1230 A TI1EATISE ON METAMORPHISM. 



the precipitation of metals as sulphides from their oxidized salts by iron 

 sulphide. 



In still other instances the wall rock itself furnishes solutions contain- 

 ing metalliferous material which is precipitated in the trunk channels or 

 furnishes solutions capable of precipitating metalliferous material in the 

 trunk channel. Either of these may be true for igneous rocks. Also 

 igneous wall rocks may furnish heat and thus in a very important measure 

 promote segregation, as explained on pages 1015-1016. 



An excellent illustration of the influence of the wall rock is exhibited 

 by the Lake vein of the San Juan district, near Telluride. This vein, 

 profitable in the volcanic tuff, extends through the tuff into the limestones 

 below, and through them into the sandstones. In passing from the tuff 

 into the limestones the vein changes from a quartz vein to a calcite vein, 

 and as it goes into the sandstone it gradually changed again to a quartz 

 vein. There can be no better illustration than this of the influence of the 

 wall rock and of the domination of the law of mass action. The vein 

 material deposited in the limestone and sandstone has essentially the same 

 composition as the rock through which it passes, showing that the minerals 

 deposited in this vein were dependent upon the solutions derived from the 

 adjacent formations. Each formation contributed as its chief material 

 compounds which could be derived from it. Solutions entering- the vein 

 from other sources were subordinate to those derived from the wall rock. 

 The influence of the wall rock is further illustrated by the fact that so long 

 as the vein remains in the tuffs it is of economic importance, and as soon 

 as it passes into the sedimentary rocks the amounts of gold and silver 

 become so small that it is valueless. 



A third class of ore shoots are those produced by the process which has 

 been so fully explained in this paper, viz, the secondary enrichment of 

 deposits by descending waters, the first concentration of which was pro- 

 duced by ascending- waters. This process produces rich ore bodies, either 

 oxidized or sulphureted, or partly each, which are limited in depth by the 

 distance to which the descending- waters are effective. 



GENERAL STATEMENTS. 



Of necessity, in this analysis, the various factors which may produce 

 ore shoots have been separately treated. In a given case it is rare, indeed, 

 to find that the entire explanation lies in the application of a single one of 



