1036 A TREATISE ON METAMOKPHISM. 



study may not enable the investigator to determine the source of the metals. 

 This is especially likely to be true of ore deposits produced by ascending' 

 waters from a somewhat deep circulation. The underground waters may 

 have their sources of supply in rocks which do not reach the surface, and 

 have not been penetrated by the mine workings. 



In concluding this part of the subject it may be suggested that in 

 many instances mistakes have been made in assuming that some one for- 

 mation, sedimentary or igneous, is the sole source of the valuable metals. 

 Such an assumption is particularly prevalent in papers descriptive of gold 

 and silver deposits. In many districts where there are a number of sedi- 

 mentary and igneous rocks I have no doubt that the silver and gold are 

 partly derived from two or several formations. 



PART II. SEGREGATION OF ORES. 



GENERAL STATEMENTS. 



We are now prepared to consider each of the divisions of ores. It 

 will be seen that the different divisions, groups, classes, and subclasses 

 of ores are based upon the last dominant genetic process concerned in 

 their production. It has just been explained that there has been a steady 

 redistribution of the metals, each of them being taken away from some 

 of the formations, and, corresponding with this, segregated elsewhere. 

 Looked at from this point of view, an existing ore deposit may represent 

 the result of segregations by many processes throughout geological time. 

 Thus, if the full history of a metal in a given ore deposit were known it 

 might be found that at different times parts of it had been segregated by 

 various sedimentary, igneous, and metamorphic processes. Not only is 

 this so, but the segregations by these various processes may have occurred 

 and recurred. It is therefore manifestly impossible to give the full history 

 of ores, or to apply a genetic classification to them which does not restrict 

 itself to the later dominant processes which resulted in the segregation of 

 the ores at the particular places where they now are. Even thus restricted 

 it will be found that some ore deposits placed in one division or class could, 

 with almost equal plausibility, be placed in another. 



