of stones and grains of sand as the whole mass of debris was being steadily pushed on in one given direction by the resistless advance of the ice (ante, p. 282).

Interglacial Beds.-The boulder-clay is not one uniform mass of material. In a limited section, indeed, it usually appears as an unstratified mass of stiff stony clay. But it is found on further examination to be split up with various inconstant and local interstratifications. Beds of sand, gravel, fine clay, and peaty layers occur in this way in different platforms in the boulder clay. In Scotland and elsewhere these interpolated beds bear witness to intervals when the ice retired from the area, and the land, so far as uncovered, was clothed with vegetation and traversed by herds of the hairy mammoth, reindeer, and muskOX. Hence the long glacial period must have been interrupted by episodes probably of considerable duration when a milder climate prevailed. Such an alternation of conditions is explained on the hypothesis discussed in previous pages (ante, pp. 218-220).

Evidences of Submergence.-After the ice had attained its greatest development, the land, which perhaps in northwestern Europe stood at a higher level above the sea than it has done since, began to subside. The ice-fields were carried down below the sea-level, where they broke up and cumbered the sea with floating bergs. The heaps of loose debris which had gathered under the ice, being now exposed to waves, ground-swell, and marine currents, were thereby more or less washed down and reassorted. Coast-ice, no doubt, still formed along the shores, and was broken up into moving floes, as happens every year now in northern Greenland. The proofs of this phase of the long glacial period are contained in the sands, gravels, erratic blocks, and stratified clays which overlie the coarse older till. It is difficult to determine the extent of the submergence, for when the land rose the more elevated portions continued to be the seats of glaciers, which, moving over the surface, destroyed the deposits which would otherwise have remained as witnesses of the presence of the sea. The most satisfactory evidence is undoubtedly that supplied by beds of marine shells. These have been observed on Moel Tryfan, in North Wales, at a height of no less than 1350 feet, but as the same kind of deposits in which they occur extend to a much greater height, the submergence probably considerably exceeded the limit at which the shells occur. In Cheshire beds of shells have been met with at a height of

In these natural reservoirs

1200 feet. In Scotland the highest level from which they | direction by the col at the head.
have yet been obtained is 524 feet; and in this instance
they lie in one of the interstratifications in the boulder-clay.
The coast of Scotland is fringed by a succession of raised
beaches which, up to that at 100 feet above the present
mean tide-level, are often remarkably fresh. The 100-feet
terrace forms a wide plateau in the estuary of the Forth,
and the 50-feet terrace is as conspicuous on the Clyde.
The elevation of the land has brought up within tide-marks
some of the clays deposited over the sea-floor during the
time of the submergence. In the Clyde basin and some
of the western fjords these clays (Clyde beds) are full of
shells. Comparing the species with those of the adjacent
seas, we find them to be more boreal in character; nearly
the whole of the species still live in Scottish seas, though
a few are extremely rare. Some of the more characteristic
northern shells in these deposits are Pecten Islandicus,
Tellina calcarea, Leila truncata, L. lanceolata, Saxicava
rugosa, Panopaa Norvegica, Trophon clathratum, and
Natica clausa.

the level at which the water stood for a time was marked
by a horizontal ledge or platform due partly to erosion of
the hill-side and partly to the arrest of the descending
debris when it entered the water.

That ice continued to float about in these waters is shown by the striated stones contained in the fine clays, and by the remarkably contorted structure which these clays occasionally display. Sections may be secu where, upon perfectly undisturbed horizontal strata of clay and sand, other similar strata have been violently crumpled, while horizontal beds lie directly upon them. These contortions must have been produced by the horizontal pressure of some heavy body moving upon the originally flat beds. No doubt the agent was ice in the form of large stranding masses which were driven aground in the fjords or shallow waters where the clays accumulated. Another indication of the presence of floating ice is furnished by large boulders scattered over the country, and lying sometimes on the stratified sands and gravels, though no doubt many of the so-called erratics belong to the time of the chief glaciation.

One of the most puzzling members of the Drift is the series of sands and gravels which cover the low grounds in many places, and rise up to heights of 1000 feet and more. These deposits are sometimes spread out in undulating sheets, which, in the lower districts, seem to merge insensibly into the marine terraces and raised beaches. But they are also very commonly disposed in remarkable mounds and in ridges which run across valleys, along hill-sides, and even over watersheds. These ridges are known in Scotland as kames, in Ireland as eskers, and in Scandinavia as osar. Thoy consist sometimes of coarse gravel or earthy detritus, but more usually of clean, well-stratified sand and gravel, the stratification towards the surface corresponding with the external slopes of the ground, in such a manner as to prove that the ridges are original forms of deposit, and not the result of the irregular erosion of a general bed of sand and gravel. Some writers have compared these features to the submarine banks formed in the pathway of tidal currents near the shore. Others have supposed them rather to be of terrestrial origin, due to the melting of the great snow-fields and glaciers, and the consequent discharge of large quantities of water over the country. But no very satisfactory explanation has yet been given.

Re-emergence. Later Glaciers.-When the land began to rise again, the temperature all over central and northern Europe was still severe. Vast sheets of ice still held sway over the mountains, and continued to descend into the lower tracts and to go out to sea. To this period are ascribed certain terraces or "parallel-roads" which run along the sides of valleys in the Scottish Highlands. It is believed that the mass of ico descending from some of the loftier snowfields of this time was so great as to accumulate in front of literal valleys, and to so choke them up as to cause the water to accumulate in them and flow out in an opposite

Every group of mountains nourished its own glaciers; even small islands, such as Arran in Scotland, had their snowfields, whence glaciers crept down into the valleys and shed their moraines. It would appear indeed that some of the northern glaciers of Scotland continued to reach the sea-level even when the land had risen to within 50 feet or less of its present elevation. On the east side of Sutherlandshire the moraines descond to the 50feet raised beach; on the west side of the same county they come down still lower. The higher mountains of Europe still show the descendants of these later glaciers, but the ice has retreated from the lower elevations. In the Vosges the glaciers have long disappeared, but their moraines remain still fresh. In Wales, Cumberland, and the southern uplands and Highlands of Scotland, moraines, perched blocks, and roches moutonnées attest the abundance and persistence of the last glaciers. It is sometimes possible to trace the stages in the gradual retreat of the ice towards its parent snow-fields, for the crescer t-shaped moraine mounds lie one behind another until they finally die out about the head of the valley, near what must have been the edge of the snow-field.

We know as yet very little of the fauna and flora of the land during the Glacial Period in Europe. The vegetation was doubtless in great measure the same as that of arctic Norway at the present day. The animals included the musk-ox, lemming, reiu-deer, and other still living arctic forms, but included some which have become extinct, such as the hairy mammoth and woolly rhinoceros. During the milder inter-glacial periods denizens of warmerregions found their way northwards into Europe. Thus the hippopotamus haunted the rivers of the south of England. By degrees, as the climate ameliorated, the arctic vegetation was finally extirpated from the lower grounds of central and western Europe, and plants loving a milder temperature, which had doubtless been natives of Europe before the period of great cold, were enabled to reoccupy the sites whence they had been driven. On the higher mountains, where the climate is still not wholly uncongenial for them, colonies of this once general arctic flora still survive. The arctic animals have also been driven away to their northern homes, or have become wholly extinct. And thus, as imperceptibly as it began, the long and varied Ice Age came to an end as it merged into the next succeeding geological period.

RECENT OR HUMAN PERIOD.

That long succession of ages to which the name of the Glacial Period has been given shaded without abrupt change of any kind into what is termed the Human or Recent Period. Indeed it may be said with truth that the Glacial Period still exists in Europe. The snow-fields and glaciers have disappeared from Britain, but they still linger among the Pyrenees, remain in larger mass among the Alps, and spread over wide areas in northern Scandinavia. This dove tailing or overlapping of geological periods has been the rule from the beginning of time, the apparently abrupt transitions in the geological record being due to imperfec tions in the chronicle.

The question has often been asked whether man was coeval with the Ice Age. To give an answer, we must know within what limits the term Ice Age is used, and to what particular country or district the question refers. For it is

evident that even to-day man is contemporary with the Ice Age in the Alpine valleys and in Finmark. There can

be no doubt that he inhabited Europe after the greatest extension of the ice, but while the rivers were still larger than now from the melting snow, and flowed at higher levels. The proofs of the existence of man in former geological periods are not to be sought for in the occurrence of his own bodily remains, as in the case of other animals. His bones are indeed now and then to be found, but in the vast majority of cases his former presence is revealed by the implements he has left behind him, formed of stone, metal, or bone. Many years ago the archeologists of Denmark, adopting the subdivisions of the Latin poets, classified the prehistoric traces of man in three great divisions-those of (1) the Stone Age, (2) the Bronze Age, and (3) the Iron Age There can be no doubt that, on the whole, this has been the general order of succession. Men used stone and bone before they had discovered the use of metal. Nevertheless, the use of stone long survived the introduction of bronze and iron. In fact, in many European countries where metal has been known for many centuries, there are districts where stone implements are still employed, or where they were in use until quite recently. It is obvious also that, as there are still barbarous tribes unacquainted with the fabrication of metal, the Stone Age is not yet extinct in many parts of the world. In this instance we again see how geological periods run into each other. The nature or shape of the implement cannot therefore be always a very satisfactory proof of antiquity. We must judge of it by the circumstances under which it was found. From the fact that in north-western Europe the ruder kind of stone weapons occurs in what are certainly the older deposits, while others of more highly finished workmanship are found in later accumulations, the Stone Age has been subdivided into an early or Palæolithic and a later or Neolithic epoch. There can be no doubt, however, that the later was in great measure coeval with the age of bronze, and even to some extent of iron.

The deposits which contain the history of the human period are cavern-loam, river-alluvia, lake-bottoms, peatmosses, sand-dunes, and other superficial accumulations.

Most calcareous districts abound in underground tunnels and caverns which have been dissolved by the passage of water from, the surface (ante, p. 271). In a large number of cases these cavities have communicated with the outer surface, so that terrestrial animals, including man himself, have made use of them as places of retreat, or have fallen or been washed into them. The floors of some of them are covered with loam or cave-earth, resulting from the deposit of the silt carried in the water which formerly flowed through them. Very commonly also, a deposit of stalagmite has formed from the drip of the roof. organic remains which may have found their way to these floors have been sealed up and admirably preserved.

feet.

Hence any

Above the present levels of the rivers there lie platforms or terraces of alluvium, sometimes to a height of 80 or 100 These deposits are fragments of the river gravels and loams laid down when the streams flowed at that elevation, and therefore before the valleys were widened and deepened to their present form. River action is at the best but slow. To erode the valleys to so great a depth beneath the level of the upper alluvia, must have demanded a period of many centuries. There can therefore be no doubt of the high antiquity of these deposits. They have yielded the remains of many mammals, some of them extinct, together with the flint flakes made by man. From the nature and structure of some of the high-lying gravels, there can be little doubt that they were formed at a time when the rivers were larger than now, and were liable to be frozen and to be obstructed by large accumulations of ice. We are thus able to connect the formations of the human period with some of the later phases of the Ice Age in the west of Europe.

From the evidence of caverns and river-alluvia of Britain, Germany, and France, Dawkins enumerates about fifty mammals as among the chief inhabitants of Europe during the early human period. They include many of the animals still found wild in Europe, Asia, or Africa, such as the marmot, hare, beaver, lemming, leopard, lynx, wild cat, otter, brown bear, wolf, fox, African elephant, musk sheep, chamois, wild boar, horse, reindeer, and hippopotamus. It will be observed what a remarkable mixture there is in this list of forms now found in alpine or arctic regions, on the one hand, with others only now to be seen in warm latitudes. Probably there continued to be great alternations of climate and changes in the distribution of food, so that migrations successively from opposite quarters took place into central and western Europe. But among the denizens of these regions were some that have been long extinct, such as the mammoth, Elephas antiquus, Machairodus latidens, the woolly rhinoceros, the Irish elk, and others. That man was the contemporary of these extinct forms is shown both by the association of his flint weapons with their remains, and by the discovery of a tusk of the mammoth with an admirable outline of the animal carved upon it. This valuable relic, with bones of the reindeer and stag admirably sculptured into likenesses of these animals, was found in one of the caves of Perigord in central France.

From the beds of lakes, and from peat-mosses, evidences of more advanced civilization have been obtained. In Switzerland and elsewhere, remains of wooden pile-dwellings have been observed during times when the water has been low. Associated with these are weapons and implements of stone, in other cases of bronze and of iron, pottery, rude kinds of cloth, seeds of different cereals, and bones of domesticated animals. The dog, swine, goat, horned sheep, and other familiar animals appear as the companions of man, But there is evidence that some of the creatures which he tamed to his use were not natives of Europe, but had their original stocks in central Asia, and that some of his grains must likewise have been introduced. Hence we have glimpses into some of the early human migrations from that eastern centre whence so many successive waves of population have invaded Europe.

The later chronicles of the geologist merge insensibly into those of the archeologist. The latter claims as his field whatever belongs to the history of man on the globe; the former includes the history of man in that larger history of the earth of which a brief outline has been given in the foregoing pages.

POST-TERTIARY SERIES IN NORTH AMERICA.

The general succession of events in post-Tertiary times appears to have been nearly the same over the northern hemisphere both in the New and the Old World. In North America we have the same sharply-defined line between the older post-Tertiary deposits and previous forma tions, due to the glacial conditions which, overspreading these regions, in great measure destroyed the superficial accumulations of the immediately preceding eras. Quaternary or post-Tertiary formations are grouped by American geologists in the following subdivisions:4. Recent and Peat, alluvium, blown sand, "alkali deposits, geyser deposits, cave deposits, artificial mounds. River-terraces, loess. Raised beaches.

Prehistoric

3. Terrace....

[ocr errors]

The

2. Champlain... Saricava sand, Champlain clays, Leda clay. 1. Glacial Boulder clays, unstratified clays, sands, and gravels. 1. Glacial.-As in Europe, the glacial deposits increase in thick. ness and variety from south to north. The southern limit of the unstratified drift lies somewhere in the neighbourhood of the 39th parallel of north latitude, and the deposit ranges from the Atlantic restward to the meridian of 98°. It spreads, therefore, across British North America, and is found over a considerable area of the north-eastern States. It rises to a height of 5800 feet among

the White Mountains. The absence of any true boulder-drift on the Rocky Mountain slopes, where it might have been looked for, is remarkable.

Underneath the boulder-clay the solid rocks, as in Europe, are often well striated. The direction of the striae is generally southward, varying to south-east and south-west according to the form of the ground. In recent years extensive ice-worn rock-surfaces have been observed among the Rocky Mountains by Hayden, King, and others, proving that these elevations formerly possessed their glaciers, if they were not buried under the great ice-sheet.

The drift bears witness to a general-southerly transport of material, and, in conjunction with the striated rocks, shows that the great icesheet moved from north to south at least as far as about the latitude of Washington. Logan mentions that in some parts of Canada the glacial drift and boulders run in ridges north and south, thus corresponding with the general direction of transport, like the "drums in Britain. As in Europe, the coarse boulder-clay at the base of the Quaternary deposits is essentially unfossiliferous.

"

2. Champlain. Under this name American geologists class the Bands, gravels, and clays which overlie the lower boulder-drift. These deposits include coarse unstratified gravels, as well as finely stratified clays. In eastern Canada they are well developed, and show the following subdivisions :

Upper.

Lower.

St Maurice and Sorel sands; Saricava sand of Montreal; upper sand and gravel of Beauport; upper Champlain clay and sand of Vermont. Leda clay of the St Lawrence and Ottawa; lower shellsand of Beauport; lower Champlain clay of Vermont. The lower subdivisions consist chiefly of clays, which rise to a height of 600 feet above the sea. They have some interstratified beds of siliceous sand, but few boulders. They contain marine organisms, such as Leda truncata, Saxicava rugosa, Tellina Grænlandica, bones of seals, whales, &c. On the banks of the Ottawa, in Gloucester, the clays contain numerous nodules which have been formed round organic bodies, particularly the fish Mallotus villosus or capeling of the Lower St Lawrence. Dawson also obtained numerous remains of terrestrial marsh plants, grasses, carices, mosses, and algae. This writer states that about 100 species of marine invertebrates have been obtained from the clays of the St Lawrence valley. All except four or five species in the older part of the deposits are shells of the boreal or Arctic regions of the 'Atlantic; and about half are found also in the glacial clays of Britain. The great majority are now living in the Gulf of St Lawrence and neighbouring coasts, especially off Labrador.1

3. Terrace. This division includes the terraced deposits of alluvial material so marked along the river valleys and lake margins in the northern part of the United States and in Canada, and found also in some degree along the sea-coast. These deposits occur in successive platforms or terraces, marking the contraction in volume of the lakes and rivers, consequent, probably, upon intermittent upheavals of the land. They are well developed round the great lakes. Thus in the basin of Lake Huron deposits of fine sand and clay containing fresh-water shells rise to a height of 40 feet or more above the present level of the water, and run back from the shore sometimes for 20 miles. Regular terraces, corresponding to former water

tially the same with those in Europe; ani, as in that continent
in America, no definite lines can be drawn within which they should
be confined. They cannot be sharply separated from the Terrare
series, on the one hand, nor from modern accumulations, on the
other. Besides the marshes, peat-bogs, and other, organic deposits
which belong to an early period in the human occupation of America,
some of the younger alluvia of the river-valleys and lakes can no
doubt claim a high antiquity, though they have not supplied the
same copious evidence of early man which gives so much interest to
the corresponding European formations Heaps of shells of edible
species occur on the coasts of Nova Scotia, Maine, &c. The large
mounds of artificial origin in the Mississippi valley have excited

much attention.

PART VIL-PHYSIOGRAPHICAL GEOLOGY. In the investigation of the geological history of any We have first country, two questions present themselves. to consider the nature and arrangement of the rocks which underlie the surface, and to ascertain from them what has been the march of events, what changes in geography have animals have come and gone. The gradual geological successively taken place, and what races of plants and evolution of the earth has been sketched in the foregoing part of this article. But besides the history of the solid rocks beneath the surface of the land, there is that of the surface itself. Mountains and plains, valleys and ravines, cliffs, peaks, passes, lakes, and the many other features of a country demand attention. By what processes have these varied outlines been impressed upon the surface of the globe? Are they of different ages, and if so, how can their history be ascertained?

The branch of geological inquiry which endeavours to answer these questions has been termed Physiography or Physiographical Geology. Its investigations evidently demand an acquaintance with Stratigraphical Geology. We must be able to trace out the former geographical conditions of the globe before we can adequately reason on the origin of those now existing. Hence the consideration of this branch of the subject has necessarily been reserved for this concluding section.

The stratified formations, of which the succession and history have been traced in the previous pages, were chiefly laid down on the sea-floor in wide horizontal or gently inclined sheets. They have since been upraised into land; their horizontality has been in great part destroyed; and they have been enormously wasted by denuding agents. In con

levels of the lake, run for miles along the shores at heights of 120, sidering therefore how they have acquired their present ex

150, and 200 feet. Shingle beaches and mounds or ridges, exactly like those now in course of formation along the exposed shores of Lake Huron, can be recognized at heights of 60, 70, and 100 feet. Unfossiliferous terraces occur abundantly on the margin of Lake Superior. At one point mentioned by Logan, no fewer than seven of these ancient beaches occur at intervals up to a height of 331 feet above the present level of the lake. Most of the rivers are bordered with lines of terraces, as in the well-known example of the Connecticut valley described by Hitchcock. The rivers are believed to have had their maximum volume at the beginning of the Terrace epoch, swollen doubtless by the melting of the still existing icesheets and snow-fields. Their work consisted partly in depositing fine alluvium or loess over their flood-plains, partly in scouring their channels out of the Champlain formations. Greater elevation towards the interior, by augmenting their slope, increased their

excavating power.

In

Terraces of marine origin likewise occur both on the coast and far inland. On the coast of Maine they occur at heights of 150 to 200 feet, round Lake Champlain at least as high as 300 feet, and at Montreal nearly 500 feet above the present level of the sea. the absence of organic remains, however, it is not always possible to distinguish between terraces of marine origin marking former seamargins, and those left by the retirement of rivers and lakes. In the Bay of Fundy evidence has been cited by Dawson to prove subsidence, for he has observed there a submerged forest of pine and beech lying 25 feet below high-water mark."

4. Recent and Prehistoric.-The deposits in this group are essen

[blocks in formation]

ternal forms, we have to deal with the effects of two kinds of forces, one acting from below, the other on the surface.

These stratified rocks were, on the whole, deposited in shallow water, and have been repeatedly upraised and denuded, so that the younger have been formed out of the waste of the older. They have their modern counterparts, not in the deposits of the great ocean-basins, but in those of comparatively shallow seas. The inference to be drawn from these facts is that the present continental regions, through many local oscillations, have existed as terrestrial ridges from a remote geological antiquity, and that the ocean basins in like manner have, on the whole, retained their identity. When the geologist asks himself how the present distribution of sea and land is to be accounted for, he finds that the answer to the question goes back to early Paleozoic times, whence he cap in some cases trace the gradual growth of a continent downward through the long cycles of geological time. But there still remains the problem to account for the original wrinkling of the surface of the globe, whereby the present great ridges and hollows were produced.

It is now generally agreed that these inequalities have been produced by unequal contraction of the earth's mass, the interior contracting more than the outer crust, which must therefore have accommodated itself to this dimi ution of diameter by undergoing corrugation. But there spars

« EelmineJätka »