Page:Africa by Élisée Reclus, Volume 3.djvu/24

8 WEST AFEICA. almost immediate vicinity of the West India Islands, where, about 90 miles north of St. Thomas, the Challenger reported nearly 4,000 fathoms, supposed to be the greatest depth till the Blake recorded 4,350 fathoms some sixty miles farther west. In the Cape Verde waters also, and even between that archipelago and the African mainland, extensive tracts occur with 2,700 fathoms and upwards. Altogether the Azorian Atlantic presents the form of a double valley, one skirt- ing the African the other the American seaboard, with a long dividing ridge, which runs to the south-west of the Azores in the direction of Guiana. This " Dolphin's Back," as it is called by English geographers, would appear to be prolonged in the southern Atlantic by the so-called " Junction Back," in the direction of a third ridge which takes the name of the Challenger. But the recorded soundings are not yet sufficiently numerous to determine this point, although the connecting ridge is already indicated on most bathymetric charts of the ocean. Except near the islands, where coral beds occur, the matter brought up from the bottom during the sounding operations consists mainly of mud presenting little diversity of character. In the shallower sections it contains the remains of globi- gerines and other animalculae ; but in the abysses of over 2,000 fathoms the frag- ments of shells are so minutely ground and altered by the enormous pressure, that it becomes impossible to detect the mingled remains of organisms in this impalpable substance, whose composition is similar to that of chalk. At still lower depths the characteristic deposit is a sort of red clay. The naturalists of the Talisman have noticed three perfectly distinct colours : a reddish yellow on the Morocco coast, a green in the neighbourhood of Senegal, and a white mud round about the Azores. Thus are being formed strata analogous to those of the upheaved rocks belonging to the successive geological epochs of the earth's crust. Most of these muddy de- posits on the bed of the Azorian Atlantic contain volcanic elements, especially pumice, which must have come from the centres of explosion in the archipelagos, as they are met in larger quantity round about the islands containing active craters. Besides these products of eruptive origin, the Talisman has fished up from depths of 2,000 or 3,000 fathoms specimens of other rocks, such as granite, gneiss, schists, sandstones and limestones. The cavities of these rocks were for the most part filled with a bluish mud composed chiefly of globigerines. In these tropical seas, as in the northern oceanic waters, the temperature falls with great uniformity. On the surface the water, exposed to the incessantly changing influence of the seasons and atmospheric currents, undergoes corresponding changes of temperature, being alternately cooled by the north-eastern trade-winds and warmed by the land breezes. In the Azorian Atlantic the mean annual tem- perature oscillates within a range of 33° F., although at times rising to 38° or even 42°, and falling to 27° and under. But the action of external climatic influences diminishes rapidly under the surface, and at a depth of 400 feet the water ceases to be affected by the alternating hot and cold atmospheric changes. Within this thin surface layer the temperature falls with the greatest rapidity, so that 200 miles to the south of the Cape Verd Islands the thermometer indicating 77° F. at the surface falls to 53° at a depth of 300 feet. Lower down the fall is extremely