1889.] Contact Metamorphism in Silurian Rocks. 197 



brown, indeed often it becomes deep brownish- black and opaque. So 

 far as we know this mica has not been isolated, and made the subject 

 of quantitative analysis, but an inference may be legitimately drawn 

 from the published analyses of clays and of rocks affected by con- 

 tact metamorphism, in which the presence of this mineral has been 

 determined by microscopic examination. 



The researches of Carius, Fuchs, and Unger* may be held to have 

 demonstrated that, as a rule, no change of any importance occurs in 

 the chemical composition of a mass of rock affected by contact 

 metamorphism. If, then, we examine a series of analyses of ordinary 

 argillaceous sediments, and of the rocks resulting from contact 

 metamorphism of the same,f we are struck with their comparative 

 poverty in magnesia and richness in iron oxides. For instance, in 

 a group of nine analyses, we find the following ranges of consti- 

 tuents : — 



MgO, from 0-06 to 2*596 ; the average being 1185. 

 FeO, from 0*495 to 6*010 1 the two together commonly forming 

 Fe 3 3 , from 3*380 to 8"165 / from 9 to 10 per cent, of the whole. 

 K 3 0, from a trace to 3'765, but generally more than 1*2 per cent. 

 Na 3 0, usually in much smaller quantity, but once rising to 2*170 

 per cent. 



Every analysis, except one, gives more than 2 per cent, of alkalies, 

 but this records only traces both of soda and of potash. The author 

 estimates that in one of these rocks — the richest in magnesia — there 

 is 32*4 per cent, of mica, but even in this example the percentage of 

 Fe to Mg is 11 : 9. Most of the analyses indicate less than half this 

 quantity of magnesia, so that the amount of the iron would be three 

 or four times that of the other constituent ; indeed, we may say that 

 in eight of these rocks the iron must largely exceed the magnesia, and 

 in one of them almost wholly replace it. 



The inferences thus suggested are confirmed by other examples 

 from the Pyrenees, from the Lake District, and elsewhere. It is 

 then evident that very many — probably most — argillaceous rocks do 

 not contain a sufficient amount of MgO to form an ordinary biotite 

 in any quantity, so that we must suppose either that the constituent 

 is subsequently introduced — which is highly improbable — or that the 

 mica is an iron mica, not a ferro-magnesian mica. Such micas are 

 known to exist. We do not indeed remember to have seen a separate 

 analysis of the mica from a case of contact metamorphism, but in the 

 table of analyses of black mica given by Mr. Teall, J we find one with 

 MgO as low as 1*50 (FeO, 18*06 : Fe 2 3 , 7*19), and similar micas are 



* Fuchs, ' Neues Jahrb. f. Mineral.,' 1870, p. 742 ; Unger, ibid., 1876, p. 785. 

 f Such as are given by Unger, ' Neues Jahrb. f. Mineral.,' 1876, p. 785. 

 X ' British Petrography.' p. 302. 



