774 REVIEWS 



the theoretical chemical changes, those taking place in each of the 

 several zones are considered in detail. 



Leaching out of the metals from the portion of the vein lying 

 above groundwater-level is considered as the main source of the enrich- 

 ing materials. The alteration at the surface leaves the iron as a gossan, 

 while the waters carrying the gold, silver, copper, and other metals in 

 solution trickle downward through the partially altered ores into cracks 

 and water-courses which penetrate the ore-body below the water-level. 

 The first of the process is, therefore, the leaching of the lean ores 

 which occurs in the superficial alteration of the vein. In weathering, 

 the sulphides oxidize according to their relative affinity for oxygen and 

 inversely as their affinity for sulphur. It is concluded from the evi- 

 dence that ore bodies lacking in iron pyrite will not show enrichment, 

 thus explaining the absence of any such phenomena in the pure silver- 

 lead bodies of the Coeur d'Alene district and elsewhere. 



The observations of the author on the effects of physiographic and 

 climatic changes, and on the changes of water-level, are of exceptional 

 significance : "Active degradation favors the accumulation of enrich- 

 ment, while prolonged degradation of a region, resulting from physio- 

 graphic revolutions, may result in successive migrations of material and 

 the accumulation in a relatively shallow zone of the metals derived 

 from many hundreds, and possibly thousands, of feet of the vein worn 

 away in the degradation of the land. Climatic conditions, rainfall or 

 aridity, warmth and rapid alteration of vein fractures, are agents affect- 

 ing surface weathering, and hence, also, enrichment. 



"Active degradation of a region, that is, rapid weathering, favors 

 enrichment by the quickness with which it removes the upper already 

 leached part of the vein, so that a larger amount of the vein-matter 

 is lixiviated in a given time than would result from the slower wast- 

 ing of the land. Such enrichments are favored by high latitudes. 

 Moreover, the mountainous regions are those in which secondary frac- 

 tures are most apt to be found. 



"Prolonged degradation is favorable for a similar reason, since time 

 is a factor in enrichment and changes in elevation, etc., affect the rate 

 and the progress of decay of the vein ; while the crustal movements 

 accompanying the physiographic changes favor fracturing of the 

 earlier deposit, increasing facility of leaching and place for deposition. 

 If a region passes through several cycles of erosion and elevation, it is 

 evident that their result is likely to be a succession of enrichments in 



