COMPLEXITY OF CONCENTRATION. 1193 



It follows from the foregoing that one of the most important classes of 

 ore deposits is that produced by the joint action of ascending and descending 

 waters. First comes the action of the downward-moving, lateral-moving 

 waters, mainly of meteoric origin, which take into solution metalliferous 

 material. In regions of recent volcanism these waters may be joined by 

 subordinate amounts of water exuded during the crystallization of magmas; 

 and such waters may be rich in metallic material. The waters, after 

 collecting- metals from any or all of the rocks with which they come into 

 contact, are converged into trunk channels and there, while ascending, 

 may deposit ore of the first concentration. During and after this first 

 concentration many of the ore deposits which are worked by man have 

 undergone a second concentration not less important than the first as a 

 result of descending lateral-moving waters. In some cases a concentration 

 by descending lateral-moving waters alone is sufficient to explain ore 

 deposits. It, therefore, appears more clearly than heretofore that an 

 adequate view of ore deposits must not be a descending-water theory, a 

 lateral secreting water theory, or an ascending-water theory alone. The 

 descending, lateral-moving, and ascending waters alike are driven mainly 

 by gravity. Each performs its own work. 



SUBCLASS 3. ORES PRECIPITATED FROM DESCENDING AQUEOUS SOLUTIONS. 



For the sake of simplicity and continuity of exposition, the effects 

 produced by descending waters have been applied to deposits which have 

 been first concentrated by ascending waters. However, it is perfectly clear 

 that a concentration by descending waters alone may be adequate to 

 produce ore deposits. The more important ores which are produced by 

 descending waters alone are those of iron and manganese. The ores of 

 these metals thus formed are chiefly oxides, and so far as iron and 

 manganese are concerned are largely hydrated oxides. 



IRON ORES. 



The ores of iron, unlike those of the other metals, occur in such large 

 masses that they properly belong with the rocks, hence their general 

 development has been already considerered in Chapters IX and XL It 

 remains here only to summarize the special factors which have been of 

 consequence in their segregation. Of the ore deposits which it has been 



