Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/30

Rh recently obtained of the continuity of animal life on the Atlantic sea-bed from the Cretaceous epoch to the present time. Important information as to the changes which the sea bed of the Atlantic has undergone within the later geolo gical periods, may be gathered from the structure of the islands which lift themselves above its surface. Along its eastern border, at no considerable distance from the coast of North Africa, there are three principal groups, the Madeiras, Canaries, and Cape Verd, all of which have an evidently volcanic origin, and rise up from the eastern slope of the basin, where it is progressively shallowing towards its continental shore-line. Further out, in mid- ocean, lies the group of the Azores, which also is volcanic, and rises from the plateau already spoken of ; but between this area and the slope from which the Madeiras and Canaries are based is a very deep channel, ranging down wards to at least 15,000 feet; and a like depth is also found between the Azores and the coast of Portugal. The structure of all these groups of islands gives obvious indications of their formation by separate igneous eruptions in a sea of great depth ; and the earliest of these eruptions seems to have taken place in the later Miocene period. As soon as the first solid lavas raised their heads above water, and were thus exposed to the action of the waves, fragments were detached and rounded on the shore ; and these being swept off, with the debris resulting from their attrition, formed deposits of various kinds upon the slope of the cone, in which corals, shells, &amp;lt;tc., were embedded. These fossiliferous deposits have been subse quently elevated to heights of from 1500 to 2000 feet above the level of the sea, showing a rise of the base of the craters; progressive additions have been made to their upper part by the piling up of basaltic andtrachytic lavas. 1 That this state of activity still continues is proved by the fact that in 1811 a new island was temporarily formed in the Azores group, off St Michael, by the throwing-up of ashes, and the formation of a cone about 300 feet high, with a crater in the centre. This island, to which the name Sabrina was given, was soon washed away by the waves. And only a few years since, another submarine eruption in this neighbourhood was indicated by earth quakes, jets of steam and columns of smoke, and floating masses of scoriae. All these considerations concur (as Sir Charles Lyell, loc. cit., justly urges) to negative on geolo gical grounds the hypothesis which has been advocated by some eminent naturalists, that the Azores, Madeiras, and Canaries are the last remaining fragments of a continuous area of land which once connected them with the west of Europe and North Africa. Proceeding to the south of the equator, we meet with similar evidence of volcanic activity in the structure of the only two islands, Ascension and St Helena, which lie near the line stretching from the Cape Verd group to the Cape of Good Hope ; and these also arise from a plateau of considerably less depth than the circumjacent area whose eastern slope gradually shallows to the coast of South Africa. This plateau stretches in a north-westerly direction towards the equator, so as to meet it in from 20 to 22 W. long.; and here indications of volcanic activity earthquakes, troubled water, floating scoriae, and columns of smoke have been several times observed since the middle of the last century, betokening the probable formation of an island or an archipelago in that locality. Nearly midway between the southern prolongations of the African and American continents, the solitary peak of Tristan da Cunha (fig. 2) lifts itself above the ocean : this also is volcanic, and seems to rise from a broad base 1 See Sir C. Lyell s account of them in his Principles of Geology, llth ed. p. 407, 22- of general elevation, resembling the plateau of the North Atlantic. The entire chain of the Greater and Lesser Antilles, which stretches from the delta of the Orinoco to the peninsula of Florida, and forms the eastern boundary of the Caribbean Sea, seems to have been in like manner elevated by vol canic action. That this elevation, like that of the groups of islands on the eastern side of the Atlantic, took place for the most part during the later Tertiary period, is shown by the occurrence of shells, corals, &c., of upper Miocene age, in the upraised sedimentary beds of several of the islands ; while the presence of &quot; fringing reefs &quot; of coral around the shores of many of the West India islands is an indication that they lie in an area in which elevation is still proceeding. The channels by which they are separated are so deep as to render it very unlikely that there was ever a continuity of land between them ; and the occa sional recurrence of earthquakes and volcanic eruptions at different points of this &quot; line of fire,&quot; shows that the plutonic action by which the islands were raised is still going on beneath. The case is very different, however, in regard to the Bermuda group, which constitutes a singular exception to the general fact of the absence in the Atlantic of those coral islands that are so numerous in the Pacific. This group consists of about 300 islands, of which, however, only five are of any considerable size ; and these rise from a shoal or platform of about 23 miles long by 13 miles broad, the channels between the islands being very shallow, while at a small distance from the edge of the shoal, the bottom rapidly deepens to 15,000 feet. The islands are entirely composed of upraised beds of coral, shells, &c. (the highest elevation being only about 180 feet above the sea-level) ; and the shoal itself appears to have the like structure throughout, no traces of any other rock than a limestone formed by the metamorphoses of coral being anywhere met with. Hence, as this insular platform proves to be the summit of a submarine column of 15,000 feet high, rising from a very small base, and as nothing we know of the structure of mountains volcanic or other would justify us in supposing that a column of such a height could be formed in any other way than by coral growth, the structure of the Bermuda group would seem to indicate a progressive subsidence of the bed of this part of the Atlantic during its formation, corresponding to that which (according to the well-known views of Mr Darwin) is at present in progress over a large area of the Pacific. It is probable that this coral growth was deter mined in the first instance by the existence of a submarine mountain, of which the summit lay near the surface, or lifted itself above it ; that as soon as this came to be submerged, the coral formation commenced ; and that by its continued growth at the summit, at a rate equal to that of the subsidence of its base, the platform has been kept up to the sea-level. The slight elevation which has raised its highest portion above that level may not impro bably have taken place in connection with the much larger recent elevations already referred to. Thus, then, we have evidence of considerable recent local modifications in the level of the Atlantic sea-bed, without any such change as would affect its general cha racter as an ocean basin ; while all geological probability seems in favour of the remoteness of the principal depres sion of the Atlantic area, even if we do. not regard it as dating back to the period when the surface of the globe was first undergoing solidification.

Currents of the Atlantic. By the term &quot; current&quot; will be here meant that sensible movement of ocean water in particular directions which can be generally traced, directly or indirectly, to the action of wind upon its surface. A

