Page:Encyclopædia Britannica, Ninth Edition, v. 18.djvu/141

 PACIFIC OCEAN 125 that deep-sea water contains more lime than surface water. This is a confirmation of the theory which regards carbonic acid as the agent concerned in the total or partial solution of the surface shells before or immediately after they reach the bottom of the ocean, and is likewise in relation with the fact that in high latitudes, where fewer calcareous organisms are found at the surface, their remains are removed at lesser depths than where these organisms are in greater abundance. It has been shown that sea water itself has some effect in the solution of carbonate of lime, and further it is probable that the immense pressure to which water is subjected in great depths may have an influence on its chemical activity. Objections have been raised to the explanation here advanced, on account of the alkalinity of sea water, but it may be remarked that alkalinity presents no difficulty which need be here considered (Dittmar, Plnjs. L hem. Chall. Exp., parti., 1884). This interpretation also explains how the remains of Diatoms and Radiolarians (surface organisms like the Furaminifcra] are found in greater abundance in the red clay than in a Gloltiyerina ooze. The action which suffices to dissolve the calcareous matter has no effect upon the silica, and so the siliceous shells accumulate. Nor is this view of the case opposed to the distribution of the Fteropod ooze. At first it would be expected that the Foraminifira shells, being smaller, would disappear from a deposit before the Pteropod shells; but if it be remembered that the latter are very thin and delicate, and, for the quantity of carbonate of lime present, offer a larger surface to the action of the solvent than the thicker, though smaller, Globigerina shells, this apparent anomaly will be explained. The origin of these vast deposits of clay is a problem of the highest interest. It was at first supposed that these sediments were com posed of microscopic particles arising from the disintegration of the rocks by rivers and by the waves on the coasts. It was believed that the matters held in suspension were carried far and wide by currents, and gradually fell to the bottom of the sea. But the uni formity of composition presented by these deposits was a great objection to this view. It can be shown that mineral particles, even of the smallest dimensions, continually set adrift upon dis turbed waters must, owing to a property of sea water, eventually be precipitated at no great distance from land. It has also been supposed that these argillaceous deposits owe their origin to the inorganic residue of the calcareous shells which are dissolved away in deep water, but this view has no foundation in fact. Everything seems to show that the formation of the clay is due to the decomposition of fragmentary volcanic products, whose presence can be detected over the whole floor of the ocean. These volcanic materials are derived from floating pumice, and from volcanic ashes ejected to great distances by terrestrial volcanoes, and carried far by the winds. It is also known that beds of lava and of tufa are laid down upon the bottom of the sea. This assemblage of pyrogenic rocks, rich in silicates of alumina, decomposes under the chemical action of the water, and gives rise, in the same way as do terrestrial volcanic rocks, to argillaceous matters, according to re actions which can always be observed on the surface of the globe, and which are too well known to need special mention here. The universal distribution of pumice over the floor of the ocean is very remarkable, and would at first appear unaccountable ; but when the fact that pieces of pumice have been known to float in sea water for a period of over three years before becoming suffici ently waterlogged to sink is taken into consideration, it will be readily understood how fragments of this material may be trans ported by winds and currents to an enormous distance from their point of origin before being deposited upon the bottom. Frag ments of pumice are dredged in the greatest profusion in the red clay of the Central Pacific, and much less abundantly in the or ganic oozes and terrigenous deposits. This is owing to the rate of deposition being much slower in the former than in the latter, where the rapid accumulation of calcareous and siliceous organisms and continental debris masks their presence. The detailed microscopic examination of hundreds of soundings has shown that the presence of pumice, of lapilli, of silicates, and of other volcanic minerals in various stages of decomposition can always be demonstrated in the argillaceous matter. In the places where the red clay attains its most typical develop ment, the transformation of the volcanic fragments into argillaceous matter may be followed step by step. It may be said to be the direct product of the decomposition of the basic rocks, represented by volcanic glasses, such as hyalomelan and tachylite. This decom position, in spite of the temperature approximating to zero (32 F.), gives rise, as an ultimate product, to clearly crystallized minerals, which may be considered the most remarkable products of the chemical action of the sea upon the volcanic matters undergoing decomposition. These microscopic crystals are zeolites lying free in the deposit, and are met with in greatest abundance in the typical red-clay areas of the Central Pacific. They are simple, twinned, or spheroidal groups, which scarcely exceed half a millimetre in diameter. The crystallographic and chemical study of them shows that they must be referred to christianite. It is known how easily the zeolites crystallize in the pores of eruptive rocks in process of decomposition ; and the crystals of christianite, which are observed in considerable quantities in the clay of the centre of the Pacific FIG. 5. Crystals of Christianite from the deep water of the Pacific. (fig. 5), have been formed at the expense of the decomposing volcanic matters spread out upon the bed of that ocean. In connexion with this formation of zeolites, reference may be made to a chemical process which gives rise to the formation of nodules of manganiferous iron. These nodules are almost uni versally distributed in oceanic sediments, but are met with in the greatest abundance in the red clay. This association tends to show a common origin. It is exactly in those regions where there is an accumulation of pyroxenic lavas in decomposition, containing sili cates with a base of manganese and iron, such for example as augite, hornblende, olivine, magnetite, and basic glasses, that manganese nodules occur in greatest numbers. In the regions where the sedimentary action, mechanical and organic, is, as it were, suspended, and where everything shows an extreme slowness of deposition, in these calm waters favourable to chemical reac tions, ferro - manganiferous substances form concretions around organic and inorganic centres. These concentrations of ferric and manganic oxides, mixed with argillaceous materials whose form and dimensions are extremely variable, belong generally to the earthy variety or wad, but pass sometimes, though rarely, into varieties of hydrated oxide of manganese with distinct indications of radially fibrous crystalliza tion. 1 The interpretation necessary, in order to explain this formation of manganese nodules, is the same as that admitted in explanation of the formation of coatings of this material on the surface of terrestrial rocks. These salts of manganese and iron, dissolved in water by carbonic acid, then precipitated in the form of car! onate of protoxide of iron and manganese, become oxidized, and give rise in the calm and deep oceanic regions to more or less pure ferro -manganiferous concretions. At the same time it must be admitted that rivers may bring to the ocean a contribution of the. same substances. Among the bodies which. in certain regions where red clay predominates, serve as centres for these mangani ferous nodules are the re mains of vertebrates. These remains are the hardest parts of the skeleton tympanic bones of whales, beaks of Ziyhius, teeth of sharks ; and, just as the calcareous FIC.R. SectionofaManganeseXodule.enclos- shells are eliminated in ing tympanic bone of a whale, from 2300 great depths, so all the re- fathoms, South Pacific, mains of the larger vertebrates are absent, except the most resistant portions. These bones often serve as a centre for the manganese iron concretions, being frequently surrounded by layers several 1 For the composition of these manganese nodules, see MANGANESB, vol. xv. p. 479.