DEEP-SEA DEPOSITS 91 



have varied considerably with the general oceanic 

 circulation, the strength and extension of the trade 

 winds, and so forth. Apart from the accruement of 

 calcareous tests from the surface layers, there is a 

 reverse process going on in the sea, namely the resolu- 

 tion of lime by the water through which the shells are 

 sinking and especially by the bottom water, where they 

 come to rest. 



Now, the lime-dissolving power of the bottom water 



25* S 20' 



5* 10° 15* 20° N 25* 



Fig. 32. Vertical circulation near the Equator 



has been ascribed partly to its low temperature plus 

 a high content of carbon dioxide set free by the decom- 

 position of organic matter, and partly to its movement 

 with the Arctic and, especially, Antarctic bottom 

 currents. A third such factor, the importance of which, 

 in my opinion, has hitherto been underestimated, is the 

 action of magmatic volatiles, that is, carbon dioxide 

 and mineral acids — like hydrochloric acid, sulphuric 

 acid, etc. — which are set free during submarine vol- 

 canic eruptions. 



