Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/302

Rh 288 given part of a coast-line must depend either upon the ch-tracter of the shore-rocks which at that locality are broken up by the waves, or upon the set of the shore-cur- rents aml the kind of detritus they bear with them. Coasts exposed to heavy surf, especially where of a rocky charac- ter, are apt to present beaches of coarse shingle between their projecting promontories. Sheltered bays, on the other hand, where wave action is comparatively feeble, afford a gathering ground for ﬁne sediment such as sand and mud. Estuaries and inlets into which rivers enter frequently show wide muddy ﬂats at low water; The mud brought down by the fresh water is allowed to sink to the bottom when the motion of the current is checked as it enters the sea. (2.) Infra-Littoral and Deeper-Water Deposits.—These extend from below low-water mark to a depth of sometimes as much as 2000 fathoms, and reach a distance from land varying up to 200 miles or even more. Near land, and in comparatively shallow water, they consist of banks or sheets of sand more rarely mixed with gravel. The bottom of the North Sea, for example, which between Britain and the continent of Europe lies at a depth never reaching 100 fathoms, is irregularly marked by long ridges of sand en- closing here and there hollows where mud has been deposited. In the English Channel large banks of gravel extend through the Straits of Dover as far as the entrance to the North Sea. These features seem to indicate the line of the chief mud-bearing streams from the land, and the general disposition of currents and eddies in the sea which covers that region, the gravel ridges marking the tracks of the more rapidly moving currents, while the muddy hollows point to the eddies where the ﬁne sediment is permitted to settle on the bottom. It is possible, how- ever, that the inequalities on the ﬂoor of the North Sea, and their peculiarities of sediment, may not be due wholly to modern accumulations, but partly to the contour of the ground before it was submerged and the land connexion between Britain and Europe was destroyed During the course of the voyage of the “Challenger,” the approach to land could always be foretold from the character of the bottom, even at distances of 150 and 200 miles from land. The deposits were found to consist of blue and green muds derived from the degradation of older crystalline rocks. At depths of 100 to 700 fathoms they are often coloured green by glauconite. At greater depths they consist of blue or dark slate-coloured mud with a thin upper layer of red or brown. Throughout these land-derived sediments particles of mica, quartz, and other minerals are distributed, the materials becoming coarser towards land. Pieces of wood, portions of fr11its, and leaves of trees occur in them, and further indicate the reality of the transport of material from the land. Shells of pteropods, larval gastero- pods, and lamellibranchs are tolerably abundant in these muds, with many infra-littoral species of Foran2z'n2_'fera, and diatoms. Below 1500 or 1700 fathoms pteropod shells seldom appear, while at 3000 fathoms hardly a foraminifer or any calcareous organism remains (Hurray, Proc. Roy. Soc. Edin., 1876, p. 519). Round volcanic islands the bottom is found to be covered with grey mud and sand derived from the degradation of volcanic rocks. These deposits can be traced to great distances, as at Hawaii for 200 miles or more. Pieces of pumice, scoriae, &c.-, occur in them, mingled with marine organisms, and more particularly with abundant grains, incrustations, and nodules of an earthy peroxide of manganese. Near coral-reefs the sea-ﬂoor is coated with a white calcareous mud derived from the abrasion of the coral. The east coast of South America supplies a peculiar red mud which is spread over the Atlantic slope down to depths of more than 2000 fathoms. (6.) Ab_2/ssal.—l’-assing over at present the organic de- posits which form so characteristic a feature on the ﬂoor GEOLOGY [11I. DY.'AMI(‘A 1.. of the deeper aml more open parts of the ocean, we come to certain red and grey clays found at depths of more than 2000 fathoms down to the bottoms of the deepest abysses. These consist of exceedingly tine clay, coloured sometimes red by iron-oxide, sometimes of a chocolate tint from manganese oxide, with grains of quartz, mica, pumice, scoriae, peroxide of manganese, and other mineral substances, together with 1"ora7m'm:fer((, and in some regions a large proportion of siliceous 1.'ucIio[arz'a. Mr Murray has shown the high probability that these clays result from the decom- position of pun1ice and fine volcanic dust transported from volcanic islands into mid-ocean. The extreme slowness of their deposit is strikingly brought out in the tracts farthest. removed from land. From these localities great nuinbers of sharks’ teeth, with ear—bones and other bones of whales. were dredged up in the “ Challenger” expedition,—some of them quite fresh, others partially crusted with peroxide of manganese, and some completely and thickly surrounded by that substance. We cannot suppose that sharks aml whales so abound in the sea as to cover the floor of the ocean with a continuous stratum of their remains. No doubt each haul of the dredge which brought up so many bones represented the droppings of many generations. The successive stages of manganese incrustation point to a long, slow, undisturbed period, when so little sediment accumu- lated that the bones dropped at the beginning remained at the end still uncovered, or only so slightly covered as to be easily scraped up by the dredge. In these deposits, moreover, Mr Murray has found numerous minute spherul-.1 r particles of metallic iron which there is every reason to believe are of cosmic origin——portions of the dust of meteo- rites which in the course of ages have fallen upon the sea- bottom. Such particles no doubt fall all over the ocean; but it is only on those parts of the bottom which, by their distance from any land, receive accessions of deposit with extreme slowness, and where therefore the present surface may contain the dust of a long succession of years, that it has been possible to detect them. The abundant deposit of peroxide of manganese over the ﬂoor of the deep sea is one of the most singular features of recent discovery. It occurs as an earthy incrustation round bits of pumice, bones, and other objects. The nodules possess a concentric arrangement of lines not unlike those of urinary calculi. That they are formed on the spot, and not drifted from a distance, was made abundantly clear from their containing abyssal organisms, and enclosing more or less of the surrounding bottom, whatever its nature might happen to be. "Mr Murray refers their origin to the decom- position of the manganese—bearing minerals in the univer- sally ditfused volcanic detritus. Quite recently Mr J. Y. Buchanan has dredged similar manganese concrctions from some of the deeper parts of Loch Fyne. In connexion with the chemical reactions indicated by these nodules as taking place on the sea-bottom, reference may be made to a still more remarkable but yet unpublished discovery made by Mr Murray in the course of his examinations of the materials brought up from the same abyssal deposits. He has detected abundant minute concrctions or bundles of crystals which on analysis are found to resemble olivine in composition. These silicates (there may be several of them) have certainly been formed directly on the sea-bottom, for they are found gathered round abyssal organisms. It is difficult to overestimate the importance of this fact in re- ference to the chemistry of marine deposits. From a comparison of the results of the dredgings made in recent years in all parts of the oceans, it is impossible to resist the conclusion that there is nothing in the character of the deep-sea deposits which ﬁnds a parallel among the marine geological formations visible to us on the land. It is only among the comparativelyshallowwater accumulations