Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/28

Rh whilst, on the other hand, the projection of the American coast at Newfoundland answers to the Bay of Biscay. Further south, the great rounded prominence of Northern Africa corresponds with the vast bay that stretches from Nova Scotia to St Thomas ; whilst the angular projection of South America towards the east corresponds with that receding portion of the mid-African coast-line which is known as the Gulf of Guinea. This correspondence suggested to Humboldt the idea that the Atlantic basin was originally excavated by a very violent rush of water from the south, which, being repulsed by the mountain ranges of Brazil, was directed by them towards the coast of Africa, and formed the Gulf of Guinea; being there checked and turned to the west by the mountains of Upper Guinea, the stream excavated the Caribbean Sea and the Gulf of Mexico ; and issuing thence, it ran between the mountains of North America and Western Europe, until it gradually diminished in velocity and force, and at length subsided. Another writer speaks of the basin of the Atlantic as an immense rift, made by some terrible force, which rent the surface-land asunder, but left the edges of the ravine to show by their form that they had once been connected. For neither of these specula tions, however, is there the smallest foundation in fact. What has to be accounted for, indeed, in regard to either of the great areas at present covered by water, is not so much the excavation of its sea-bed, as its segregation from an ocean originally universal by the boundaries that now enclose it ; in other words, not so much the depression of the bottom of its basin as the elevation of its sides. Not only is the proportion of the laud-surface of the globe to its water- surface scarcely more than one-third (being as 1 to 2 78), but the entire mass of the land which thus covers little more than one-fourth of the surface of the globe is quite insignificant in comparison with that of the water which covers the remaining three-fourths. For whilst the average elevation of the whole land is certainly less than one-fifth of a mile, giving from 9 to 10 millions of cubic miles as the total mass of land that rises above the sea-level, the average depth of the sea (so far as at present known) may be taken at about 2 miles, giving a total of nearly 290 millions of cubic miles of water, which is therefore about thirty times the mass of the land. From the computation of Keith Johnston, it appears that, &quot; if we conceive an equalising line, which, passing around the globe, would leave a mass of the earth s crust above it, just sufficient to fill up the hollow which would be left below it, this line would then fall nearly a mile below the present level of the sea.&quot; This is tantamount to saying that, if the solid crust of the earth could be conceived to be smoothed down to one uniform level, its entire surface would be covered with water to the depth of about a mile. Hence it is obvious that as the elevation of that crust into land over certain areas must be accompanied by a corresponding depression of the sea-bed over other areas, such depression, augmenting in those areas the previous depth of the aqueous covering of the globe, would be quite sufficient to account for the existence of the great oceanic basins, without any excavating action. And a confirmation of this view is found in the fact, ascertained by recent soundings, that the deepest local depressions of the sea-bed are met with in the neighbourhood of islands that have been raised by volcanic agency. Further, as the quantity of solid mat ter that must have been removed (on Humboldt s hypo thesis) in the excavation of the Atlantic valley must have been nearly four times as great as that which forms the whole known land of the globe, and as it is impos sible to conceive of any mode in which such a mass can have been disposed of, we may dismiss that hypothesis as not only untenable in regard to the Atlantic basin, but as equally inapplicable to any other valley of similar width and depth. 1 The general direction of geological opinion, indeed, has of late been, on physical grounds, towards the high anti quity of the great oceanic basins, not exactly as at present bounded, but as areas of depression having the same rela tion as they have now to the areas of elevation which form the great continents. Thus Sir Charles Lyell was strongly impressed by the fact that the mean depth of the sea is not improbably fifteen times as great as the mean height of the land ; and that depressions of the sea-bottom to a depth of three miles or more extend over wide areas, whilst elevations of the land to similar height are confined to a few peaks and narrow ridges. Hence, he remarked, &quot; while the effect of vertical movements equalling 1000 feet in both directions, upward and downward, is to cause a vast transposition of land and sea in those areas which are now continental, and adjoining to which there is much sea not exceeding 1000 feet in depth, movements of equal amount would have no tendency to produce a sensible alteration in the Atlantic or Pacific Oceans, or to cause the oceanic and continental areas to change places. Depressions of 1000 feet would submerge large areas of the existing land; but fifteen times as much movement would be required to convert such land into an ocean of average depth, or to cause an ocean three miles deep to replace any one of the existing continents.&quot; 2 And Professor Dana, who, more than any other geologist, has studied the structure of the existing continents and the succession of changes concerned in their elevation, has been led, by the consideration of the probable direction of the forces by which that elevation was effected, to conclude that the defining of the present con tinental and oceanic areas began with the commencement of the solidification of the earth s crust. &quot; The continental areas are the areas of least contraction, and the oceanic basins those of the greatest, the former having earliest had a solid crust. After the continental part was thus stiffened, and rendered comparatively unyielding, the oceanic part went on cooling, solidifying, and contracting throughout ; consequently, it became depressed, with the sides of the depression somewhat abrupt. The formation of the oceanic basins and continental areas was thus due to unequal radial contraction. &quot; In the opinion of Professor Dana, there has never been any essential change in the relations of these great features. &quot;It is hardly possible,&quot; he says, &quot;to conceive of any conditions of the contracting forces that should have allowed of the continents and oceans in after time changing places, or of oceans, as deep nearly as exist ing oceans, being made where are now the continental areas; although it is a necessary incident to the system of things that the continental plateaus should have varied greatly in their outline and outer limits, and perhaps thousands of feet in the depths of some portions of the overlying seas, and also that the oceans should have varied in the extent of their lands.&quot; ...&quot; The early defining, even in Archsean times, of the final features of North America, ;nid the con formity to one system visibly marked out in every event through the whole history in the positions of its outlines and the formations of its rocks, in the character of its oscillations, and the courses of the mountains from time to time raised sustain the statement that the American con tinent is a regular growth. The same facts also make it evident that the oceanic areas between which the continent 1 The case of such a shallow trough as that of the English Channel, of the former continuity of whose sides there is ample evidence, whilst its bottom is nowhere 500 feet beneath the surface, is obviously alto gether different. The extraordinary depth of the Mediterranean basin, on the other hand, affords strong reason for regarding it as, like the Atlantic, a portion of the original area of depression, cirsumscribed by the elevation of its borders. 2 Principles of Geology, llth ed. vol. i. p. 2G9. 