CONCLUSIONS 331 



have been the source, through allcraliuii and segregation, of the iron ore 

 of the region. 



Serpentine 



Serpentine, according to the latest determinations,^ is not worthy of 

 perpetuation as a name of a mineral species. It is rather a rock, and 

 embraces mixtures of various greenish and usually ferrous, silicates of 

 magnesia, or magnesia and alumina, combined with a large percentage 

 of water, such as steatite (or talc), chrysotile, picrolite, antigorite, clino- 

 chlore, and sometimes pennine,'^ with other forms of chlorite. Serpen- 

 tine is abundantly produced by the decay of the Archean greenstones, 

 whether the greenstones were of igneous and crystalline nature aiul 

 massive in structure or fragmental and stratified, in which latter case 

 they should rather be called greenwacke. 



Conclusions 



From the foregoing it is apparent that the decay of a basic igneous 

 rock gives rise characteristically to a group of green minerals, the com- 

 position of which varies from the silicates of iron and magnesia to sili- 

 cates of alumina, iron, and magnesia, all of them hydrated and rather 

 soft, and it is evident that the prevailing green color of the KecAvatin 

 greenstones is due to the predominance of these secondary minerals 

 rather than to the existence of amphiboles and pyroxenes. The am- 

 phiboles indeed are plainly secondary after these greenish products, and 

 can be seen to have been formed in microscopic spicules in the midst of 

 the yellowish green isotropic field or to form directly by (;rystallint^ 

 change from the original pyroxene. 



If the question arises as to the whereabouts of the lime and soda, 

 which were the alkaline elements in the original feldspars of the dia- 

 base, it can be answered by stating that they entered into the waters of 

 the ocean, being more soluble, where they still remain, and that the 

 existence of accessory quantities of lime in several of the secondary green 

 minerals mentioned accounts for that portion of the lime which escaped 

 such removal. 



While a green product is characteristic of a change of basic igneous 

 rocks undergoing weathering, it appears to be true also that the different 

 insoluble elements when present in too large quantity for the secondary 

 minerals are sometimes segregated l)v tliemselves. Thns were formed 



* Lacroix : Min. de France et de les i-olonios. part i. p. 417. 

 » Final report. Minnesota Oeological Survey, vol. v. p. .S29. 



