1042 A TEEATISE ON METAMORPHISM. 



placer deposits, (2) of simultaneous chemical precipitation witli mechanical 

 accumulation of the gravel, and (3) of introduction of the gold into the 

 conglomerate by underground waters since its formation. He rejects 

 the idea of a combination of these, saying that "this would only uselessly 

 complicate the hypothesis." It seems to me that the only hypothesis 

 which explains the facts is a combination of the placer and metamorphic 

 processes. 



Where two men differ so radically as do Becker and Hammond, the 

 truth may lie between, as is so often the case. Each is possibly partly 

 right and partly wrong. It is natural to think that a given ore deposit can 

 not be produced partly by the processes of sedimentation and partly by 

 underground water. These two classes of deposits are automatically 

 assumed not to grade into each other, whereas it is perfectly clear that such 

 gradation may and probably does exist. It may therefore be suggested 

 that a closer study of the Rand' deposits will show that they are partly in 

 the nature of mechanical sediments and partly in the nature of deposits 

 segregated by underground water. If this be the fact, study should be 

 directed to determine the quantitative importance of each. 



The placers of California and New Zealand furnish an instance in 

 which it is certain that both mechanical concentration and later chemical 

 modification by underground water have played a part. a In these placer 

 deposits, and especially in the deep placers, crystallized gold occurs not 

 infrequently, and it must be held that at least the rearrang*ement of this 

 gold has been due to the work of the underground water since the formation 

 of the placers. If this be admitted, the question at once arises as to what 

 extent gold has been added by the circulating waters since the placers were 

 formed. The added gold may have been derived from the leaner upper 

 portion of the placers, from the surrounding rocks, including the veins, 

 or in part from both sources. 



It is clear from the foregoing that mechanical concentration or second- 

 ary metamorphic action may be the dominant factor in producing' a suffi- 

 cient amount of segregation to make an ore deposit, or both may be 

 necessary in order to accomplish this work. 



The chemical and mechanical ore deposits particularly well illustrate 

 the principle that ores can be classified only upon the basis of the last 



"Gordon, H. A., Hysterornorphqus auriferous deposits of the Tertiary and Cretaceous periods in 

 New Zealand: Trans. Am. Inst. Min. Eng., vol. 25, 1896, pp. 292-301. 



