THE FELDSPARS: TSCHER]\IAK'S THEORY. 37 



Petersen objected to Tschermak's theory on the ground that orthoclase 

 feldspars containing soda do not show any ol' the striations peculiar to tri- 

 clinic feldspars, which, if the theory is correct, must be mechanically mixed 

 with the soda-bearing orthoclase ; also, that some potash-bearing plagioclases 

 exhibit no trace of orthoclase.* 



Professor J D. Dana, in 18G7, also opposed Tschermak's theory, holding 

 that the variations from the normal analyses Avere caused by, — 



(«) Incorrect analyses. 



(h) Impurities; and often, mixtures of diflerent feldspars through inter- 

 crystallization. 



(c) Alteration ; caused either (1) by the infiltration of ordinary waters, 

 carbonated or not — the rocks containing feldspars having been exposed to 

 this action through long ages past — or (2) through the same process aided 

 by mineral ingredients in the waters, resulting in the introduction of mag- 

 nesia, oxide of iron, etc., and in other changes.f 



In the meanwhile Tschermak's theory assumed great prominence, and in 

 1874, Dr. T. Sterry Hunt put forward the claim that he was the originator 

 of it. In support of this assertion he c|uoted from a published abstract of 

 his original paper (aute, p. 35), which had given his views in an indefinite 

 manner, and in his direct quotation from this abstract a hypothetical state- 

 ment was altered to a positive one. t 



As pointed out in the preceding pages, Hunt's theory of the triclinic 

 feldspars is nearly the same as Dana's (/>) given above. Hunt held that 

 they were indefinite, variable, mechanical aggregates, or intercrystallizations; 

 while Tschermak held that they were formed by isoinorphous molecular 

 unions in definite proportions. Further, Hunt's theory does not seem to 

 be at all oriijinal with him. 



Yet a number of writers have acknowledged Hunt's claim, presumably 

 because they have never read his original papers, or else have misunderstood 

 Tschermak. The use of the term "mixture" with two distinct meanings 

 — 1st, for mechanical aggregation (Hunt), 2d, for molecular combination 

 (Tschermak) — has probably added to the confusion. 



* Neues Jahr. Miu., 1S72, pp 570-5SC ; Jour. Prakt. Cliemie, 1S73 (2), vi. 200-212. 



t Amer. Jour, Sci., 1807 (2), xliv. 260, 399. See also System of Miueralogy, 5tli ed., 1SG8, p. 336. 



X Chem. Geol. Essays, pp. 4oS, 413-415. 



