1897] DISTRIBUTION OF PELAGIC FORAMINIFFRA 25 



with only traces of carbonate of lime in its composition. Again, if 

 we suppose a basin-like depression on the floor of the ocean, the 

 centre of which descends to 4000 or 5000 fathoms, while the rim 

 of the basin has only a depth of 1000 or 2000 fathoms, then, on 

 the rim deposits of Pteropod and Globigerina Oozes will be found 

 with 70 or 80 per cent, of carbonate of lime, while the centre of 

 the basin will be occupied by a Eed Clay with probably not a trace 

 of these carbonate of lime shells. The gradual disappearance of 

 these calcareous shells with increasing depth is evidently due to the 

 solvent action of sea-water, and especially of deep-sea water. In 

 the lesser depths a very large proportion of these surface shells 

 seem to reach the bottom before they are completely dissolved, and 

 accumulation takes place. With increasing depth the more delicate 

 shells are dissolved before reaching the bottom, and accumulation 

 becomes slower and slower, the last traces of these shells observed 

 in the deposits with increasing depth being broken fragments of 

 large Pulvinidinae and Sphaeroidinac. The greater quantity of lime 

 in solution which Dittmar found in the Challenger samples of deep- 

 sea water is apparently a consequence of the solution of the pelagic 

 shells here referred to. 



During the early part of the Challenger Expedition, Wyville 

 Thomson was much puzzled to account for the origin of the fine 

 Eed Clay which occupies the basin-like depressions of the sea-bed 

 far from land, and he suggested that this was an ash x left behind 

 after the solution of the carbonate of lime shells. He was led to 

 this view by observing that when the shells were taken from the 

 purest samples of Globigerina Ooze, and, after being carefully 

 washed with pure water, were dissolved with dilute acid, a small 

 clayey residue of a red colour remained behind. I was not satisfied 

 with this experiment, for I observed that the colour of the residue 

 varied in different samples, and it seemed to me that the fine clayey 

 matter had infiltrated the shells after they had reached the bottom. 

 I accordingly collected, in the course of several months, about 10 

 grammes of pelagic Foraminif era from the surface of the sea. When 

 these shells were dissolved in dilute acid not a vestige of residue 

 was observed. It was subsequently shown that the Eed Clay came 

 from a variety of sources, and that in the deep sea far from con- 

 tinents it was chiefly derived from the trituration and decomposi- 

 tion of floating pumice. 2 



During the past year or two I have carefully collected all the 

 available temperatures of the surface waters of the ocean, and from 

 these have constructed a map showing the annual range of tempera- 



1 Proc. Roy. Soc, vol. xxiii., p. 45. 1874. 



2 See Murray, Proc. Roy. Soc. Edin., vol. ix., p. 247. 1876; also Murray and 

 Renard, Deep-Sea Deposits Chall. Exp., p. 294. 1891. 



