HOW THE WATERS OP THE OCEAN BECAME SALT. 55 



The Chairman (the Venerable Archdeacon Robinson Thornton, 

 D.D., Y.P.) — I am sure all have listened to this paper with much 

 interest and desire to return the author their hearty thanks. 



Captain F. Petrie (Hon. Sec). — Some communications have 

 been sent by those unable to be present to-day. The first is from 

 Professor John Tyndall, F.R.S. : — 



" Hind Head House, Haslemere. 

 " Dear Sir, 



" I have I'ead with interest the paper by Professor Hull which 

 you have been kind enough to send to me. 



" The theory which he enunciates is set forth with lucidity and 

 scientific truth, 



" Tours very faithfully, 



" John Tyndall. 

 " To Captain F. Petrie." 



The next is from Professor Joseph Prestwich, D.C.L., F.R.S. : — ■ 



" There is one point in the interesting address of Professor Hull 

 in which I cannot agree. Speaking of the great deposits of salt 

 in the Triassic and other strata, he says : ' Another fatal objection 

 to the view of the marine origin of rock-salt is to be found in the 

 fact that this rock generally consists of nearly pure chloride of 

 sodium, while ocean water contains large proportions of the 

 chlorides of calcium, magnesium, and potassium, the precipitation 

 of which would result in a deposit very different from that of the 

 rock-salt of Cheshire and Woi'cestershire, which is composed of 

 9S"30 per cent, of chloride of sodium and only small traces of 

 other salts.' As I have explained elsewhere (Geology, vol. ii, 

 p. 160) it seems to me on the contrary probable that these salt 

 beds were formed by the deposition on the evaporation of sea 

 water in lagoons or inland lakes, the cause of the difference 

 of composition being the different solubility of the various salts 

 existing in sea water. Thus the sulphate of lime, which is the 

 most insoluble, is deposited first, and this substance is always 

 found associated with rock-salt, while the more soluble salts of 

 potash and magnesia, which require a greater degree of concen- 

 tration than the chloride of sodium (rock-salt), resist deposition 

 till the last. Thus in the salterns on the coasts of the Channel 

 the sea water let in first deposits the more insoluble sulphate, and 

 then, when removed to other pans, deposits its chloride of sodium 

 with but traces of the other ingredients, whilst in the mother 



