DISTRIBUTION OF DISSOLVED MATERIAL. 539 



cementation, containing soluble material derived directly or indirectly from 

 the belt of weathering. In so far as the streams empty into lakes with no 

 outlet the material is not permanently lost to the belt of weathering, but 

 rejoins it, as will be shown below. But the vast majority of streams reach 

 the ocean and the dissolved material of the run-off is contributed to the 

 mother water. The material thus contributed is the source of the salts of 

 the sea. 



MATERIAL DISSOLVED, TRANSPORTED, AND REPRECIPITATED IN BELT OF 



WEATHERING. 



A portion of the compounds dissolved in the various ways in the belt 

 of weathering is transported to other parts of the belt and precipitated. 

 The salts which are reprecipitated depend upon (a) the abundance of the 

 compounds, (b) their relative and changing solubility, and (c) the humidity 

 of the region. 



(a) The more abundant a compound is in a solution the more likely is it 

 to be reprecipitated. This law is almost self-evident, for the more material 

 there is in solution the more likely, under the changing conditions which 

 obtain in the belt of weathering, are the solutions to become supersaturated 

 and thus precipitation take place. The super saturation may result from 

 any of the causes given on pages 113-123. This principle of precipitation 

 of the abundant compounds is one of the utmost importance in metamor- 

 phism. It applies not only to the belt of weathering but to all belts and 

 zones, and is correlative with the principle already explained, that the more 

 abundant a substance is the more likely it is to be dissolved. Both are 

 but special cases of the law of mass action. (See p. 94.) 



Both of these principles are well illustrated by the solution and precip- 

 itation of calcium carbonate in the limestone regions, although the relative 

 solubilities of the compounds present also enter into the result. Where 

 the dominant material is calcite or dolomite, or any combination of these, 

 these materials preponderate in the underground solutions. Also to an 

 equal degree they preponderate in the precipitates. The extensive solution 

 of limestone has already been considered. (See pp. 485-486). But wher- 

 ever there is a minute cavity, a cleft, a cave, or an opening of any other 

 kind in limestones in the belt of weathering, there calcite, aragonite,. or 

 dolomite, or some combination of these, may be precipitated. The small 



