Page:Encyclopædia Britannica, Ninth Edition, v. 18.djvu/138

 122 PACIFIC OCEAN Terrigenous deposits. Shore formations. Blue mud. Cireen mud and sand. Hed mud. Coral mud and sand. ) f ) Found in inland seas and ailing the shores of continents. Found around oceanic Coralline mud and siiml. I islands and along the Volcanic mud and sand. j shores of continents. Red clay. 1 Pelagic Globigfrina ooze 1. Pteropod ooze. 1 Found in the abysmal regions of deposits. Diatom ooze. the oceanic basins. ^ Kadiolitiian ooze. J Terri- The terrigenous deposits are found in more or less close proximity genous to the land, and are chiefly made up of the triturated fragments deposits, carried down into the ocean by rivers, or worn away from the coasts by waves or currents. Those found in the deeper water surrounding the land dill er from the sands, gravels, and shingles of the shore and shallow water chiefly in the smaller size of the grains and the greater abundance of clayey matter and remains of oceanic organisms. As, however, the water becomes still deeper and the distance from land greater, the deposits assume, more and more, a deep-sea character until they pass into a true pelagic deposit. The principal minerulogical constituents of the terrigenous deposits near continental land are isolated fragments of rocks and minerals coming from the crystalline and schisto-crystalline series, and from the clastic and sedimentary formations ; according to the character of the nearest coasts they belong to granite, diorite, diabase, porphyry, &c. , crystalline schists, ancient limestones, and the sedimentary rocks of all geological ages, with the minerals which come from their disintegration, such as quartz, monoclinic and tri- clinic felspars, hornblende, augite, rhombic pyroxene, olivine, muscovite, biotite, titanic and magnetic iron, tourmaline, garnet, epidote, and other secondary minerals. The tiituration and decomposition of these rocks and minerals give rise to materials more or less amorphous and without distinctive characters, but the origin of which is indicated by association with the rocks and minerals just mentioned. Mixed with these are found in many places phosphatic nodules, large quantities of glauconitc, and minerals arising from chemical action probably in presence of organic matter. Blue Blue mud is the most extensive deposit now forming around the mud. great continents and continental islands, and in all enclosed or partially enclosed seas. It is characterized by a slaty colour, which passes in most cases into a thin layer of a reddish colour at the upper surface. These deposits are coloured blue by organic matter in a state of decomposition, and frequently give off an odour of sulphuretted hydrogen. When dried, a blue mud is greyish in colour, and rarely or never has the plasticity and compactness of a true clay. It is finely granular, and occasionally contains fragments of rocks 2 cm. in diameter ; generally, however, the minerals which are derived from the continents, and are found mixed up with the muddy matter in these deposits, have a mean diameterof 5 mm. and less. Quartz particles, often rounded, play the principal part ; next come mica, felspar, augite, hornblende, and all the mineral species which come from the disintegration of the neighbouring lands, or the lands traversed by rivers which enter the sea near the .place where the specimens have been collected. These minerals make up the principal and characteristic portion of blue muds, sometimes forming 80 per cent, of the whole deposit. Glauconite, though generally present, is never abundant. The remains of calcareous organisms are at times quite absent, but occasionally they form over 50 per cent. The latter is the case when the specimen is taken at a considerable distance from the coast and at a moderate depth. These calcareous fragments consist of bottom-living and pelagic Forarninifera, Molluscs, Polyzoa, Serjmlas, Echinoderms, Alcyonarian spicules, Corals, &c. The remains of Diatoms and Radiolarians a -e usually present. Generally speaking, as the shores are approached the pelagic organisms disappear ; and, on the con trary, as we proceed seawards the size of the mineral grains diminishes, and the remains of shore and coast organisms give place to pelagic ones, till finally a blue mud passes into a true deep- sea deposit. In those regions of the ocean affected with floating ice, the colour of these deposits becomes grey rather than blue at great distances from land, and is further modified by the presence of a greater or less -abundance of glaciated blocks and fragments of quartz. These deposits are found along the coasts of North and South America, and in all the enclosed and partially enclosed seas, such as the Japan Sea, China Sea, Arafura Sea, Sulu Sea, Banda Sea, Celebes Sea, Sea of Okhotsk, &c. Green At some points in the same regions are found green muds and muds sands, which, as regards their origin, composition, and distribution r.nd near the shores of continental land, resemble the blue muds. They sands. are largely composed of argillaceous matter and mineral particles of the same size and kind as the blue muds. Their chief character istic is the presence of a considerable quantity of glauconitic grains, either isolated or united into concretions by a brown argillaceous matter. The Foraminifcra and fragments of Echinoderms and other organisms in these muds are frequently filled with glauconitic substance, and beautiful casts of these organisms remain after treatment with weak acid. At times there are few calcareous organisms in these deposits, and at other times the remains of Diatoms and Radiolarians are abundant. When these muds are dried they become earthy and of a grey-green colour. They frequently give out a sulphuretted hydrogen odour. The green colour appears sometimes to be due to the presence of organic matter, probably of vegetable origin, and to the reduction of peroxide of iron to protoxide under its influence. The green sands dill er from the muds only in the comparative absence of the argillaceous and other amorphous matter, and by the more important part played by the grains of glauconite, to which the green colour is chiefly due. Red mud is found where quantities Red of ochreous matter are brought down by rivers and deposited along mud. the coast, as in the Yellow Sea, but it is most characteristic in the Atlantic off the Brazil coast of America. In addition to the terrigenous deposits above referred to, volcanic Volcani muds and sands and coral muds and sands are found around the muds a shores of oceanic islands either of volcanic or coral origin. The sands. volcanic muds and sands are black or grey, and when dried are rarely coherent. The mineral particles are generally fragmentary, and consist of lapilli of the basic and acid series of modern volcanic rocks, which are scoriaceous or compact, vitreous or crystalline, and usually present traces of alteration. The minerals are sometimes isolated, sometimes surrounded by their matrix, and consist principally of plagioclases, sanidine, amphibole, pyroxene, biotite, olivine, and magnetic iron ; the size of the particles diminishes with distance from the shore, but the mean diameter is generally 5 mm. Glauconite does not appear to be present in these deposits, and qiiartz is also very rare or absent. The fragments of shells and rocks are frequently covered with a coating of peroxide of manganese. Shells of calcareous organisms are often present in great abundance, and render the deposit of a lighter colour. The remains of Diatoms and Radiolarians are usually present. Coral muds frequently contain as much as 95 per cent, of Coral carbonate of lime, consisting of fragments of Corals, calcareous muds alga3, Foraminifera, Serpulee, Molluscs, and remains of other lime- and secreting organisms. There is a large amount of amorphous sands, calcareous matter, which gives the deposit a sticky and chalky character. The particles may be of all sizes according to the distance from the reefs, the mean diameter being 1 to 2 mm., but occasionally there are large blocks of coral and large calcareous concretions ; the particles are white and red. Remains of siliceous organisms seldom make up over 2 or 3 per cent, of a typical coral mud. The residue consists usually of a small amount of argillaceous matter, with a few fragments of felspar and other volcanic minerals ; but oft barrier and fringing reefs facing continents there may be a great variety of rocks and minerals. Beyond a depth of 1000 fathoms off coral islands the debris of the reefs begins to diminish, and the remains of pelagic organisms to increase ; the deposit becomes more argillaceous, of a reddish or rose colour, and gradually passes into a Globigerina ooze or a red clay. Coral sands con* tain much less amorphous matter than coral muds, but in other respects they are similar, the sands being usually found nearer the reefs and in shallower water than the muds, except inside lagoons. In some regions the remains of calcareous algje predominate, and In these cases the name coralline mud or sand is employed to point out the distinction. The extent and peculiarities of the region in which these terri genous deposits are laid down are interesting. It extends from high-water mark down, it may be, to a depth of over 4 miles, and in a horizontal direction from 60 to perhaps 300 miles sea wards, and includes all inland seas, such as the North Sea, Norwegian Sea, Mediterranean Sea, Red Sea, China Sea, Japan Sea, Caribbean Sea, and many others. It is the region of change and of variety with respect to light, temperature, motion, and biological conditions. In the surface waters the temperature ranges from 80 F. in the tropics to 28 F. in the polar regions. From the surface down to the nearly ice-cold water found at the lower limits of the region in the deep sea there is in the tropics an equally great range of temperature. Plants and animals are abundant near the shore, and animals extend in relatively great abundance down to the lower limits of the region, now marked out by these terrigenous deposits. The specific gravity of the water varies much, and this variation in its turn affVcts the fauna and flora. In the terrigenous region tides and currents produce their maximum effect, and these influences can in some instances be traced to a depth of 300 fathoms, or nearly 2000 feet. Tho upper or continental margin of the region is clearly defined by tho high-water mark of the coast-line, which is constantly changing through breaker action, elevation, and subsidence. The lower or abysmal margin passes in most cases insensibly into the abyemal region, but may be regarded as ending where the mineral particles from the neighbouring continents begin to disappear from the deposits, which then pass into an organic ooze or a red clay. The area covered by terrigenous deposits has been called the estimated at about transitional &quot; or by critical deposits and is