486 A TREATISE ON METAMORPHISM. 



square mile of area." He calculates that throughout the entire globe there 

 is removed annually in solution 96 tons (about 86 tonneaus) of material 

 per square mile, which he divides as follows: Calcium carbonate, 50 tons 

 (45 tonneaus); calcium sulphate, 20 -tons (18 tonneaus); sodium chloride, 

 8 tons (7.2 tonneaus); silica, 7 tons (6.3 tonneaus); alkaline carbonates and 

 sulphates, 6 tons (5.4 tonneaus); magnesium carbonate, 4 tons (3.6 ton- 

 neaus); oxide of iron, 1 ton (0.9 tonneaus). The order of the larger of 

 these amounts is much what one would expect. The strong, abundant 

 base, calcium, is largely united with the weak abundant acid, carbonic. 

 The strong acids, such as sulphuric and hydrochloric, are largely united 

 with the alkaline metals, sodium and potassium, but a residuum is left over 

 which is supposed to be united with the calcium. 



We now know that in solutions all the bases are united with all the 

 acids except so far as dissociation occurs, according to the various factors 

 of strength, mass, and solubility, and we can see that it would have been 

 better to have estimated the various compounds as oxides or elements rather 

 than as salts. But it would still be true that the most abundant salt is 

 calcium carbonate, that that standing next in abundance is probably cal- 

 cium sulphate, and that those standing next in abundance are the alkaline 

 sulphates and chlorides. 



We conclude from the foregoing that in the belt of weathering all the 

 elements in the minerals are being dissolved all the time, but with greatly 

 varying rates, depending upon the strength and abundance of the active 

 compounds in solution and upon the solubility of the various minerals upon 

 which the solvents are acting. While the more readily soluble substances 

 are dissolved many times more rapidly than those which are usually spoken 

 of as insoluble, even those substances which are least soluble may be taken 

 into solution on a large scale. For instance, silica in the form of quartz is 

 regarded as exceedingly insoluble, yet it is certain that in the iron-bearing- 

 formations of the Lake Superior region quartz has been dissolved on 

 an enormous scale. Also the evidence is conclusive that such extremely 

 refractory substances as hematite and limonite are dissolved. (See pp. 

 548-549.) While all minerals in the belt of weathering are soluble, after 

 a sufficient length of time the constituents remaining undissolved are, of 

 course, those which are relatively insoluble. 



«Keade : T. Mellard, Chemical denudation in relation to geological time. Also Merrill, cit, p. 194. 



