24 ART. 12. B. KOTO I 



secondary enrichment of ores took place later tlirough oxidation 

 (within the limit of about 450 m from the surface). This now 

 affords an opportunity for economic enterprise, though not a very 

 promising one. Whether the poor ore-deposit within the granite 

 had been developed by means of the descending waters of a metallic 

 solution, or by the decomposition of auriferous pyrite originally 

 contained in it, could not be definitely settled. 



The occurrence of basalt ^' in the form of a long narraw dyke 

 at the junction of the porphyritic granite and the sedimentaries 

 seemingly lends support to the thermal spring theory ^* of ore- 

 deposits ; but it seems to the writer that, as already stated, the 

 ore-deposit was formed at a later phase of the granitic intrusion 

 (its formation having been brought to an end with the final consoli- 

 dation of the plutonic), and has no connection whatever with the 

 basaltic eruption which happened at a still later geologic period. 

 The basalt contains no ores. 



E. Analyses of Ores 



The sample ores contained in the contact limestone, taken by 

 Mr. Nakashima from the crushed ore heaps near the stamp mill 

 sites around the Hol-gol mine, gave the following percentage con- 

 tents of gold, silver, and copper according to the assay done in 

 the laboratory of the Mitsui Mining Co. : — 



Nos. Gold Silver Copper 



1 0.0012 0.0014 — 



2 0.0130 0.0082 3.16 



2) See pp. 18 and 22. 



3) Dr. K. Nakashima is, indeed, quite right when he says that the effusion of basalt 

 was followeJ by a preat development of thermal springs as elsewhere; but this rock is not 

 usually an " ore-bringer " excepting in the case of mercury deposits as along tlxe Pacific 

 coast of the United Staler. 



