86 



NA TURE 



\_May 22, i< 



varieties of these lithological families, for instance, pumice, and 

 loose, incoherent, volcanic particles of recent eruptions, will) 

 ~ their characteristic minerals. All these mineral substances being 

 usually extremely fine or areolar in structure, are easily attacked 

 by the sea water at the place where they are deposited. This 

 chemical action brings about an alteration of the minerals and 

 vitreous fragments, which soon passes into complete decompo- 

 sition, and in special circumstances gh i-s rise to the formation of 

 secondary products. In some places the bottom of the sea is 

 ed with deposits due to this chemical action, principal among 

 which is clayey matter, associated with which there are often 

 concretions composed of manganese and iron. In other regions 

 the reactions which result in the formation of argillaceous 

 matter from volcanic products give rise also to the formation of 



:i ilites. 



Among other products arising from chemical action, pro- 

 bably combined with the activity of organic matter, may be 

 mentioned the formation of glauconite and phosphatic nodules, 

 with, in some rare and doubtful examples, the deposition of 

 silica. The decomposition of the tissues, shells, and skeletons 

 of organisms adds small quantities of iron, fluorine, and phos- 

 phoric acid to the inorganic constituents of the deep-sea de] 



Finally, we must mention extra-terrestrial substances in the 

 form of cosmic dust. 



We now pass to the consideration of the rSle played by organ- 

 isms in the formation of marine deposits. Organisms livii 

 the surface of the ocean, along the coasts, and at the bottom of the 

 sea are continually extracting the lime, magnesia, and silica held in 

 solution in sea water. The shells and skeletons of these, af 

 leath of the animals and plants, accumulate at the bottom and 

 give rise to calcareous and siliceous deposits. The calcareous 

 deposits are made up of the remains of coccospheres, rhabdo- 

 spheres, pelagic and deep-sea Foraminifera, pelagic and deep-sea 

 Mollusks, Corals, Alcyonarians, Polyzoa, Echinoderms, Anne- 

 lids, Fish, and other organisms. The siliceous deposits are 

 formed principally of frustules of Diatoms, skeleton, of Radio- 

 larians, and spicules of Sponges. 



While the minute pelagic and deep-sea organisms above men- 

 tioned play by far the most important part in the formation of 

 deep-sea deposits, the influence of Vertebrates is recognisable 

 only in a very slight d le special regions by The pre- 



sence of large numbers of sharks' teeth, and the ear-bones and 

 a few other bones of whales. The otoliths offish are usually 

 present in the deposits, but, with the exception of two vertebrae 

 and a scapula, no other bones of fish have been detected in t he- 

 large amount of material we have examined. 



Agents. — Having passed in review the various materials whii h 

 go to the formation of deposit, in the deep water immediately 

 surrounding the land and in the truly oceanic area . attenl on 

 must now lie directed to the agents which are concerned in the 

 transport and distribution of these, and to the sphere of their 

 action. The relations existing between the organic and inorganic 

 elements of deposits to whii h we have just referred, and th 

 which determine their distribution, will be pointed out ai the 

 -same time. 



The fluids which envelop the solid crust of the globe are in- 

 mly at work disintegrating the materials of the land, which, 

 becoming loose and transportable, are carried away, sometimes 

 by the atmosphere, sometimes by water, to lower regions, and 

 are eventually borne to the ocean in the form of solid partii les 

 or as matter in solution. The atmosphere when agitated, aftei 

 having broken up the solid rock, transports the particles from 

 the continents, and in m carries them far out to ., ,, 



where they form an appreciable portion of the deposit ; as, for 

 instance, off the west coast of North Africa and the soulh wi 

 coast of Australia. Again, in times of volcanic eruption,, the 

 dust and scoria which are shot into the air are carried immi ,- 

 distances by winds and atmospheric currents, rid no tmall por- 

 tion eventually falls into the sea. 



Water is. however, the most powerful ned in the 



formation and distribution of marine sediments. Running water 

 corrodes the surface of the land, and carries the triturated frag- 

 ments down into the ocean. The waters of the ocean, in the form 

 of waves and tides, attack the coasts and distributi ! 



a lower level. Independently of the action of waves, there exist 

 along most coasts currents, more or les s constant, which have an 

 effect in removing sand, gravel, and pebbles further from theii 

 origin. ( Generally, terrestrial matters appear to lie distributed by 

 these means to a distance of one or two hundred miles from the 

 coast. Waves and currents probably have no erosive or trans- 



porting power at depths greater than 200 or 300 fathoms, and 

 even at such depths it is necessary that there should be some 

 local and special conditions in order that the agitated water 

 may produce any mechanical effect. However, it is not im- 

 probable that, by a peculiar configuration of the bottom and 

 ing oceanic islands, the deposit on a ridge may be dis- 

 the tidal wave even at loco fathoms ; and this may be 

 the cause of the hard ground sometimes met with in such posi- 

 tions. By observations off the coast of France it has been shown 

 that fine mud is at times disturbed at a depth of 150 fathoms ; 

 but, while admitting that this is the ca,e on exposed coasts, the 

 majority of observations indicate that beyond 100 fathoms it is 

 an oscillation of the water, rather than a movement capable of 

 exerting any geological action, which concerns us in this 

 connection. 



Although the great oceanic currents have no direct influence 

 upon the bottom, yet they have a very important indirect effect 

 on deposits, because the organisms which live in the warm equa- 

 torial currents form a very large part of the sediment being de- 

 posited there, and this in consequence differs greatly from the 

 deposits forming in regions where the surface water is colder. 

 In the same way a high or low specific gravity of the surface 

 water lias an important bearing on the animal and vegetable life 

 of the ocean, and this in its turn affects the character of the 

 1 



The thermometric observations of the Ckattmger show that a 

 slow movement of cold water must take place in all the greater 

 depths of the ocean from the poles, but particularly from the 

 southern pole, towards the equator. It could be shown from 

 many lines of argument that this extremely slow massive move- 

 ment of the water can have no direct influence on the distribu- 

 tion of marine sediments. 



s, which eventually become icebergs that are carried 

 ,ea by currents, transport detrital matter from the land 

 to the ocean, and thus modify in the Arctic and Antarctic regions 

 the deposits taking place in the regions affected by them. The 

 detritus from icebergs in the Atlantic can be traced as far south 

 36 off the American coast, and in the southern hemi- 

 sphere as far north as latitude 40°. 



The fact that sea water retains fine matter in suspension for a 

 much shorter time than fresh water should lie referred to here as 

 having an important influence in limiting the distribution of fine 

 argillaceous and other materials borne down to the sea by rivers, 

 tli'i, giving a distinctive chare ter to deposits forming near land. 

 1 d out the influence of temperature and salinity 

 upon the distribution of the surface organisms whose skeletons 

 rge part of some oceanic deposits, and may state also 

 that the bathymetrical distribution of calcareous organisms is 

 influenced by the chemical action of sea water. We will return 

 to these influences presently when describing the distribution of 

 the various kinds of deposits and their re 1 ■ ins, espe- 



cially in those regions of the deep sea far removed from the 

 I action of rivers, waves, and superficial currents. The 

 action of life as a geological agent ha 1 lie n indicated under the 

 heading Mali-rials. 



-We give here an example showing the order fol- 

 lowed in describing the deposits examined: — 



Station 338 ; lat. 21° 15' S., long. 14 2' W. ; March 21, 

 1 , : face temperature 76 -"5, bottom temperature 36 0- 5, 



depth 1990 fathom,. 



Globigerina Ooze, white with slight rosy tinge when wet ; 

 granular, homogeneous, and very slightly coherent when dry; 

 resembles chalk. 



Cat n, o:c;S p r cent., consists of pelagic 

 Foraminifera (80 per cent.) ; coccoliths and rhabdoliths (9 per 

 1 • mas, and other Foraminifera. I I 



in nl, .I Echini spines, and one or two small frag- 

 Pteropods ( 1 38 per cent.), 

 ii. Residue, q'62 per cent., reddish brown ; consists of — 

 I. Minerals [1*62] m. di. C45 mm., fragments of feldspar, 

 hornblende, magnetite, magnetic spherules, a few ,111 ill grains 

 of manganese, and pumice. 



■us Organisms\l 'oo], Radiolarians, spicules of Spong* ,. 

 -is of Foraminifera. 

 3. Fine Washings [7 '00], Argillaceous matter with small 

 mineral particles and fragments of pumice and siliceous 

 organisms. 



the description of the deposits has been made upon this plan, 

 which was adopted after many trials and much consideration. 



