IMetallogeny of the Japanese Islands. 21 



mon in Japan, but in the Satsuma Province it is frequently rich 

 in the precious metals, notwithstanding the fact that the andésite 

 of other provinces is generally barren of these metals. I made a 

 microscopic study of the ores in the replacement vein in the and- 

 ésite of tliC (Jkuchi Gold Mine. 



This andésite is generally fresh, but the felspar phenocrysts 

 are decomposed from the central portion into brownish chloritic 

 matter. By the penetration of a mineral solution into the 

 fissures of the rock, felspar is kaolinized, and augite and a part 

 of the felspar are chloritized. The groundmass is at the same 

 time transformed into a mixture of chlorite, kaolin, and 

 quartz. Then sihca, separated from tlie constitution of the rock- 

 forming minerals, is deposited in the rock masses in the shape 

 of veinlets or rounded masses. Magnetite is dissolved, and its 

 iron together with that secreted from the other components is 

 deposited again as pyrite by the chemical action due to the 

 introduction of hydrogen sulphide. The auriferous solution is 

 reduced by the ferrous oxide formed during the decomposition of 

 the felspar and augite, and is deposited in colonies around the 

 chlorite pseudomorph after the augite and felspar, or is imbedded 

 in tlie kaolin-silica mixture. Thus gold in rich ores exists in 

 colonies, as in the case of the Hasami Gold Mine already referred 

 tu. In ores in the replacement veins, gold is never found in tlie 

 chlorite pseudomorphs or in the quartz veinlets running through 

 the decomposition-products of the mother-rock. It will be seen, 

 therefore, that the introduction of gold began after the chloritiza- 

 tion of the rock-forming minerals, and finished before the forma- 

 tion of the veinlets. It is, however, not well estabhshed whether 

 tlie metasomatic cliange took place at the same time as the forma- 

 tion of the mineral veins, or after the completion of the vein- 

 making and during the period of its secondary enrichment. I am 

 of opinion that the latter is the more probable theory. 



10. Summary. 



The above statements may be summarized as follows: — 



a. During the consolidation of magma, various magmatic 



