SEA 



273 



thus produced. The present writer has shown that 

 a similar vertical circulation takes place in the lochs 

 of the west of Scotland. 



Composition of Sea-water. \i is probable that 

 every element is in solution in sea- water ; the great 

 majority are, however, present only in exceedingly 

 minute traces. If the average density of sea-water 

 be taken at 1027, pure water being 1000, then the 

 following would represent the composition of 1000 

 cubic centimetres of sea-water : 



Sodium chloride SS'flOSO 



Magnesium chloride 4D668 



Magnesium sulphate 1'7665 



Calcium sulphate 1 3425 



Potassium sulphate 0-9193 



Magnesium bromide 0-8809 



Calcium carbonate OT2>7 



Water 989 7073 



1027-0000 



Each base is probably in combination with each 

 acid, so that tliere are really sixteen salts alto- 

 gether from tin 1 mixture of the four buses and four 

 acids. The total amount of sea-salts may vary 

 greatly in different samples of sea-water, but it 

 has been shown by hundreds of carefully conducted 

 experiments that the ratio of the constituents of 

 sea-salts is nearly everywhere constant, with one 

 significant exception, that of lime, which is in 

 slightly greater proportion in the water from the 

 deeper parts of the ocean basins. Owing to the 

 constant circulation in the ocean, the gases of the 

 atmosphere, which are everywhere al>sorl>ed at the 

 surface of the sea, are carried down to the greatest 

 deptlis, and thus living organisms may flourish 

 throughout the whole extent of the ocean. Nitro- 

 gi-n remains at all times and places nearly con- 

 stant ; not infrequently the proportion of oxygen is 

 much reduced in deep water, owing to tin- processes 

 of oxidation ami respiration. CarUmic acid free or 

 loosely combined is abundant, and plays a most 

 imimrtant role in the economy of the ocean, com- 

 billing with, and rendering soluble normal carlmn- 

 ates of lime and magnesia to solution in the form 

 of bicarlionates. Water, as is well known, is but 

 slightly compressible, and almost any siilistancc that 

 will fall to the bottom of a tumbler of water will 

 in time fall to the bottom of the deep ocean. Still 

 the compressibility of water must not lie neglected 

 in oceanographical questions. In the deeper parts 

 of the ocean the pressure amounts to four or live 

 tons per square inch ; hence, in an ocean with a 

 depth of 5 miles, were the action of gravity suddenly 

 to cease, the ocean waters would rise 500 feet 

 alwve their present level from expansion. There 

 is evidence of very extensive chemical action on 

 some regions of the sea-Moor, and it has been 

 suggested that this action is much intensified 

 by the great pressure in the deeper parts of the 

 ocean. It is probable, however, that all the re- 

 actions here alluded to may be accounted for by 

 the dei ipo*ition of organic substances on the sea- 

 bed in the presence of the sulphates in sea-water, 

 and the long periods of time to which the materials 

 on the sea !-il have been exposed to the action of 

 sea-water in regions where there is a slow rate of 



deposition. 



I. iff. The colour of pure sea-water is a light 

 shade of blue; it has, however, frequently various 

 shades of green and brown, owing to the presence 

 of organisms ami matters in sus|>ensioii. It has 

 been definitely established that life in some of its 

 many forms is universally distributed throughout 

 the ocean. It has long been known that marine 

 plants and animals abound in the shallow waters 

 snrroiiniling continents and islands. Algie ill-- 

 appear from the sea-l>ed at depths between 100 

 ami 200 fathoms, but a great abundance of animals 

 have lieen procured in the greater depths. A 

 434 



Challenger trawling in a depth of orer a mile ( 1000 

 fathoms) yielded 200 specimens of living animals 

 belonging to 79 species and 55 genera. A haul in 

 about two miles (1600 fathoms) yielded 200 speci- 

 mens belonging to 84 species and 75 genera. A 

 trawling in about three miles depth (2600 fathoms) 

 yielded 50 specimens belonging to 27 species and 25 

 genera, not counting Protozoa. Even in depths of 

 over four miles fishes and animals belonging to all 

 the chief invertebrate groups have been procured. 

 The term 'Benthos' is now used for all the animals 

 and plants which live attached to or creep over the 

 bottom of the ocean, ' Plankton ' being the term for 

 all the plants and animals which live in, and are 

 carried along by the currents of, the ocean. In the 

 great body of oceanic waters life is most abundant 

 in the surface and sub-surface waters down to about 

 100 fathoms. Pelagic alga-, such as diatoms and 

 oscillatoria, are abundant in this region, and are 

 the principal and original source of food for many 

 pelagic and nearly all deep-sea animals. In the 

 intermediate depths of the ocean life though 

 present is less abundant. Within a few hundred 

 fathoms of tin- bottom life again becomes more 

 abundant, crustaceans and cuttle-fish being espe- 

 cially numerous. A very large number of the 

 organisms which belong to the pelagic Plankton, 

 such as diatoms, radiolaria, foraminifera, and 

 molluscs, secrete silica or carbonate of lime to 

 form their shells and skeletons : these in falling to 

 the bottom after the death of the organisms make 

 up a large part of the marine deposits in many 

 regions of the ocean. 



Deposits. The explorations of the Challenger 

 and other expeditions have resulted in a great 

 extension of our knowledge of marine sediments, 

 esi>eciallv of those now forming in the deep sea. 

 All marine deposits may be divided into two 

 classes viz. those made up principally of the 

 debris from the solid land of the globe, laid 

 down in greater or less proximity to the shores of 

 continents and islands, called 'terrigenous' de- 

 posits, and those in which this continental debris 

 is nearly or quite alisent, laid down in the abysmal 

 regions" of the ocean, called ' pelagic ' deposits. 

 Commencing with the former, tliere are first the 

 littoral and shallow- water deposits, forming around 

 the land-masses from the shore down to a depth 

 of about 100 fathoms, consisting of sands, gravels, 

 ami minis derived almost entirely from the disin- 

 tegration of the neighbouring lands. The littoral 

 dcjMisits, laid down between tide-marks, cover 

 alMiut 03,000 sq. m., and the shallow-water 

 deposits, ln'twecn low-water mark and 100 

 fathoms, about 10,000,000 sq. m. Proceeding 

 seawards from an average depth of about 100 

 fathoms, the deposits gradually change in char- 

 acter, the proportion of land detritus decreasing, 

 while the remains of oceanic organisms increase in 

 abundance, until at a considerable distance from 

 land and in comparatively deep water the terri- 

 genous deposits pass insensibly into truly pelagic 

 dcpo-its. The terrigenous deep-sea deposits i.e. 

 those formed at depths greater than 100 fathoms 

 may be briefly summarised as follows : 



Blue Mud, the most extensive, is grayish or 

 bluish in colour, with usually a thin reddish upper 

 layer, and is characterised by the presence of frag- 

 ments of rocks and mineral particles coming from 

 the disintegration of the land, of which quartz is 

 the principal specie- ; the remains of marine organ- 

 isms may l>e present in varying proportions, in- 

 creasing with depth and distance from the land. 

 Blue mud is found along the coasts of continents 

 and continental islands, and in all enclosed and 

 partially enclosed seas ; in some places, as in the 

 Yellow Sea, but notably off the coast of Brazil, 

 the mud may be of a red colour from the large 



