1234 A TREATISE ON METAMORPHISM. 



gold and silver. This subclass is of very great importance. It is illustrated 

 by the copper ores of the Lake Superior region, by all but the superficial 

 portion of the pyritiferous gold veins of the Sierra Nevada, and by the 

 deposits of many other regions. 



The ores which are deposited by descending waters alone are the most 

 important subclass, both as to volume and as to importance to mankind. 

 These ores are chiefly oxides, anhydrous and hydrous. The dominant ores 

 here placed are those of iron. While subordinate amounts of iron ores 

 belong in other divisions, certainly more than 90 per cent and probably 

 more than 95 per cent of the iron ores yet exploited are in this subclass. 

 The ores of manganese also are mainly oxides produced by descending 

 waters. 



The ores deposited by ascending and descending waters combined con- 

 stitute a very important subclass. The materials of this subclass include 

 oxides, carbonates, anhydrous and hydrous silicates, and chlorides, chiefly 

 above the level of ground water; sulphurets and tellurides chiefly below, 

 but often also above it, and native metals both above and below it. There 

 is frequently a transition belt of considerable breadth between the various 

 products. At or near the level of ground water all of these products are 

 often intermingled. Important ores of copper and zinc are included in the 

 oxides; of lead, zinc, and copper in the carbonates; of zinc, copper, and 

 nickel in the hydrous silicates ; of silver in the chlorides ; of iron, zinc, lead, 

 copper, nickel, mercury, and silver in the sulphides; of gold and silver in 

 the tellurides. The metals include important ores of gold and silver, and 

 subordinate amounts of copper. 



It is believed that the ores deposited by ascending and descending 

 waters combined are more numerous than those of any other subclass, even 

 if they do not occur in such great volume and are not of the same impor- 

 tance to man as the ores deposited by descending water alone. I suspect 

 that a close study of the origin of ore deposits will show that of ores formed 

 by underground water the most numerous subclass is not made up of ores 

 deposited by ascending waters alone, but is composed of ores which have 

 undergone a first concentration by this process and a second concentration 

 by descending waters. As a result of this there is found in the upper 50 to 

 500, or possibly even 1,000, meters of an ore deposit a large portion of 

 the metalliferous material which originally had, as a result of the first 

 concentration, a much wider vertical distribution. 



