Page:Encyclopædia Britannica, Ninth Edition, v. 18.djvu/137

PACIFIC OCEAN 121 connected with the shallow water surrounding the Low Archipelago and the Marquesas Islands (groups which are bisected by the 140th meridian of west longitude) and the Society Islands. If this be the case there is an almost continuous area of elevation stretching between Patagonia and Japan. It has been remarked that many of the sub merged plateaus of the Pacific have a south-east to north west trend. The " Challenger " examined the depth of the eastern half of the Pacific in 1875, along a line which extended fro n 33 1ST. lat. on the 160th meridian south-east to the Sandwich Islands, and then as nearly as possible along the 150th meridian to the Society Islands in 23 S. lat. From this point the course was again south-east to the 40th parallel of south latitude, which was followed eastward to the Patagonian coast, a visit to Juan Fernan dez forming a northward digression. The depth was ascertained at fifty points along this route, and it was found to vary on the whole from 2000 to 3000 fathoms. There were two soundings of over 3000 fathoms between latitudes 38 and 36 N., and one a little to the south of the Sandwich Islands. Between the meridian of 120 W. and the coast of America the soundings showed the depth to vary considerably as the ship was in deep water or over the submerged Patagonian plateau. The actual numbers observed proceeding eastward from 120 W. long, were in fathoms : 2250, 1600, 2025, 2270, 1500, 1825, 1775, 1375, 2160, 2225, 1450, 1325. The soundings made by the United States ship " Tuscarora " during 1874 were much more numerous, closer together, and extended along several lines, but the general result was similar to that of the " Challenger " observations. The results of all recent observations are shown on Plate III. The western half of the Pacific Ocean is a complete con trast to the eastern. Archipelagos and scattered islands are exceedingly numerous ; the depth of the ocean is by no means uniform, for shallows and areas of unusual depth occur scattered over it at irregular intervals. Along the Asiatic coast and between the island groups there are a number of partially enclosed seas, and these are separated from the great ocean by submarine plateaus of sufficient extent and height to warrant the supposition that a moderate upheaval would extend the Asiatic continent as far south as Australia, transforming the seas into inland salt lakes. Considerations of the peculiar animal and vegetable life of New Zealand and Australia lend some degree of probability to the speculation that these islands were joined to the main continent of Asia at some very remote period ; and it is even possible to trace the sub merged coast-line of the great continent which then existed. This line separates the very deep water of the West Pacific from the shallower water of the inland seas and archipel agos ; it runs from Kamchatka, over Japan, Formosa, the Philippines, New Guinea, to Australia and New Zealand. The most conspicuous peculiarity of the West Pacific is the very deep water lying in a crescent shape to the east of the Kurile Islands and Japan. It extends from 50 N. lat. to nearly 20 N. lat., although it is of no great breadth. The average depth of this area is nearly 4000 fathoms, and a narrow strip of still more abysmal depths runs along its western margin, like a ditch across the entrance to the Sea of Okhotsk : here the United States ship " Tuscarora " found depths of over 4600 fathoms. The course of the " Challenger " led her to explore the seas of the Eastern Archipelago pretty thoroughly, and she carried a line of soundings from the archipelago to Japan, and thence east ward across the Pacific, crossing the area of great depth about the centre, off Nippon, where two soundings of 3950 and 3*325 fathom? respectively were obtained. Like the East Pacific, the western division of the ocean has an average depth of from 2000 to 3000 fathoms, although a great number of small depressions exist where the depth is greater, and detached areas of shallower water occur still more frequently. Many of the islands rise from depths of about 3000 fathoms, forming isolated mountains springing from the bed of the ocean, and several peaks which do not rise to the surface have been detected. More usually a number of islands are bound together by submarine elevations, frequently within a few hundred fathoms of the surface, over wide areas. Although the greater part of the sea surrounding New Zealand, the north of Australia, and the adjacent islands is under 1000 fathoms in depth, there are areas of great depression amongst the islands, and some very deep channels. In 1875 when sounding in the channel between the Carolines and Ladrones, the "Challenger" met with the deepest water of the cruise, 4475 fathoms, or about five miles and a quarter ; and this is the greatest depth from which a specimen of the bottom has hitherto been obtained. This abysmal depth only extends over a relatively small area, for the two nearest " Challenger " stations, one to the north and one to the south, had depths of 2300 and 1850 fathoms respectively. The seas branching off from the Pacific are usually relatively shallow. Behring Sea on the north has ex tremely Shallow water in its north-eastern half, where there is a depth of under 100 fathoms ; in the south-western por tion the depth increases rapidly to between 1000 and 2000 fathoms, except round the coasts and the Aleutian Archi pelago. The Sea of Okhotsk is still shallower : much of it is within the 100 fathom line ; and in its deepest part it does not attain 1000 fathoms. The Yellow Sea is entirely within the hundred fathom line ; while the Sea of Japan, only separated from it by the Corean Peninsula, is not inferior in depth to the open ocean, its average depth being from 2000 to 3000 fathoms. The western portion of the Pacific, which lies between the Philippines and the Carolines and Ladrones, is also very deep, its mean depth approaching 3000 fathoms. This sea is of importance, since it is to the Pacific what the Gulf of Mexico is to the Atlantic the source of its great northern thermal current. The fact that the temperature at 1500 fathoms over the whole of the North Pacific does not differ by more than 5 F. from that at the bottom appears to indicate that this portion is cut off from the southern division by a ridge rising to within 1500 fathoms of the surface. The existence of such a barrier cannot be said to be proved, but the indications lead to the supposition that it may extend from Japan to the equator, through the Bonin, the Ladrone, and the Caroline Islands. Taken altogether, so far as present knowledge goes, the bed of the Pacific is more uniform than that of the Atlantic, and its changes of level are less abrupt. Its depth is, on an average, greater, and appears to be more evenly distributed than in the Atlantic, but this appar ent greater uniformity may be partly due to the fact that the latter ocean, both on account of its smaller size and its greater commercial importance, has been much more care fully surveyed, and its bathymetrical conditions more exactly ascertained. DEPOSITS. The explorations of the "Challenger," "Tuscarora," and other Deposit* surveying ships have in recent years given a great amount of information respecting the nature of the deposits now forming over the floor of the ocean, and the specimens collected by these expeditions have been made the subject of a careful investigation by Messrs Murray and Renard. The great extent and depth of the Pacific Ocean make it the most suitable field for the study of the varieties of deep-sea deposits and the conditions under which they are found. The various kinds of deposits, all of which are found in the Pacific Ocean, are classed as follows : XVIII. 1 6