Continental drift

“ From want of attention to this, the permanence of continents is still a debated question, and it is difficult for many to understand how the frequent dips of the continental plateaus and margins under the sea, and their re-elevation, often along with portions of the shallower sea bottom, can be consistent with a general permanence of the position of the continents and of the corresponding ocean abysses; yet, when this is properly understood, it becomes plain that the union of fixity with changes of level has been a main cause of the continuity and changes of organic beings. ”

“ Within a narrow belt, which rarely exceeds two or three hundred miles, except near the mouths of muddy rivers such as the Amazon and Congo, nearly all the waste of the continent, whether worn from its surface by the weather, by streams, by glaciers, or by the wind, or from its edge by the chafing of the waves, comes at last to its final resting place. The agencies which spread the material of the continental delta grow more and more feeble as they pass into deeper and more quiet water away from shore. ”

“ For example, it is evident that we can gain nothing by adding to the continents those submerged margins delineated by Murray in the Challenger reports, and which have in periods of continental elevation themselves formed portions of the land. Nor do we establish a case in favour of perished oceanic continents by the argument that they are needed to furnish the materials of marginal mountains which are due to the continuous sweeping of arctic material to the south by currents, as we see in the coast of North America to-day. ”

“ In this respect the American record differs somewhat from our own, for in Britain we have evidence of a submergence of over 1000 feet, which supervened in times immediately preceding the latest great extension of continental ice. [P] But nowhere in middle Europe, and nowhere in North America, in the region south and west of the great lakes, is there any trace of a general marine submergence. The “Palæocrystic Sea” is as idle a dream for the northern States of America as it is for any part of Europe. ”

“ If it be true that cold climatic conditions were contemporaneous in our latitude with submergence, it is equally true that an extensive land-surface in north-west Europe has, sometimes at least, co-existed with markedly genial conditions. In Tertiary times, for example, as the Oligocene deposits of Scotland, the Faröe Islands, Iceland, and Greenland testify, a land-connection existed between Europe and the North American continent. Again, it has been shown that during the interglacial phase of the Pleistocene period Britain was continental, and enjoyed at the time a peculiarly genial climate. ”

“ Experiment with a compass on a globe shows that their dimensions agree accurately.
This phenomenon was the starting-point of a new conception of the nature of the earth’s crust and of the movements occurring therein; this new idea is called the theory of the displacement of continents, or, more shortly, the displacement theory, since its most prominent component is the assumption of great horizontal drifting movements which the continental blocks underwent in the course of geological time and which presumably continue even to-day. ”

“ In short, from what we do know of the geological structure — 363 — of the continents in question, we can hardly doubt that they have passed through geographical revolutions of a like kind with those of Europe and North America. Everywhere over the great continental plateau elevation appears, in the long-run, to have been in excess of depression, so that, in spite of many subsidences, the tendency of the land throughout the world has been to extend its margins, and to become more and more consolidated. ”

“ If there were no compensating action in the differential movement of land and sea in the transitional area, the whole of the land would be gradually planed down to a submarine platform, and all the globe would be covered with water. There are, however, periodical warpings of this transitional area by which fresh areas of land are raised above sea-level, and fresh continental coast-lines produced, while the sea tends to sink more deeply into the great ocean basins, so that the continents slowly increase in size. ”

“ When one takes into consideration the fact that, notwithstanding many oscillations of level, the land during Cainozoic times was gradually extending, and the sea disappearing from wide regions which it had formerly — 365 — covered, one can hardly doubt that the seemingly gradual change from tropical to temperate conditions was due, in large measure, to that persistent continental growth. ”

“ Since the reconstruction itself is a very restricted one, on account of the well-marked course of the continental margins, and leaves no room for adjustment to fit this requirement, we are dealing with a quite independent criterion which is of the greatest value for the estimation of the accuracy of the displacement theory.
The Atlantic rift is broadest in the south, where it first broke open. Its width amounts here to 6220 km. ”

“ It is true it is possible that it has been passively carried along by a stream of sima. Africa and South America lie to-day on the equator, and undergo but slight meridional displacement. The great displacements which South America suffered in the Tertiary period, and which led to the uplifting of the South American Andes, were directed towards the north-west relatively to the position of the poles of that period, and likewise exhibits the drift from the poles. ”

“ To sum up, then, we may say that the chief agents concerned in the development of coast-lines are crustal movements, sedimentation, and marine erosion. All the main trends are the result of elevation and depression. Considerable geographical changes, however, have been brought about by the silting up of those shallow and sheltered seas which, in certain regions, overflow wide areas of the continental plateau. ”

“ The adherents to the doctrine of the bridging-continents have up to the present made no differentiation between the bridges over the oceans and those over the continental shelves. It must be especially emphasized that the displacement theory only deals with the question of land connection over the present-day deep sea areas for which new ideas are developed, whilst for the shelf-bridges, as the Bering Straits between North America and Siberia, the earlier view of submergence and re-emergence of dry land remains absolutely undisputed. ”

“ Small alterations of level, possibly as much as several hundred metres, such as should lead to the emergence or submergence of an area of continental shelf, are obviously possible, for example, in the case of the wandering of the poles due to the lag of the earth in its adjustment to the new ellipsoid of rotation. ”

“ Geologists, however, are beginning to doubt whether upheaval of the latter kind can affect a broad continental area. Probably, in most cases, the apparent elevation of continental regions is only negative. The land appears to have risen because the floor of the oceanic basin has become depressed. Even the smaller plateau-like elevations which occur within some continental regions may in a similar way owe their dominance to the sinking of contiguous regions. ”

“ Since the African rift valleys seem clearly to be the result of normal faulting resulting from extension of the crust, Heezen and Ewing conclude that the topography of the Mid-Atlantic Ridge is largely the result of normal faulting. Whether the forces are the result of horizontal extension or vertical uplift remains the most important unsolved problem in connection with the origin of the continental as well as the suboceanic rift-valley systems. ”

“ If their re-elevation were not compensated by an equivalent re-submergence in another place, the remaining much diminished ocean basins would not have nearly enough room for the total amount of water of the oceans. The re-elevation of those former continental bridges would raise the surface of the water so much that all the continents, old as well as new, with the exception of lofty mountains, would be completely flooded. In other words, the assumption of continental bridges does not lead to the desired end, which consists of a land connection between extended continents. ”

“ 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. ”

“ General Stability of Continents with Constant Change of Form.—It will be observed that the very same evidence which has been adduced to prove the general stability and permanence of our continental areas also goes to prove that they have been subjected to wonderful and repeated changes in detail. Every square mile of their surface has been again and again under water, sometimes a few hundred feet deep, sometimes perhaps several thousands. Lakes and inland seas have been formed, have been filled up with sediment, and been subsequently raised into hills or even mountains. ”

“ During the genial epochs that intervened between the three great glacial movements from the north, the continental area was open to incursion on either side, the climate of the Northern Hemisphere was everywhere equable, and the topographic conditions were such as to encourage migration along the Atlantic and Pacific shores of the Eurasian continent into the arctic peninsulas of the New World. As the arctic glaciers became more and more restricted to the northern regions, primeval men were probably able to hold their own in the continental area and migrate east and west across Eurasia. ”

“ It is true that at each oscillation of level the solid rocks composing the original continent suffer some fresh denudation, and do not remain unimpaired like the wooden and glass framework of the hour-glass, still the wear and tear suffered by the larger area exposed to subaërial denudation consists either of loose drift or of sedimentary strata, which were thrown down in seas near the land, and subsequently upraised, the same continents and oceanic basins remaining in existence all the while. ”

“ During that period the mainland seems to have been, to some extent at least, higher above the level of the sea than now; as we observe, on the shores of Great Britain, Norway, and Sweden, as well as on the eastern shores of North America, the polished surfaces dipping under the level of the ocean, which encroaches everywhere upon the erratics proper, effaces the polished surfaces, and remodels the glacial drift. During these periods, large terrestrial animals lived upon both continents, the fossil remains of which are found in the drift of Siberia, as well as of this continent. ”

“ North America, or Europe, if subjected to such an experience as the continental areas of the moon have passed through, would be, in proportion, worse wrecked than the most fearfully battered steel victim of a modern sea fight, and one can readily understand that, in such circumstances, those now beautiful and populous continents would exhibit, from a distance, scarcely any token of their present topographical features, to say nothing of any relics of their occupation by living creatures. ”

“ The continent, as one might call Greenland, does not shed the bulk of its central waters in fluid rivers, but discharges them to the ocean in solid, crystalline, slowly progressing streams. They flow, or rather march, with irresistible, mighty force, and far-resounding footsteps, crossing the shore line, a perpetual procession of block-like masses, flat or diversified with hill and hollow on the top, advancing upon the sea until too deeply immersed longer to resist the buoyant power and pressure of the surrounding waters, when they break upwards, and float suspended in the vast oceanic abyss. ”

“ In this way every part of a continent may again and again have sunk beneath the sea, and yet as a whole may never have ceased to exist as a continent or a vast continental archipelago. And, as subsidence will always be accompanied by deposition, of sediments from the adjacent land, piles of marine strata many thousand feet thick may have been formed in a sea which was never very deep, by means of a slow depression either continuous or intermittent, or through alternate subsidences and elevations, each of moderate amount. ”

“ Alternate Overflow and Emergence of Continents. In a broad way these agencies of elevation and of erosion have caused in their age-long struggle an alternation of periods of overflow and periods of continental emergence during geologic time. During the periods of overflow, great portions of the low-lying parts of the continents were submerged, and formed extensive but comparatively shallow seas. ”

“ The shelf break ranges from 50 to 80 fathoms. The continental-slope gradients range from 1:15 to 1:40 and are thus somewhat less steep than in the American sectors. Prominent benches are common at 300, 600, 850, 1200, and 1600 fathoms. The continental rise is well developed and is compound. The main contrast between the North African and American sectors is the greater width of the African continental rise. Off northeastern United States a line of isolated volcanic peaks cuts across the continental rise and abyssal plain. ”

“ Very little can be empirically determined regarding the causal connection of the phenomena of the formation of continents, or of the analogies and contrasts presented by their p 294 configuration. All that we know regarding this subject resolves itself into this one point, that the active cause is subterranean; that continents did not arise at once in the form they now present, but were, as we have already observed, increased by degrees by means of numerous oscillatory elevations and depressions of the soil, or were formed by the fusion of separate smaller continental masses. ”