Page:Encyclopædia Britannica, Ninth Edition, v. 18.djvu/139

PACIFIC OCEAN 123 two-eighths of the earth s surface, while the continents cover three- eighths, and the deep-sea deposits of the abysmal regions, which will now be considered, cover the remaining three- eighths. The true deep-sea deposits may be divided into two classes, viz. , those in which the organic elements predominate, and those in which the mineral constituents play the chief part. Belonging to the former class there are Globigerina, Pteropod, Diatom, and Radiolarian Odzes, and to the latter Red Clay. Globigerina oo~e is the name given to all those truly pelagic deposits containing over 40 per cent, of carbonate of lime which con sist principally of the dead shells of pelagic Foraminifcra (Globige- rina, Orbulina, Pulvinulina, Pullenia, Sphasroidina] and coccoliths and rhabdoliths. In some localities this deposit contains 95 per cent. of carbonate of lime. The colour is milky white, yellow, brown, or rose, the varieties of colour depending principally on the relative abundance in the deposit of the oxides of iron and manganese. This ooze is fine grained ; in the tropics some of the Foraminifcra shells are macroscopic. AVhen dried it is pulverulent. Analyses show that the sediment contains, in addition to carbonate of lime, phosphate and sulphate of lime, carbonate magnesia, oxides )f of iron and manganese, and argillaceous mat ters. The residue is of a reddish-brown tinge. Lapilli, pumice, and glassy fragments, often altered into palagonite, seem always to be pre sent, and are frequent ly very abundant. The mineral particles are generally angular, and rarely exceed 08 mm. in diameter ; mono- clinic and triclinic fel spars, augite, olivine, FIG. 1. The finer particles of a GloUgerina O.-zo, hornblende, and mag- showing Coccolitlis, Coecospheres, and Hhab- netite are the most fre- dohths. quent. When quartz is present, it is in the form of minute, rounded, probably wind-borne grains, often partially covered with KIG. 2. Glolngrrina Ooze from 1000 fathoms. oxide of iron. More rarely there are white and black particles of mica, bronzite, actinolite, chromite, glauconite, and cosmic dust. Siliceous organisms arc probably never absent, sometimes forming 20 per cent, of the deposit, while at other times they are only recognizable after careful microscopic examination. In some regions the frustules of Diatoms predominate, in other the skeletons of Radiolarians. Pteropod ooze differs in no way from a Globigcrina ooze except in the presence of a greater number and variety of pelagic organisms, and especially in the presence of Pteropod and Heteropod shells, such as Diacria, Atlanta, Stijliola, Carinaria, &c. The shells of the more delicate species of pelagic Foraminifcra and young shells are also more abundant in these deposits than in a Globigcrina ooze. It must be remembered that the name "Pteropod ooze" is not intended to indicate that the deposit is chiefly composed of the shells of these Molluscs, but, as their presence in a deposit is char acteristic and has an important bearing on geographical and bathy- mctrical distribution, it is desirable to emphasize the presence of these shells in any great abundance. It may be pointed out that there is a very considerable difference between a Globigerina ooze or a Pteropod ooze situated near continental shores and deposits bearing the same names .situated towards the centres of oceanic areas, with respect both to mineral particles and to re mains of organisms. Diatom oo~e is of a pale straw colour, and is composed princi- Diatom pally of the frustules of Diatoms. When dry it is a dirty white ooze, siliceous flour, soft to the touch, taking the impression of the fingers, and contains gritty particles which can be recognized by the touch. It contains on an average about 25 per cent, of carbo nate of lime, which exists in the deposit in the form of small Glo bigerina shells, frag ments of Echinoderms ami other organisms. The residue is pale white and slightly plastic ; minerals and fragments of rocks are in some cases abun dant ; these are vol canic, or, more fre quently, fragments and minerals coming from continental rocks and transported by glaciers. The fine washings consist essen tially of particles of Diatoms along with argillaceous and other * IG - 3 - Diatom Ooze from 19.00 fathoms in the T , Antarctic Ocean. amorphous matter. It is estimated that the frustules of Diatoms and skeletons of siliceous organisms make up more than 50 per cent, of this deposit. It has been already mentioned that Radiolarians are seldom, if ever, completely absent from marine deposits. In some regions they make up a considerable portion of a Globigcrina ooze, and are also found in Diatom ooze and in the terrigenous de posits of the deeper water surrounding the land. In some regions of the Pa cific, however, the skele tons of these organisms make up the principal part of the deposit, to which the name Radio larian conchas been given. The colour is reddish or deep brown, due to the presence of the oxides of iron and manganese. The mineral particles consist of fragments of pumice, lapilli, and volcanic mine rals, rarely exceeding 07 mm. in diameter. There is not a trace of carbonate of lime in the form of Fie. 4. Uud;olari;in Ooze from 4475 fathoms in Central Pacific. shells in some samples of Radiolarian ooze, but other specimens contain 20 per cent, of carbonate of lime derived from the shells of pelagic Foraminifcra. The clayey matter and mineral particles are the same as those found in the red clays, which will now be described. Of all the deep-sea deposits red day is the one which is distri- Reel buted over the largest areas in the modern oceans. It might be clay said that it exists everywhere in the abysmal regions of the ocean basins, for the residue in the organic deposits which have been described under the names Globigerina,, Pteropod, Diatom, and Radiolarian oozes is nothing else than the red clay. However, this deposit only appears in its characteristic form in those areas where the terrigenous minerals and calcareous and siliceous organisms disappear to a greater or less extent from the bottom. It is in the central regions of the Pacific that the typical examples are met with. Like other marine deposits, this one passes laterally, according to position and depth, into the adjacent kind of deep-sea ooze, clay, or mud. The argillaceous matters are of a more or less deep brown tint from the presence of the oxides of iron and manganese. In the- typical examples no mineralogical species can be distinguished by the naked eye, for the grains are exceedingly fine and of nearly uniform dimensions, rarely exceeding O Oo mm. in diameter. It is plastic and greasy to the touch ; when dried it forms lumps so coherent that considerable force must be employed to break them. It gives the brilliant streak of clay, and breaks down in water. The pyrognostic properties show that it is not a pure clay, for it fuses easily before the blowpipe into a magnetic bead. Under the term red clay are comprised those deposits in which the characters of clay are not well pronounced, but which are mainly composed of minute particles of pumice and other volcanic material