540 



Prof. T. G. Bonney—The Effect of Pressure 



What then is this mineral, and what was the rock originally ? 



I have examined a considerable number of analyses of minerals 

 which have a general resemblance to that dominant in the above 

 described rocks, and are quoted in Dana's " Mineralogy," Heddle's 

 " Chloritic Minerals," 1 and other works. 2 Now in the case of the 

 Gorner Grat rock the microscope shows us that it is mainly com- 

 posed of one mineral. Hence the composition of this must be 

 roughly represented by the bulk analysis of the rock (omitting most 

 of the Fe 2 3 ). Therefore the mineral cannot be pennite, ripidolite, 

 or chlorite (clinochlore), i.e. not one of the chlorites as defined by 

 Professor Heddle. But this analysis nearly approaches those of 

 chloritoid, given by him (I. and II.) except in the presence of 

 alkalies and the much lower percentage of FeO. 



99-70 ... 100-41 ... 101-87 ... 99-44 

 But the hardness of chloritoid is 5-5 to 6, while in this case it 

 is about 2 ; it has also a higher specific gravity, viz. about 3 - 5. 

 However, there can be little doubt that it is very nearly related to 

 chloritoid and a member of Tschermak's ' Clintonite ' group, 5 which 

 contains this mineral with Ottrelite, Xanthophyllite, etc. 



I have not been more successful in endeavouring to ascertain the 

 original nature of the rock. A peridotite is obviously out of the 

 question ; the excess of the alumina over the silica is much greater 

 than in any analysis given by Both. If, however, we suppose that 

 silica has been removed (perhaps with some magnesia and lime), 

 we are perplexed at the percentage of alkalies, which is about that 

 of a normal basalt. 



The adjacent schist adds to our difficulty. 6 Its softness and 

 microscopic structure justify us in regarding it as a talc-scbist, and 

 the field evidence is in favour of its being only an altered condition 

 of the slaty serpentine. This was my own opinion at the time, 

 and Mr. Eccles, who kindly undertook to re-examine the question, 

 informs me that though the change from the one rock to the other 

 is abrupt, this conclusion appears the more probable. 7 To convert 



1 Trans. Royal Soc. Edin. vol. xxix. (1879) p. 55. 



2 I am indebted to Miss C. A. Eaisin for much help in this search. 



3 Analysis of Chloritoid, Tschermak and Sipocz, Sitz. k.k. Akad. "Wiss. 1879. 



4 Analysis of ' Sismondine ' from Zermatt, Des Cloiseaux, Bull. Soc. Min. vol. vii. 

 P- 80. _ 5 Xoc. cit. 



6 Curiously enough there is a similar association in Anglesey, Q.J.G.S. vol. 

 xxxvii p. 44. 



7 After microscopic examination of a specimen collected by Mr. Eccles, I think 



