DEPOSITS OF THE DEEP CIRCULATION. 



1135 



of different minerals or different combinations of minerals along the walls 

 of a vein or along one wall is evidence that the waters have risen from 

 a deep-seated source, the mineral solutions changing- from time to time and 

 so resulting- in the banded or comb structure. I follow Posepny in the 

 belief that, in most cases, such regular crustification is especially charac- 

 teristic of ores deposited by ascending waters. But undoubtedly, in many 

 cases there are superimposed upon the primary crustification later similar 

 minerals deposited by descending- waters, as fully explained on subsequent 

 pages. (See pp. 1146, 1151, 1152, 1154-1157.) Such secondary sulphides 

 have been described at many localities, but so far as I know no one has 

 claimed that such descending- waters have added definite continuous sec- 

 ondary crusts, one above the other. Certainly it has never been shown that 

 an ore deposit produced solely by descending waters shows a definite 



Fig. 28. — Cross section of banded vein near the London shaft. Mineral Point, Colo, a, country rock; b, quartz and 

 , chalcopyrite; c tetrahedrite; d d', quartz; e, galena. After Eansome. 



crustification. It must be remembered that the existence of crustification 

 is no evidence that the alternating minerals contained in the veins have not 

 been modified and enriched by descending waters. Indeed, this is very 

 frequently the case. Therefore crustification is an evidence of a first con- 

 centration by ascending waters, but is not evidence that descending waters 

 have not since greatly modified the character of the deposit. 



A third .criterion which is of great importance in showing that ores are 

 deposited from the deep circulation is the somewhat even tenor of values 

 vertically. It is explained (pp. 1139-1143) that a special characteristic of 

 descending waters is to produce a rich upper belt which diminishes in value 

 with depth. It is shown, also, that one of the most characteristic effects of 



«Posepny, F., The genesis of ore deposits: Trans. Am. Inst. Min. Eng., vol. 23, 1894, pp. 254-262. 



