Page:Encyclopædia Britannica, Ninth Edition, v. 18.djvu/140

 124 PACIFIC OCEAN which, owing to their relatively recent deposition, have not under gone great alteration. It the analyses of red clay are calculated, it will be seen, moreover, that the silicate of alumina present as clay (2SiOj, AljOs + 2HjO) comprises only a relatively small portion of the sediment ; the calculation shows always an excess of free silica, which is attributed chiefly to the presence of siliceous organisms. Microscopic examination shows that a red clay consists of argillaceous matter, minute mineral particles, and fragments of siliceous organisms. The mineral particles are for the greater part of volcanic origin, except in those cases where continental matters are transported by floating ice, or where the sand of deserts has been carried to great distances by winds. These volcanic minerals are the same constituent minerals of modern eruptive rocks enum erated in the description of volcanic muds and sands ; in the great majority of cases they are accompanied by fragments of lapilli and of pumice more or less altered. Vitreous volcanic matters belonging to the acid and basic series of rocks predominate in the regions where the red clay has its greatest development ; and it will be seen presently that the most characteristic decompositions which there take place are associated with pyroxenic lavas. Associated with the red clay are almost always found concretions and microscopic particles of the oxides of iron and manganese, to which the deposit owes its colour. -Again, in the typical examples of the deposit, zeolites in the form of crystals and crystalline spherules are present, along with metallic globules and silicates which are regarded as of cosmic origin. Calcareous organisms are so generally absent that they cannot be regarded as characteristic. On the other hand, the remains of Diatoms, Eadiolarians, and Sponge spicules are generally present, and are sometimes very abundant. The ear-bones of various Cetaceans, as well as the remnants of other Cetacean bones and the teeth of sharks, are, in some of the typical samples far removed from the continents, exceedingly abundant, and are often deeply impregnated with, or embedded in thick coatings of, the oxides of iron and man ganese. Over six hundred sharks teeth, belonging to the genera Carcharodon, Oxyrhina, and Lamna, and one hundred ear-bones of whales, belonging to Ziphius, Bal&noptera, Kal&na, Orca, and Delphiniis, along with fifty fragments of other bones, have been obtained in one haul of the dredge in the Central Pacific. The remains of these vertebrates have seldom been dredged in the organic oozes, and still more rarely in the terrigenous deposits. The abysmal region, in which the true pelagic deposits above described are laid down, shows a marked contrast with the &quot; tran sitional&quot; or &quot;critical area&quot; where the terrigenous deposits are found. The former area comprises vast undulating plains from 2 to 5 miles beneath the surface of the sea, the average being about 3 miles, here and there interrupted by huge volcanic cones (the oceanic islands). No sunlight ever reaches these deep cold tracts. The range of temperature over them is not more than 7, viz., from 31 to 38 F., and is apparently constant throughout the whole year in each locality. Plant life is absent, and, although animals belonging to all the great types are present, there is no great variety of form nor abundance of individuals. Change of any kind is exceedingly slow. Distri- Leaving out of view the coral and volcanic muds and sands which bution are found principally around oceanic islands, the blue muds, green of dc- muds and sands, red muds, together with all the coast and shore posits, formations, are situated along the margins of the continents and in enclosed and partially enclosed seas. The chief characteristic of these deposits is the presence in them of continental debris. The blue muds are found in all the deeper parts of the regions just in dicated, and especially near the embouchures of rivers. Red muds do not differ much from blue muds except in colour, due to the presence of ferruginous matter in greater abundance, and they are found under the same conditions as the blue muds. The green muds and sands occupy, as a rule, portions of the coast where detrital matter from rivers is not apparently accumulating at a rapid rate, viz., on such places as the Agulhas I ank, off the east coast of Australia, off the coast of Spain, and at various points along the coast of America. In the tropical and temperate zones of the great oceans, which occupy about 110 of latitude between the two polar zones, at depths where the action of the waves is not felt, and at points to which the terrigenous materials do not extend, there arc now forming vast accumulations of Globigcrinn and other pelagic Foraininifcm, coccoliths, rhabdoliths, shells of pelagic Molluscs, and remains of other organisms. These deposits may perhaps be called the sediments of median depths and of warmer zones, because they diminish in great depths and tend to disappear towards the poles. This fact is evidently in relation with the surface temperature of the ocean, and shows that pelagic Fur ami infer a and Molluscs live in the superficial waters of the sea, whence their dead shells fall to the bottom. Globigerina ooze is not found in enclosed seas nor in polar latitudes. In the southern hemisphere it has not been met with south of the 50th parallel. In the Atlantic it is deposited upon the bottom at a very high latitude below the warm waters of the Gulf Stream, and is not observed under the cold descending polar current which runs south in the same latitude. These facts are readily explained if it be admitted that this ooze is formed chiefly by the shells of surface organisms, which require an elevated temperature and a wide expanse of sea for their existence. The distribution of oceanic deposits may be summarized thus. (1) The terrigenous deposits blue muds, green muds and sands, red muds, volcanic muds and sands, coral muds and sands are met with in those regions of the ocean nearest to land. With the exception of the volcanic muds and sands and coral muds and sands around oceanic islands, these deposits are found only lying along the borders of continents and continental islands, and in enclosed and partially enclosed seas. (2) The organic oozes and red clay are confined to the abysmal regions of the ocean basins ; a Pteropod ooze is met with in tropical and subtropical regions in depths less than 1500 fathoms, a Glubiyc.rina ooze in the same regions between the depths of 500 and 2800 fathoms, a Kadiolarian ooze in the central portions of the Pacific at depths greater than 2500 fathoms, a Diatom ooze in the Southern Ocean south of the latitude of 45 south, a red clay anywhere within the latitudes of 45 north and south at depths greater than 2200 fathoms. As long as the conditions of the surface are the same, it might be expected that the deposits at the bottom would also remain the same. In showing that such is not the case, an agent must be taken into account which is in direct correlation with the depth. It may be regarded as established that the majority of the cal careous organisms which make up the Globigerina and Pteropod oozes live in the surface waters, and it may also be taken for granted that there is always a specific identity between the cal careous organisms which live at the surface and the shells of these pelagic creatures found at the bottom. Globigerina ooze is found in the tropical zone at depths which do not exceed 2400 fathoms, but when depths of 3000 fathoms are explored in this zone of the Atlantic and Pacific there is found an argillaceous deposit without, in many instances, any trace of calcareous organisms. Descending from the &quot;submarine plateaus&quot; to depths which exceed 2250 fathoms, the Globigerina ooze gradually disappears, passing into a greyish marl, and finally is wholly replaced by an argillaceous material which covers the bottom at all depths greater than 2900 fathoms. The transition between the calcareous formations and the argil laceous ones takes place by almost insensible degrees. The thinner and more delicate shells disappear first. The thicker and larger shells lose little by little the sharpness of their contour and appear to undergo a profound alteration. They assume a brownish colour, and break up in proportion as the calcareous constituent disappears. The red clay predominates more and more as the calcareous element diminishes in the deposit. Recollecting that the most important elements of the organic deposits have descended from the suj er- iicial waters, and that the variations in contour of the bed of the sea cannot of themselves prevent the debris of animals and plants from accumulating upon the bottom, their absence in the red clay areas can only be explained by the hypothesis of decom position. Pteropod ooze, it will be remembered, is a calcareous organic deposit, in which the remains of Pteropods and other pelagic Mollusca are present, though they do not always form a preponderat ing constituent, and it has been found that their presence is in cor relation with the bathymetrical distribution. In studying the nature of the calcareous elements which are deposited in the abysmal areas, it has been noticed that, like the shells of the Foraminifera, those of the Thecosomatous Ptcropoda, which live everywhere in the superficial waters, especially in the tropics, become fewer in number in the deposit as the depth increases. It has just been observed that the shells of Fora minifera disappear gradually along a series of soundings from a point where the Globigerina ooze has abundance of carbonate of lime, towards deeper regions ; but it is also noticed that, when the sounding-rod brings up a graduated series of sediments from a declivity descending into deep water, among the calcareous shells those of the Pteropods and Hetcropods disappear first in pro portion as the depth increases. At depths less than 1400 fathoms in the tropics a Pteropod ooze is found with abundant remains of Heteropods and Pteropods ; deeper soundings then give a (Uobi- ijcrina ooze without these Molluscan remains ; and in still greater depths, as has been said above, there is a red clay in which cal careous organisms arc nearly, if not quite, absent. In this manner, then, it is shown that the remains of calcareous organisms arc completely eliminated in the greatest depths of the ocean. For if such be not the case, why are all these shells found at the bottom in the shallower depths, and not at all in the greater depths, although they are equally abundant on the surface at both places? There is reason to think that this solution of calcareous shells is due to the presence of carbonic acid throughout all depths of ocean water. It is well known that this substance, dissolved in water, is an energetic solvent of calcareous matter. The investiga tions of Buchanan and Dittmar have shown that carbonic acid exists in a free state in sea water, and Dittrnar s analyses also show