Dec. 10, 1874] 



NA TURE 



117 



Nature of the Bottom. 



from Cape Firiistcrre lo 

 Ti ncriffc. 



From Tcneriffe to St. Thomas. 



2740 

 2950 

 2750 

 2800 



3150 

 2720 



2575 



2325 

 2435 

 2385 

 2675 

 3CO0 

 2975 

 3025 



From St. Thomas to Birmuiias. 



2850 

 27CO 

 2600 



m Bel mtidas to Halifax. 

 I 2650 

 2600 

 2850 



... ! 2425 



1 1700 I ... 



m Halifax to Btrmudas. 

 1250 

 2200 

 2S00 

 2650 

 2650 

 2500 



Biriinidas to the Azores. 



37 

 38 

 39 

 42 



44 I 

 Fn 



50 



5' 



52 



53 



54 



55 



Frc 



58 



59 



60 



61 



62 



63 



65 



66 



67 



68 



69 



70 



Nature of the Bottom. 



O 



From Bermudas to the Azores 

 (continued). 



71 1675 1 ... 



72 1240 



73 1000 



74 135° 

 76 900 I 



From the Azores to Madtira. 



From Madeira to Cape Verde 

 Islands. 



From the Cape Verde Islands to 

 St. Faul Focks. 



Fi am the St. Faul Ri 

 S. Salvador. 



10 1 2275 I 



11 2475 I 



2150 



2275 



From S. Salvador lo Tristan 

 d'Acunha. 



2150 

 2350 



From Tristan d'Aainha to the 

 Cape of Good Hope. 



137 ... ... I 2550 



138 ... ... 2650 



139 ... 23i5 



140 ... 1250 1 ... 

 From the Cafe of Good Hope to 



Kcrsuelen Island. 



From Kers;ueUn Isiand to 

 Melbourne. 



The nature and origin of this vast deposit of clay is a question 

 of the very greatest interest ; and although I think there can be 

 no doubt that it is in the main solved, yet some matters of detail 

 are 'till involved in difficulty. My first impression was that it 

 might be the most minutely divided material, the ultionate sedi- 

 ment produced by the disintegration of the land, liy rivers and 

 by the action of the sea on exposed coasts, and held in suspen- 

 sion and distributed by ocean currents, and only makirg itself 

 manifest in places unoccupied by the Globigetina coze. Several 

 circumstances se:med, however, to negative this mode of origin. 

 The formation seemed too uniform ; whenever we met with it 

 it bad the same character, and it only varied in composition in 

 containing less or more carbonate of lime. 



Again, we were gradually becoming more and more convinced 

 that all the important elements of the Globigerina ooze lived on 

 the surface ; and it seemed evident that so long as the conditions 

 on the surface remained the same, no alleration of contour at the 

 bottom could possibly prevent its accumulation ; and the surface 

 conditions in the Mid- Atlantic were very uniform, a moderate 

 current cf a very equal temperature passing continuously over 

 elevations and depressions, and everywhere yielding to the tow- 

 net the ooze-forming foraminifera in the same proportion. The 

 Mid-Atlantic swarms with pelagic mollusca, and in moderate 

 depth the shells of these are constantly mixed v/ith the Globi- 

 gerina ooze, sometimes in number sufficient to make up a con- 

 siderable portion of its bulk. It is clear that these shells must 

 fall in equal numbers upon the red clay, but scarcely a trace of 

 one of them is ever brought up by the dredge on the red clay 

 area. It might be possible to explain the absence of shell- 

 secreting animals living on the bottom, on the supposition that 

 the nature of the deposit was injurious to them ; but then the 

 idea of a current sufliciently strong to sweep them away is nega- 

 tived by the extreme fineness of the sediment which is being laid 

 down ; the absence of surface shells appears to be intelligible 

 only on the supposition that they are in some way removed. 



We conclude, therefore, that the "red clay" is not an addi- 

 tional substance introduced from without, and occupying certain 

 depressed legions on account of some law legulatirg its deposi- 

 tion, but that it is produced by the removal, by some means or 

 other, over these areas of the caibonate of lime, which forms 

 probably about 98 per cent, of the mateiial of the Globigerina 

 ooze. We can trace, irdeed, every successive stage in the 

 removal of the carbonate of lime in descending the slope of the 

 ridge or plateau when the Globigerina ooze is forming to the 

 region of the clay. We find, first, that the shells of pteropods 

 and other surface mollusca, which are constantly falling on the 

 bottem, are absent, or if a few remain they are brittle and 

 yellow, and evidently decayirg rapidly. These shells of mollusca 

 decompose mere easily and disappear sooner than the smaller 

 and appaiently more delicate shells of rhizo[ cds. The smaller 

 foraminifera now give way and are found in lessening proportion 

 to the larger ; the coccoliths first lose their thin outer border 

 and then disappear, and the clubs of the rhabdolilhs get_ worn 

 out of shape and are last seen under a high power as infinitely 

 minute cylinders scattered over the field. The larger foraminifera 

 aie attached, and instead of being vividly while and delicately 

 sculptured, they become brown and worn, and finally they break 

 up, each according to its fashion ; the chamber-walls of Globi- 

 gerina fall into wedge-shaped pieces, which quickly disappear, 

 and a thick rough crust breaks away from the suiface of Orbulina, 

 leaving a thin inner sphere, at first beautifully transparent, but 

 scon becoming opaque and crumbling away. 



In the meantime the proportion of the amorphous "red clay" 

 to the calcareous elements of all kinds increases until the latter 

 disappear, with the exception of a few scattered shells of the 

 larger foraminifera, which are still found even in the most cha- 

 racteristic samples of the " red clay." 



There seems to be no room left for doubt that the red clay is 

 essentially the insoluble residue, the ash, as it were, of the cal- 

 careous organisms which foim the Globigerina ooze after the 

 calcareous matter has been by some means removed. An ordi- 

 naiy mixture of calcareous foraminifera with the shells of ptero- 

 pods, forming a (air sample of Globigerina ooze from near 

 St. Thomas, was carefully » ashed and subjected by Mr. Buchanan 

 10 the action of weak acid ; and he found that there remained 

 after the carbonate of lime had been removed, about one per cent, 

 of a reddish mud consisting of silica, alumina, and the red oxide 

 of iron. This experiment has been frequently repeated with 

 different samples of Globigerina ooze, and always with the 

 result that a small proportion of a red sediment remains which 

 possesses aU the characters of the red clay. 



