June 12, 1890] 



NATURE 



163 



statement, for in these exceptional cases the temperature of the 

 ocean water appears occasionally to fall to 66" or 64° F., and 

 there is a wider annual range than \z' F. This condition of 

 high temperature with small range in the temperature of the 

 water is only to be met with in the middle and western portions 

 of the Atlantic and Pacific Oceans and the central parts of the 

 Indian Ocean ; consequently, coral reefs flourish along the 

 eastern shores of the continents, where the coasts are bathed by 

 currents of pure oceanic water coming directly from the open 

 sea ; while, on the other hand, they are absent along the western 

 shores of the continents, where the water is colder and the annual 

 range is very much greater — for instance, off the western coasts 

 of America and Africa. The Challenger observations have also 

 shown that the layers of warm surface waters are much thicker 

 towards the western parts of the great oceans ; consequently, 

 reef-forming organisms flourish at a greater depth along the 

 eastern shores of the continents than in positions further to the 

 eastward in the open ocean, where the warm layer of water — 

 over 70° F. — is much thinner. Throughout the temperate and 

 polar regions there are no coral reefs. This is all the more re- 

 markable, seeing that organisms belonging to the same orders, 

 families, and even genera as those which build up coral reefs 

 flourish throughout colder, and even in polar, seas. In these 

 colder seas the representatives of the reef-builders either do not 

 secrete carbonate of lime in their body-walls, or.'if they do so, 

 the shells or skeletons are much less massive than in tropical 

 waters. An attentive examination of the animals procured by 

 ihe dredge and trawl from all depths shows that in descending 

 into deeper water in equatorial regions the amount of carbonate 

 of lime secreted by the animals living on the sea bottom be- 

 comes less with increasing depth, and all the calcareous struc- 

 tures of the organisms become less massive with the descent 

 into the deeper and colder water of the abysmal regions. This 

 remark does not, of course, apply to the shells and skeletons 

 of surface organisms which have fallen to the bottom from the 

 surface waters. 



Still another illustration of the same fact is furnished by the 

 study of the pelagic organisms collected in the surface and sub- 

 surface waters by means of the tow nets. In the warmest tropical 

 waters there are numerous species of Pteropoda, Heteropoda, 

 Gasteropoda, Foraminifera, and Coccospheres and Rhabdospheres 

 (calcareous Algas), which lead a purely pelagic existence, and 

 secrete carbonate of lime shells. Mr. Murray estimates from 

 his tow-net experiments that at least 15 tons of carbonate of lime 

 exists in this form at any moment of time in a mass of tropical 

 oceanic water i square mile in extent by 100 fathoms in depth. ^ 

 The number of species and individuals of these lime-secreting 

 organisms decreases and the shells become less massive with a 

 wider removal from the equator and an approach to the colder 

 water of the poles, till we find in the surface waters of the polar 

 regions only one or two thin-shelled Pteropods, and one, or at 

 most two, dwarfed species of pelagic Foraminifera. It would 

 appear then that organisms, as a whole or individually, are able 

 to, and actually do, secrete more lime in regions where there is 

 a uniformly high temperature of the ocean water than in those 

 regions where there are great seasonal fluctuations of tempera- 

 ture, or where there is a uniformly low temperature of the water, 

 as in the ])olar regions and in the deep sea. In temperate seas 

 more carbonate of lime is secreted in the warm summer months 

 than during winter months. Indeed, a high temperature of the 

 sea water is m )re favourable to abundant secretion of carbonate 

 of lime than high salinity. 



An examination of the deep-sea deposits collected by the 

 Challenger and other expeditions in all oceans shows that, after 

 the death of the pelagic organisms above referred to, their cal- 

 careous shells are rained down on the ocean's bed, and there 

 make up the larger part of the deposits known as Pteropod and 

 Globigerina oozes, as well as a very considerable part of nearly 

 . all other marine deposits. If we take the samples of deep-sea 

 deposits collected by the Challenger &% a guide, then the average 

 percentage of carbonate of lime in the whole of the depDsits 

 covering the floor of the ocean is 36-83, and of this carbonate of 

 lime, it is estimated that fully 90 per cent, is derived from 

 the remains of pelagic organisms that have fallen from the sur- 

 face waters, the remainder of the carbonate of lime having been 

 secreted by organisms that live on or attached to the bottom. 

 If coral muds and sands, together with Pteropod and Globi- 

 gerina oozes, be considered, then it is estimated that these con- 



' Murray, "Structure and Orijin of Coral Reefs," Proc. Roy. Soc. Edin., 

 1880, p. so3. 



tain an average j:>ercentage of 76*44 of carbonate of lime, and 

 cover about 51,859,400 square miles of the sea bottom. We 

 have little knowledge as to the thickness of these deposits ; still 

 such as we have goes to show that in these organic calcareous 

 oozes and muds, we have a vast formation greatly exceeding in 

 bulk and extent the coral reefs of tropical seas ; they are most 

 widely distributed in equatorial regions, but some patches of 

 Globigerina ooze are to be found even within the Arctic circle 

 in the course of the Gulf Stream. The following table shows 

 the estimated area of the various kinds of deposits, with the 

 average depth, and average percentage of carbonate of lime in 

 each: — 



Table showing the Estimated Area, Mean Depth, and Mean 

 Percentage of CaCOj of the different D. posits. 



Oceanic 



Oozes and 



Clay. 



Terrigenous 

 Deposits. 



' Red clay. 



j Radiolarian ooze. 



Diatom ooze. 

 I Globigerina ooze. 

 I Pteropod ooze. 

 ! Coral sands and 



muds. 

 1 Other terrigenous 

 j deposits, blue 



muds, &c. 



50,289,600 



2,790,400 



10,420,600 



47,752,500 



887,100 



2727 

 2894 

 1477 

 1996 

 1118 



3,219,800 I 710 



27,899,300 I 1016 



670 

 4*oi 

 22-96 

 64-53 

 79*26 



86*41 



19*20 



NO. 1076, VOL. 42] 



One of the most remarkable facts discovered by the Challenger 

 Expedition is that, although the dead shells of these pelagic 

 organisms are rained down on the sea bottom, and in shallower 

 depths accumulate so as to form calcareous deposits of immense 

 extent, still, in other contiguous but deeper areas, these shells do 

 not accumulate on the bottom, being wholly removed either 

 while falling through the water, or shortly after reaching the 

 ocean's floor. The pelagic organisms are as abundant in the 

 surface waters over the one area as over the other, the only ap- 

 parent difference in the conditions being one of depth. In the 

 shallowest deposits of the open sea, shells, representative of 

 nearly all the lime-secreting surface organisms, are to be found 

 in the deposits. With increasing depth the more delicate ones 

 disappear from the bottom, till, in 1800 or 2000 fathoms, it is 

 rare to find more than traces of Heteropod, Pteropod, or the 

 more delicate pelagic Foraminifera shells in the deposits, while 

 these same delicate shells occasionally make up fully one-half of 

 the carbonate of lime that is present in depths of 700 or lOOO' 

 fathoms. Again, in the still greater depths of 3000 and 4000 

 fathoms and deeper, the Foraminifera, Coccoliths, and Rhabdo- 

 liths are either wholly removed, or are represented only by 

 the broken fragments of the thickest and most compact shells, 

 like Pulvinulina meiiardii, Sphceroidina dehiscens, or Globis^erina 

 conglobata. This gradual decrease in the quantity of carbonate 

 of lime in the deposits with increasing depth is well illustrated 

 in the following table, showing the percentage of lime in the 

 samples of deep-sea deposits collected by the Challenger towards 

 the central parts of the ocean basins, away from the immediate 

 influence of the debi-is from continental land or volcanic islands. 



The organic oozes, including the red clays and the coral 

 deposits, make up a total of 231 samples, and are arranged as 

 follows, showing the percentage of carbonate of lime in relation 

 to depth : — 



14 cases under 500 fathoms, m. p.c. 86*04. 



7 ,, from 500 to 1000 ,, ,, 66*86. 

 24 ,, ,, 1000 to 1500 ,, ,, 70*87. 

 42 ,, ,, 1500 to 2000 ,, ,, 69-55. 

 68 ,, ,, 2000 to 2500 ,, ,, 4673. 

 65 ,, ,, 2500 to 3000 ,, ,, 17'36. 



8 ,, ,, 3000 to 3500 ,, ,, 0*88. 

 2 ,, ,, 3500 to 4000 „ ,, 0*00. 

 I ,, over 4000 ,, ,, trace. 



The fourteen samples under 500 fathoms are chiefly coral muds 

 and sands, and the seven samples from 500 to 1000 fathoms con- 

 tain a considerable quantity of mineral particles from continents 

 or volcanic islands. In all the depths greater than 1000 fathoms 



