MURRAY — DEPTH AND MARINE DEPOSITS. 373 



by weak acid, the residue resembles in nearly all essential particulars a sample of Red 

 Clay from the deeper water of an adjacent region, such, for example, as occur in the 

 deeper water of the area around Cocos-Keeling and Christmas Islands in the Indian 

 Ocean. We may take it, then, that this residue and the Red Clay have the same origin 

 when they come from approximately the same region. 



In studying the phenomenon of the disappearance of calcium carbonate structures 

 with increasing depth, there are many modifying considerations to be taken into account. 

 It would appear that solution becomes more active beyond the depth of 2000 fathoms ; 

 this may be due to the lower temperature, the increased pressure, and greater abundance 

 of carbonic acid in the water. Again, in areas of warm currents, where lime-secreting 

 organisms are more abundant than elsewhere, the dead shells are able to accumulate at 

 greater depths than is usually the case ; and the same appears to be true in localities 

 where warm and cold currents meet at the surface, and where we have reason to believe 

 that a great destruction of life continually takes place, as, for instance, to the south-east 

 of the Agulhas Bank, where the Mozambique Current and the Antarctic drift mingle 

 their waters. Here we may suppose that many more shells are falling to the bottom, 

 because of the unfavourable conditions, than elsewhere, and consequently they are found 

 on the bottom at greater depths. In those regions covered by dark chocolate-coloured 

 Red Clays, characterised by the abundance of manganese in nodules and grains (as in 

 the 'Egeria' samples above cited), it seems as though the calcareous shells were removed 

 from the deposit at lesser depths than usual, and this may possibly be due to some 

 hypogene action now going on in such regions. 



It is in areas where warm and cold currents meet at the surface that glauconite and 

 phosphatic deposits are found at the bottom, and off the Agulhas Bank characteristic 

 deposits of this nature are being laid down in abundance. 



If we now turn to the cold and less salt waters of the Indian Ocean in high southern 

 latitudes towards the Antarctic, we find a marked contrast to the conditions prevailing 

 in the warm and salter waters in the tropical and subtropical regions. I well remember, 

 when the ' Challenger ' proceeded from Heard Island to the south, I found so few 

 Foraminifera in the surface-nets that I predicted there would be no Globigerina Ooze 

 found south of the 60th parallel of latitude. When a sounding was obtained in 

 1260 fathoms, the tube was filled with a cream-white deposit not unlike a typical 

 Globigerina Ooze, and it was at first sight believed that my prediction was all wrong. 

 On examination, however, this turned out to be chiefly composed of the frustules of 

 Diatoms and Radiolarian skeletons ; hence it was named a Diatom Ooze. It contained 

 three dwarfed species of pelagic Foraminifera which made up less than 20 per cent, of 

 the deposit. The living Diatoms and Radiolaria were found abundantly in the surface 

 waters of the Antarctic. Here, again, we have an instance of how largely the nature of 

 the deposits at the bottom of the ocean are dependent on the conditions prevailing in the 

 surface waters. It is also to be observed that (as in the case of the calcareous pelagic 

 organisms) the siliceous ones — Diatoms and Radiolaria — are in like manner dissolved 

 by sea-water, although not so completely nor so rapidly. Many of the more delicate 



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