Page:Africa by Élisée Reclus, Volume 3.djvu/23

 EELl'Er, GEOLOGY, AND TEMPEEATUEE OF THE AZOEIAN BASIN. 7 The systematic exploration of the Azorian Atlantic is still far from complete, some of the recorded soundings occurring only at intervals of several hundred miles. The only section of the ocean whose relief has been accurately determined is the plateau on which have been laid the telegraphic cables between West Europe and the United States. Further south, ^he reports of vessels specially equipped for scientific expeditions are disconnected, and separated from each other by extensive unexplored spaces. The Challenger^ the Magenta^ and long before them, the Venus, traversed the waters between the Azores and the coast of Brazil in an oblique direction ; the Gazelle, the Saratoga, and the Dolphin visited the eastern section between Madeira and the Cape Verd Islands ; the Talisman and the Gettysburg confined their operations mainly to the vicinity of the archipelagoes ; while the soundings of the Silvertown were made only for the purpose of laying the cable between the Cape Yerd group and the Portuguese possessions on the neighbouring mainland. Off the American seaboard soundings have also been recorded by the Blake and several other vessels between Newfoundland and the Bermudas, and thence to Florida and the Bahamas. But from these isolated records it is impossible to prepare a complete oceanic chart, most of the bathymetric curves having still to be filled in on more or less plausible conjectures. Hence the great discrepancies in the published charts, which are, nevertheless, all based on the materials supplied by the same soundings. Fresh researches will be needed to gradually remove the unknown elements, and at some points new observations have already been begun, for the purpose of verifying or correcting former records. Thus the section between the Cape Yerd and Bissagos groups has been twice explored, the more careful soundings, made with improved appliances, revealing greater depths than those previously registered. In the same way the Talisman has corrected several of the figures supplied by the Challenger. Before the introduction of the new registering plummet, there was always a danger of the line running out indefinitely without indicating the bottom ; hence the exag- gerated depths reported, amongst others, by Denham and Parker in the Brazilian waters. At the same time the more sensitive modern apparatus is liable to the opposite danger of under-estimating the real depth, by recording the shocks pro- duced, not by contact with the bed of the sea, but by casual friction, the lurching of the vesselj a passing fish, and the like. Relief, GeologYj and Temperature of the Azorian Basin. The mean depth, calculated by Frummel for the whole depression of the Atlantic, would appear to be about 2,000 fathoms, which is probably somewhat less than that of the Azorian basin. If the Azores with their western submarine con- tinuation constitute a transverse ridge in mid- Atlantic, the prolonged axis of these partly upheaved partly still flooded elevated lands will indicate one of the deepest abysses hitherto discovered in the Atlantic. This abyss lies to the south of the New- foundland bank, where a sudden subsidence of considerably over 3,000 fathoms would still leave a vast marine basin filled with water. Another great cavity occurs in the