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MR. G. BAEEOW 0^' THE OCCUEEENCE OF [May I 898, 



it, whether it be quartz, white mica, or chlorite, or any other mineral, 

 we see, on turning to the published analyses, that the nature of the 

 enveloping material will largely account for the supposed differences 

 in composition. Briefly put, I would suggest that chloritoid, ofctre- 

 lite, etc., possessing the optical properties of the mineral here 

 described, have really the same composition — if we could only obtain 

 pure material to work upon. 



The Presence of Manganese. 



In the Kincardineshire specimen and in some crystals sent to me 

 by Dr. Barrois a special examination was made for manganese, and 

 a very small quantity ('02 per cent.) was found to be present in 

 both. In some analyses of ottrelite the manganese is present in 

 considerable quantity, as much as 8 per cent, in one case. 



In conclusion I have to thank Dr. Barrois for courteously sending 

 me a specimen of the He de Groix rock, and Prof. Bonney for the 

 loan of his slides. My thanks are also due to my colleague Mr. Teall 

 for the loan of the slide of the rock from Ottre, and for kind assist- 

 ance in the preparation of this paper. 



Postscript. 



[The appended figure shows the form and mode of occurrence 

 of the common binary twins. The crystal is immediately in 



Chloritoid from Friar Glen Burn, Kincardineshire. 



X about 80. Crossed nicols 



contact with minutely intergrown white mica and chlorite along 

 a considerable part of its margin. Great difficulty was experienced 



