Geological Survey of Canada. 95 



The ignition of the dried residue expels a portion of carbonic acid 

 from the earthy carbonates, and hence the calculated results exceed 

 fhe weight of the residue, besides which considerable portions of the 

 lime and magnesia are combined with silica, and not with carbonic acid 

 as in the calculated table. 



The comparison of the water of these two rivers shows the following 

 differences : — The water of the Ottawa, containing but little more than 

 one-third as much solid matter as the St. Lawrence, is impregnated 

 with a much larger portion of organic matter derived from the decom- 

 position of vegetable remains, and a large amount of alkalies uncombin- 

 ed with chlorine or sulphuric acid. Of the alkalies determined as 

 chlorids, the chlorid of potassium in the Otta\i a water forms 32 per 

 cent, and in that of the St. Lawrence only 16 per cent., while in the 

 former the silica equals 34 per cent., and in the latter 23 per cent, of 

 the mineral matters. The Ottawa drains a region of crystalline rocks, 

 and receives from these by far the greater part of its waters ; hence the 

 salts of potash liberated by the decomposition of these rocks are in 

 v large proportion. The extensive vegetable decomposition, evidenced 

 by the organic matters dissolved in the water, will also have contri- 

 buted a portion of potash. Tt will be recollected that the proportion of 

 potash salts in the chlorids of sea-water and saline waters generally, 

 does not equal more than two or three per cent. As to the St. Law- 

 rence, although the basin of Lake Superior, in which the river takes its 

 orign is surrounded by ancient sandstones and by crystalline rod s, it 

 afterwards flows through lakes whose basins are composed of palEeozic 

 strata which abound in limestones rich in gypsum and salt, and these 

 rocks have given the waters of this river that predominance of soda, 

 chlorine, and sulphuric acid which distinguishes it from the Ottawa. It 

 is an interesting geographical feature of these two rivers that they each 

 pass through a series of great lakes, in which the waters are enabled to 

 deposit their suspended impurities, and thus are rendered remarkably 

 clear and transparent. 



The presence of large amounts of silica in river waters is a fact only 

 recently established, by the analyses by H. Ste. Claire Deville of the 

 rivers of France.* The silica of waters had generally been entirely or 

 in great part overlooked, or had, as he suggests, from the mode of ana- 

 lysis adopted, been confounded with gypsum. The importance in an 

 agricultural point of view of such an amount of dissolved silica, where 

 river waters serve for the irrigation of the soil, is very great ; and geo- 

 logically it is not less significant, as it marks a decomposition of the 

 silicious rocks by the action of water holding in solution carbonic 

 acid, and the organic acids arising from the decay of vegetable mat- 

 ter. These acids combining with the bases of the native silicates, libe- 

 rate the silica in a soluble form. In fact silica is never wanting in na- 

 tural waters, whether neutral or alkaline, although proportionately 

 much greater in those surface waters which are but slightly charged 

 with mineral ingredients. The alumina, whose presence is not less con- 

 * Annates de Chimie et de Physique, 1848, vol. xxiii., p. 32. 



