BY LEO A. COTTON. 231 



nature, by Mr. E, C. Andrews, B.A., in his work on "The 

 Geology of the New England Plateau." 



A series of slides, covering the entire distance from the centre 

 of the vein to the country-rock, was examined, and a series of 

 four analyses, three of the vein-material and one of the country- 

 rock, were undertaken. The former, when judged by the criteria 

 for metasomatism suggested by Lindgren, were found to give 

 abundant evidence of such action. The analyses were used to 

 deduce the processes which were dominant in the formation of 

 the vein. A diagram of the analyses, after the method of 

 Pirsson, was constructed to show the relations graphically. Two 

 analyses used by Dr. Dalmer in a similar, but less detailed 

 examination, were included for comparison. From the variation 

 in the analyses it is probable that the fissure, along which the 

 solutions rose, was not more than 3 inches in width, and that the 

 total width of about 10 inches is due to metasomatic replacement 

 of the country-rock. The constituents of the solution, which 

 was probably under pneumatolytic conditions, were found to be 

 chiefly SiOs, FeO, MgO, SnOg, MoS^, and AlsO.. These acted 

 on the country-rock, which is a granite ; and, by molecular 

 replacement, formed a vein-product containing the above-men- 

 tioned constituents, plus CaO, NagO, KoO, and TiO^, probably 

 derived from the granite. The characteristic metasomatic pro- 

 duct is a paragOiiite-sericite mica, with Na2 replacing some of 

 the H.jO molecules. The exact composition of this mica is 

 indeterminate, and it quite probably contains fluorine. A second 

 metasomatic product is fluorite, probably formed by the action of 

 the fluorine-bearing solution on lime-felspars in the country-rock. 

 Finally, a comparison of the analyses and deductions with those 

 made by Dr. Dalmer in his work on the tin-deposits of Alten- 

 berg and Zinnwald in Saxony, gave quite harmonious results. 



In conclusion, I would like to express here my thanks to 

 Dr. W. Gr. Woolnough, F.G.S., Acting Professor of Geology at 

 the University of Sydney, for the kindly interest he has taken 

 in this work, for his ever-ready help, and for the unstinted. 



