BY LEO A. COTTON. 22^' 



which is itself unknown. However, it seems fairly certain that 

 this mica was formed, in great part, by the replacement of the 

 felspars, so that the alkali-molecules were retained in the new 

 product. The composition of the mica seems to be a mixture of 

 paragonite and sericite molecules, with replacement of some of 

 the H2O molecules by NaoO. Its general composition may be 

 written 3 (K, H, Na)2 O, 3 AI2O3, 6 SiOo. It is possible that 

 some Li is also present, as this was not sought in the analyses. 

 It is also more than probable that some F enters into the 

 composition of this mica. 



Criteria of Metasomatism. — If the results discussed above be 

 judged by the criteria for metasomatism given by Lindgren, it 

 will be seen, I think, that the evidence for such action is com- 

 plete. Lindgren (op. cit.) states the following. 



(1) "The only decisive criterion is that of metasoraatic pseudo- 

 morphism involving the proof (generally to be furnished by 

 microscope study) as to whether simultaneous dissolution and 

 deposition have actually taken place. The most satisfactory proof 

 is the distinct alteration of well defined crystals (or, at least, well 

 defined grains) of the original mineral into the secondary mineral 

 in such a way that the latter projects into the former in prisms 

 or fibres having crystalline outlines." 



(2) " Another proof is afforded by sharply defined crystals of 

 the secondary embedded in the primary mineral, without any 

 break -between their surfaces; but in this case it must be clear 

 that the mineral is really secondary and was not formed before 

 the primary." 



(3) " The occurrence of nuclei of unaltered rock is sometimes 

 an available criterion, but it must be used with caution; and 

 probably has given rise to misinterpretations, on account of its 

 similarity to actual inclusions of country rock in vein-filling." 



(4) " Generally .... the replacement proceeds very 

 irregularly, owing to the effect of little cracks and fissures." 



In quoting the above, I have taken the liberty of numbering 

 the paragraphs for reference. The alteration of quartz into the 

 characteristic mica is a good illustration of No.l. Plate xx., fig. 2, 

 shows this. This alteration is abundant in Slides B, 0, D and E. 



