Popular Science Monthly/Volume 36/December 1889/The Struggle of Sea and Land
|THE STRUGGLE OF SEA AND LAND.|
WE stand on a bluff at the sea-shore. The surf is undermining it. That deep cutting into the bank is its work. An over-hanging mass of earth is thrown down and becomes the toy of the waves, which reduce it to gravel. This in its turn becomes ammunition to be hurled against the shore. Wherever this process is going on, the land falls back before the advancing sea, and considerable results are evident in a short time. The Island of Heligoland has been reduced, within a thousand years, from a considerable island to a mere rock. The strings of rocky islands along many coasts are remnants of destroyed shore-land. Thus the land yields with hardly a struggle to the supremacy of the sea. Loose alluvial terrains give way in a body. The Zuyder Zee so came into being five hundred years ago, and Holland, part of which is below the level of the sea, would have been likewise overflown if it had not been defended by artificial dikes. Subsidences of ground have also been sometimes observed during earthquakes.
In other places the sea gives way. Rivers carry out masses of detritus and deposit them along the shores, causing the land to advance. By the operation of this process Roman ports on the eastern coast of Italy have been left away inland, and whole alluvial districts of the upper Italian plain have been wrested from the sea.
No doubt a very long time is required for important changes in the sea-lines to be produced by these processes; hence we must widen our view if we would find a solution of the problems which the history of the primitive seas offers us.
When the Alpine traveler finds sea-shells included in the rocks on high peaks, he says that the strata of the mountain are ocean deposits. In the great foldings of the rocks which he can follow along the naked cliffs, he recognizes the results of mighty movements which the strata have undergone since they were formed, and will correctly ascribe their elevation to the same movements. The mountain has been produced through a doubling up caused by a sidewise pressure. If he turns his steps to the adjacent lowlands, he will make the same observation in horizontal strata. He now has his choice whether to believe in an elevation of a large part of the earth's crust, or in a sinking of the level of the sea since the crust was formed. One of the most debated questions of geology turns on this point.
The successive layers of the earth's crust have sometimes been compared to the leaves of a book. We read in them a long passage in the earth's history written by the scribe Nature herself while the events were happening, and therefore even more trustworthy than the sources of ordinary history. Yet many pages of the book are obscure, and those of the first part are still waiting to be deciphered; for, in the sparkling leaves of the archaic crystalline rocks, the letters that should give us knowledge of the beginnings of life on the earth seem to have been washed out. The first volume, telling of the Palæozoic age, makes us acquainted with a lower fauna, principally marine, from which only the vertebrates are absent. Then cartilaginous fishes appear in the Silurian and Devonian, and land-inhabiting vertebrates, amphibia, and reptiles in the Carboniferous and Permian. The development of life goes on in the Mesozoic epoch. The oldest and lowest organized mammals, the marsupials, meet us in the upper Trias. The Jurassic gives us the first birds, curious creatures with teeth in their bills and lizards' tails bearing feathers. Two specimens have been found, in the Archæopteryx, of the transition form between the reptile and the bird. The Cretaceous furnishes the first bony fish and new toothed birds, the odontornithides. The Cenozoic age, the fourth and last volume of the great book, exhibits another advance in the development of animal life; and in the Tertiary the forerunners of the present mammals, and in the diluvial, man, appear. A similar process of development from lower to higher forms is shown in the vegetable world.
The story of this gradual rise of more and more highly organized beings is certainly the most important content of those stone books. But, besides that, they record that the firm lands arose out of the floods, that the sea washed over the land, left it, and covered it again, while the mass of the land constantly grew. The pages that sketch the covering of a stretch of earth by the sea are fully written up; but the periods of dry land are more frequently made known by a gap than by a continuance of the relation; and, in the latter case, the terrestrial deposits are only present when the spot has been covered by a river or a lake.
According to the best established and prevailing views, the great sea-beds, the deep basins, are the original features of the "face of the earth." They are the first depressions that were made after the planet's surface was solidified and its structure began to shrink from cooling. These depressions have probably been increasing vertically through all the geological ages, and therein lies the cause of the constant increase of the land to the present time. But the constancy of this increase is a fact only as a whole; for there have been times when the sea rose over the shore and overflowed a large part of the land. The last great transgression of this kind occurred at the beginning of the Miocene Tertiary, when extensive tracts of the Old World were covered with water. Parts of Italy, Portugal, southern France, northern Switzerland, southern and northern Germany, the Vienna basin, and the Hungarian plain, and of the lowlands of eastern Europe and the Black Sea region away into the interior of Asia to Persia, with the plains of North Africa still bear the marine deposits of that period, with the remains of organisms, mostly of extinct species, that inhabited the adjoining seas. The waters retired to their beds during the Pliocene, and the present boundary-lines of the land regions, aside from a few erosions, were shaped during the same period. Similar processes took place repeatedly in the earlier periods of the earth's growth, and to them are ascribed most of the changes that have taken place in the species of animals and plants; for these encroachments of the sea forced the living world into a narrower space, and entailed a fierce struggle for existence, in which the less valiant species succumbed. The retreat of the sea again permitted a fruitful development of life and the origination of new species.
An important circumstance has been brought to light in the investigation of the fluctuations of the ocean. The continents have been overflowed several times. Suess, who has made the most thorough study of the subject, has recognized six principal periods of submersion, and as many of dryness. But no indubitably deep-sea deposits later than the Silurian are to be found on the present continents. The great sea-beds are primitive; and primitive likewise are the socles of the continents, standing as equally sturdy champions with the briny flood, sometimes in the alternations of the contest lying under it, overcome by the sea, at other times shaking it off and sending it back within its lines.
The cause of these processes is still awaiting explanation. Celsius and Linnæus, who observed the gain of land on the Baltic coast during the last century, expressed the opinion that the sea was retreating. This view was contested at the beginning of the present century by Leopold von Buch, who thought that Scandinavia was rising. Lyell and Darwin advanced the theory of the rising and sinking of continents, and this solution of the problem has not been contradicted till very recently. There is a kind of suggestion with which great men, to whose minds the world pays deference, inspire their contemporaries when they give any view the weight of their approval, which is at the same time detrimental to progress in science. In this way many an error has been generally accepted without further proof.
Suess does not ask for an unjudicial acceptance of his theory, but has published the whole course of his investigations, with his proofs, in a great work, the "Antlitz der Erde" ("The Face of the Earth"), in which he has examined the signs of changes in the level of the ocean, so far as they have been observed in all known parts of the earth and through all the geological periods. His exposition points to a synchronism of overflows and uncoverings of the land over extensive regions. This result has impelled him to oppose the prevailing doctrine of upheavals and depressions of the land. Aside from the fact that the supposed elevation of the continents is problematical in itself, such movements could not go on over the whole earth at the same time and in the same direction. Changes in the level of the waters, on the other hand, would be of the general character which the survey of the phenomena indicates, for a free rising of the water, even under local influences, would at once make itself felt over the whole surface of the earth.
Suess's studies of the causes of the rising and falling of the waters brought him to the following conclusions: The ocean beds were produced by the sinking of those parts of the earth's surface that correspond with them. The uneven shrinking of the globe is a consequence of its continuous cooling. Every new subsidence of the sea-bottom causes a falling of the water. Elevations of the ground take place too. The bottom of the ocean is incessantly receiving detritus from the overflowed land, of which the water brings down as much as it can hold; this tends to raise the level of the ocean. Yet Suess concludes that these processes are not adequate to explain the full measure of the primitive movements, and reserves judgment on that point.
The present author has gone further into this subject, in an article in the "Zeitschrift für wissenschaftliche Geographie," although he has not concealed the existing difficulty. Unfortunately, Suess's deductions were not before him when he prepared his paper. That essay, building in part on similar researches, accepts contraction as the sufficient cause for the fluctuations of the sea. According to the now prevailing views, which have, however, been very recently contradicted by investigators of repute, the constant loss of heat from the interior of the earth produces a steady shrinkage of the globe. From time to time the tensions in the outer arches of the earth are relaxed, and the crust sinks. In that case, three views are conceivable respecting the relations of land and sea: either both parts sink alike, or the land sinks more than the sea, or the sea than the land. In the first case, the rise of the sea will be only that caused by the contraction of its bed, which, being dependent on the slighter contraction of the surface of the globe, is of relatively little importance; in the second case, the sea would spread over the lower lands, rising to appearance, while in fact its level has diminished on account of the wider diffusion of its waters; or, in the third case, the sea would retire on account of the falling of its level.
These things alone could hardly have produced the observed results, had there not been important circumstances associated with them which in times of quiet worked in their favor. The land is being uninterruptedly swept off, and is gradually as a whole becoming lower and lower from the top. The rock falls from the heights, the brooks and rivers take fragments in their course and drag the chips of weathering and of their own planing-work into the sea. The land is reduced by denudation, the sea rises by the action of the deposits on its bottom. Extensive lowlands are at last formed as the result of the washings, so that large districts may be put under water by slight shrinkings of the land. When these are overflowed by the sea, the deposits rise on its bottom, stratum by stratum, till, after a long time, they nearly reach the level of the water. Then a slight sinking of the sea-bottom suffices to reduce the water to its old level; and the alternation of a washing away of the soil and the conversion of the overflowed territory into dry land begins anew.—Translated for the Popular Science Monthly from Das Ausland.