SUMMARY AND CONCLUSIONS. 1233 



Of the sedimentary ores the important chemical precipitates are those 

 of iron, manganese, and aluminum. Of the mechanical concentrates the 

 residuary deposits include ores of iron and tin; the stream deposits include 

 ores of gold, of tin, and of platinum; the beach deposits include ores of 

 gold and magnetite. 



In igneous ores — magmatic segregations — we can only certainly include- 

 titaniferous iron ores and the aluminum ores which occur in corundum 

 syenite. 



In the metamorphic ores deposited from gaseous solutions are probably 

 to be placed those impregnation deposits associated with which there have 

 contemporaneously developed the heavy anhydrous minerals, such as gar- 

 net, wollastonite, amphibole, pyroxene, biotite, and tourmaline. Doubtless 

 many if not all of the ores here belonging developed under the conditions 

 of the zone of flowage or anamorphism. This class is a subordinate one, 

 but includes a number of mines in various districts. Copper seems to be 

 the most important metal of this class, but zinc, lead, gold, silver, etc., occur. 



The class of aqueous solution ores is believed to be more important 

 than all others combined. The criteria by which these ores are to be 

 recognized are as follows : 



(1) The major portion of the material of the ore deposits, and especially 

 the gang-ue minerals, is the same as the general cementation materials which 

 fill the openings of rocks of all kinds, from fissures to subcapillary openings. 

 General cementation is universally recognized as the work of aqueous 

 solutions. 



(2) The gangue minerals of the ores are dominantly hydrous silicates, 

 carbonates, and oxides, such as are now being produced by aqueous solu- 

 tions in innumerable localities throughout the world, as, for instance, in the 

 Yellowstone National Park, the Mississippi Valley, Iceland, etc. 



(3) At some of these localities in which the gangue minerals are being 

 formed on an extensive scale, metallic ores are also being deposited in small 

 quantities by the aqueous solutions, as at Steamboat Springs, Sulphur 

 Bank, and Boulder Hot Springs. 



All of the three subclasses of ores deposited by aqueous solutions are 

 important. Ores of the first subclass, those which at the point of precipi- 

 tation are deposited by ascending waters alone, comprise ores of gold, silver, 

 and copper, the sulphides of all of the valuable metals, and the tellurides of 

 mon xLvn — 04 78 



