2 6 SOILS. 



leached out is also seen by considering the composition of sea 

 water, since the ocean is the final reservoir for all the leaching-s 

 of the land. It might be objected that the ocean may have 

 received its salts from other sources ; but this objection is over- 

 borne by the fact that substantially the same salts are found in 

 landlocked lakes, in which, as they have no outflow, the leach- 

 ings of the adjacent regions are perforce, as a rule, the only 

 possible source of the salts. It is true that the nature of the 

 salts differs somewhat in different lakes, as might be expected ; 

 but a general statement of that nature will, after all, be the 

 same as that made in regard to sea-water. The following 

 table of the average composition of sea-water, according to 

 Regnault, illustrates these facts. 



MEAN COMPOSITION OF SEA-WATER. 



Sodium Chlorid (common salt) 2.700 



Potassium chlorid 070 



Calcium sulfate (gypsum) 140 



Magnesium sulfate (Epsom salt) .230 



Magnesium chlorid (bittern) 360 



Magnesium bromid 002 



Calcium carbonate (limestone) 003 



Water (and loss in analysis) 96.495 



100.000 



The average saline contents of sea-water would thus be 



3.505 per cent. In twenty-one determinations of the saline 

 contents of the Atlantic Ocean, the percentage ranged from 



3.506 to 3.710 per cent. Of this mineral residue, common 

 salt constitutes from about 75 to over 80 per cent. 



We see that most prominent among the ingredients mentioned here 

 is common salt (sodium chlorid), which forms nearly four-fifths of the 

 total solid contents. Next in quantity are the compounds of magnesium, 

 viz. Epsom salt and bittern, with a very small amount of the bromin 

 compound. Next come the compounds of calcium (lime), of which 

 gypsum is the more abundant, while the carbonate, so abundant on the 

 land surface in the various forms of limestone, is present in minute 

 amounts only, yet enough to supply the substance needed for the shells 



