Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/29

Rh lies have been chief among the regions of the earth s crust that have used the pent-up force in the contracting sphere to carry forward the continental developments. If this was true of the North American continent, the same in principle was law for all continents.&quot; 1

Dimensions of the Atlantic. The length of the Atlantic basin, considered as extending from the Arctic to the Antarctic circle, is nearly 8000 geographical miles. The nearest approach of its boundaries is between Greenland and Norway, whose coasts are only about 800 miles apart. They thence recede from each other towards the south, as far as the parallel of 30 N. lat., where, between the peninsula of Florida and the western coast of Marocco, there is an interval of 70 of longitude, or about 3600 geographical miles. The channel then rapidly narrows as it passes southward, so that between Cape St Roque in Brazil (5 S. lat.) and the coast of Sierra Leone (between 5 and 8 N. lat.) the African and American continents approach within 1500 miles of each other. The sudden eastward recession of the African coast as it approaches the equator, and the westward trend of the South American coast-line between Cape St Roque and Cape Horn, widen out the South Atlantic basin to the same breadth as that of the North Atlantic in the parallel of 30 N., the interval between the Cape of Good Hope and the estuary of La Plata, in the parallel of 35 S., being no less than 73|- of longitude, or about 3GOO geographical miles. The depth of the North Atlantic has been more care fully and systematically examined than that of any other oceanic basin ; and the general contours of its undulating sea-bed may now be regarded as pretty well determined. Putting aside the older soundings as utterly untrust worthy, and accepting only those taken by the modern methods, whose reliability has been amply tested by the accordance of diversified experiences, we can now assert with confidence that scarcely any portion of its floor has a depth exceeding 3000 fathoms, or about 3 &quot;4 miles, the greatest depth determined by the recent &quot;Challenger&quot; sound ings, which was that of a limited depression about a hundred miles to the north of St Thomas, having been 3875 fathoms, or about 4 4 miles. Except in the neigh bourhood of its coast-lines, and in certain shallower areas to be presently specified, the floor of the basin at its widest part seems to lie at a depth of from 2000 to 3000 fathoms, its slopes being extremely gradual. The central portion of the principal basin of the North Atlantic, however, is occupied by a plateau of irregular shape, of which a considerable part lies at a less depth than 2000 fathoms. Of this plateau the Azores may be regarded as the culmination ; and that group being taken as its centre, it may be said to extend to the north as far as lat. 50, and to the south-west as far as the tropic of Cancer. The northern extension of this plateau narrows out into a sort of isthmus, which connects it with the plateau that occupies a great part of the Atlantic basin to the north of 50 N. lat. ; and it is across this isthmus, and along the bottom of the deep narrow valley on either side of it, that the telegraph cables are laid between Ireland and Newfound land. Whether its south-western prolongation, known as the &quot;Dolphin Rise&quot; (fig. 1, infra) extends to the equator, so as to become continuous with the elevated area which cul minates in St Paul s rocks, and by a further southward ex tension becomes continuous either with the volcanic elevation of St Helena and Ascension Island, or with the elevation in the middle of the South Atlantic which culminates in the island of Tristan da Cunha (fig. 2), has not yet been ascertained. According to the view already suggested as to the formation of the Atlantic basin, the plateau might &quot; On some Results of the Earth s Contraction trom Cooling,&quot; in Amer. Journ. of Science, June 1873. be regarded as representing the original sea-bed (from which the Azores have been lifted up by volcanic action), whilst the deep valleys on either side of it are &quot; areas of subsidence &quot; answering to the &quot; areas of elevation &quot; of the land that borders them. Generally speaking, the depths of these valleys increase pretty rapidly with the distance from the shore-line, so that th contour-lines of one and two miles follow the shore-lines pretty closely. But there are two localities in which shallow water extends to a much greater distance from land than it appears to do elsewhere. One of these lies in the neighbourhood of the British Isles. For a dis tance of about 230 miles to the westward of Ireland there is a slope of only about 6 feet in a mile ; but in the next 20 miles there is a fall of 9000 feet, after which there is little change of level for 1200 miles. Hence as the depth of the sea immediately surrounding the British Isles is nowhere 100 fathoms (so that an elevation of their whole area to that amount would unite these islands not only to each other but also to the continent of Europe), it is obvious that the platform on which they rest is really, although now submerged, a part of the land-mass of Europe. Another of these extensive shallows is that of which the Banks of Newfoundland form the highest part ; and of the existence of this a probable explanation may be found in the accumulation of the rock-masses that are brought down by icebergs every summer from the coasts of Greenland and Labrador. For it is now generally admitted that these icebergs are really parts of glaciers, that were originally formed on the mountain-slopes of Greenland and Labrador, and then descended valleys which open out on their coasts, so as, on arriving at the mouths of these valleys, to detach themselves and float away, being borne southwards by the Polar Current to be presently described. Most Arctic icebergs of which a near view can be obtained are observed to have upon them a considerable number of pieces of rock, sometimes of a very considerable size ; and. these are of course deposited on the sea-bed when the icebergs melt (which they usually do on the borders of the Gulf Stream), thus forming a vast conglomerate bed, to which parallels are not improbably to be found in various geological epochs.

Geological Age of the Atlantic Basin. Guided by the principle that great oceanic basins are to be considered rather as original marine areas that have been limited by the elevation of their boundaries, than as having been formed by the excavation of terrestrial areas, we have to inquire what evidence there is that the basin of the Atlantic has undergone any considerable change within a compara tively recent period. As has been pointed out by Prof. Wyville Thomson (Depths of the Sea, p. 473), it is difficult to show that any oscillations have occurred in the north of Europe since the termination of the Secondary period, to a greater extent than from 4000 to 5000 feet, this being the extreme vertical depth between the base of the Tertiaries and the highest point at which Tertiary or post-Tertiary shells are found on the slopes and ridges of mountains. Such oscillations, while considerably modifying the boundaries of the Atlantic, would not seriously affect the condition of the deeper parts of its sea-bed ; and hence it may be concluded that the two deep valleys, one on the European side of the modern volcanic platform of the Azores, and the other on the American, each having a width of 600 or 700 miles, and an average depth of 15,000 feet, could neither have been formed by such oscillations, nor could, when once formed, have been converted into dry land. It will be presently shown that this idea of the existence of an Atlantic basin correspond ing generally to that now existing, as far back as the later Secondary period, is strongly supported by the evidence III. 3