HOW THE WATEES OF THE OCEAN BECAME SALT. 59 



replies. With regard to Mr. David Howard's remarks, he re- 

 ferred to the nitre beds of Western America. T have not seen 

 those, they are very exceptional and veiy valuable ; but I think 

 he will, perha^DS, find they are exceptional in this respect, viz., 

 that they are all situated in volcanic regions, and nitre, as we know, 

 is a volcanic product, and these are probably laid down in inland 

 lake beds which have derived their supply of that salt from volcanic 

 sources. It is quite true what he said about the possible existence 

 of substances in such small quantities that in an ordinary — or 

 indeed more than ordinary — chemical analysis they might be 

 passed over ; and I have referred in my paper to the case of free 

 silica in solution in the waters of the ocean, which we know must 

 be there, because from that the silicious sponges, diatoms, and other 

 forms have built up their skeletons. Dr. Walker has given me 

 some questions to answer, but I am afraid if I entered into them 

 exhaustively I should keep you here too long, I will endeavour, 

 therefore, to reply as shortly as possible. I think the first question 

 is whether the salinity of the different parts of the ocean varies ? No 

 doubt it does to a certain extent. The surface portions of the ocean 

 are not so saline as those of deep water; and, of course, waters which 

 are in proximity to the outlets of large rivers are necessarily less 

 saline than those which are in mid-ocean ; but as regards the absence 

 or prevalence of marine life in the ocean it is probably much more 

 due to the difference of temperature than to salinity. Now the 

 " Challenger " and other expeditions in their soundings over almost 

 all parts of the ocean, adduced this remarkable and interesting fact 

 — that the very deep parts of the ocean, even under the equator 

 and the tropics, are intensely cold — almost as cold as the Arctic 

 waters; — differing very slightly from the freezing point of 

 water, viz., 32° F. In consequence of that, temperature is, as 

 we know, the main factor in determining the presence or absence 

 of animal forms in the ocean. There are many forms which will 

 survive and flourish in warm water and will not do so in temperate 

 water — much less will they do so in cold water ; and after going 

 down, say 3,000 fathoms, the temperature of the ocean does not 

 vary much in any part of its floor area from the freezing point of 

 water. Dr. Carpenter, we know, founded on that the very beautiful 

 theory with regard to oceanic circulation which seems to be borne 

 out by this fact. He showed that not only is there a surface 

 cii'culation — the currents with which we are familiar, which 



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