1881.] 201 [Wadsworth. 



secondary or alteration one. Hornblende in the recent andesites 

 is foreign, but in the older forms, and in almost all the older rocks 

 of every kind, it is either a secondary product or is a more or less 

 altered mineral. The micas, feldspars and quartz occur as foreign, 

 indigenous and secondary products. As a rule in the modern 

 rhyolites quartz is foreign, but in the older rocks of this 

 type — felsites and quartz porphyries — it is both foreign and 

 secondary. All hydrous oxides and silicates and all carbonates 

 appear to be alteration products.. 



As a rule the different modes of occurrence of these minerals 

 can be readily distinguished from one another under the micros- 

 cope by their characters and their relations to the rock mass. 



Further, it may be pointed out that olivine, except in the more 

 recent rocks, is found, as a rule, to be more or less altered to, or 

 replaced by, serpentine, quartz, iron ore, carbonates, etc. ; augite 

 by hornblende, biotite, chlorite, etc. ; and feldspar by quartz, 

 kaolin, micaceous and chloritic minerals, etc. 



These changes are so common that it is rare to find original 

 minerals in the older rocks that remain unchanged. 



Again, almost every mineral in rocks is found to contain 

 inclusions of other minerals, glass, liquids, and gases, thus 

 vitiating conclusions drawn from the chemical analysis of the 

 mineral. 



Since ore deposits are, in general, associated with altered or 

 metamorphosed rocks, and occur in regions in which thermal waters 

 have been active, the country rock would naturally be more or less 

 changed, and sometimes completely decomposed. In the process 

 of the formation of the ore deposit, it may happen that the ore ma- 

 terial will be entirely removed from the adjacent rock, or this rock 

 may have deposited in it ores which never existed there before ; or 

 again, the ore material may have been brought from a distance by 

 the percolating waters. 



From the above it follows that chemical analyses alone, either 

 of the country rock or of its inclosed minerals, lead to unreliable 

 conclusions as to the source of the ores ; and hence it is an 

 unphilosophical procedure to build any general theory upon such 

 analyses. 



If, by chemical anatysis, any accurate deductions are to be 

 drawn regarding the original source of the ores it seems necessary 

 that we should select those rocks and minerals that are known to 



