1040 A TREATISE ON METAMORPHISM. 



superimposed upon the previous alterations. These later alterations by 

 metamorphism, like the earlier ones, may greatly diminish or greatty 

 increase the value of the original sedimentary deposit. In consequence of 

 the varying action of the secondary metamorphic processes an ore deposit 

 may be chiefly due to the direct processes of sedimentation or chiefly due 

 to metamorphic processes. There are therefore all gradations between 

 deposits produced solely by the processes of sedimentation and those 

 formed entirely by the processes of metamorphism. 



This is especially well illustrated by the gold placers. Many of these 

 deposits are in large part in the belt of weathering, but extend downward 

 into the belt of cementation. It has been repeatedly explained that in the 

 belt of weathering ferric and cupric sulphates and chlorides are produced. 

 These are capable of dissolving gold. Doubtless other solutions are 

 formed which have similar power. Wherever these solutions traverse 

 placers it is little short of certain that the gold is dissolved to some extent. 

 The material may be dissolved in the belt of weathering, transported 

 downward to the belt of cementation, and there precipitated; and thus one 

 part of the deposit may be enriched at the expense of another; or 

 solution may take place throughout a deposit and, in consequence, a 

 placer be impoverished; but the abstracted material may be precipitated 

 in another deposit, so that correlative with the depletion of some deposits 

 there may be enrichment of others. 



The formation of placer deposits has not been restricted to the present 

 geological period. It is highly probable that throughout geological time 

 placer deposits of greater or less richness have been produced, although it 

 is likely that those in early geological times were comparatively small. In 

 many cases as denudation continues the material of placers is largely 

 transported to the sea, and so forms beach deposits. It at first might be 

 thought surprising that so few ancient beach deposits of gold have been 

 found, but this is partly explained by the fact that gold is undoubtedly 

 dissolved in sea water, since the ocean contains compounds capable of 

 dissolving this metal. When the gold is transported to the seabeach, it is 

 in a very favorable position to be finely ground by attrition, and in this 

 fine condition is especially subject to the solvent action of the sulphates 

 and chlorides of the sea. It has been estimated that in sea water there is 

 a greater amount of gold than has been extracted from the earth by man. 



