﻿Microscopic Constitution of certain Cornish Rocks. 89 



serpentines and killas, and of the disseminated and venigenous 

 minerals in them, will kindle a brilliant light in the most secret 

 laboratory of nature ; but one thing is wanting, an exact de- 

 scription of all the characteristic facts observable in each particular 

 case, without the adornments of theory or the disarray of new 

 nomenclature " *. Since the foregoing was written (now fifteen 

 years ago) but little has been done with the view of elucidating 

 the composition and constitution of Cornish rocks f. It is 

 therefore hoped that the following results of a series of careful 

 analyses, and of numerous microscopical examinations, may be 

 of some value as a small instalment of that mass of exact infor- 

 mation so necessary to a more accurate knowledge of the petrology 

 of one of our most important mining districts. 



The methods which have been employed in conducting the 

 various analyses will be found fully described at the end of this 

 paper ; and with regard to the sections for microscopical exami- 

 nation, it may be observed that they have been prepared in the 

 usual way, but that the softer varieties of killas require exceed- 

 ingly careful manipulation. In the case of the more fissile 

 specimens, it was often found very difficult to obtain satisfac- 

 tory sections at right angles to the cleavage-planes ; and even 

 when procured, great care was required in mounting, to prevent 

 their entire disintegration in the balsam employed for that pur- 

 pose. Each section was examined by the aid of two different 

 arrangements of the instrument. The first, spoken of as "a 

 low power/' magnified 60, and the second, mentioned as "a 

 high power," about 400 linear. The polariscope could be con- 

 veniently used with both. Higher powers were sometimes, but 

 not often, employed. 



Killas from Polgooth Mine, adit-level, sp. gr. = 2*60. — This slate 

 is very soft, has a decidedly clayey smell, and adheres slightly 

 when applied to the tongue. Colour generally light grey, in 

 places marked with a darker shade of the same tint, and occa- 

 sionally slightly tinged with yellow. 



This rock was found, on analysis, to have the following com- 

 position : — 



* Manual of Geology, ante cit., p. 512. 



t A valuable paper relating to the constitution of the Cornish granites 

 has been published by the Rev. Samuel Haughton, F.R.S., of Trinity Col- 

 lege, Dublin, entitled " Notes of a Comparison of the Granites of Cornwall 

 and Devon with those of Leinster and Mourne," Proceedings of the Royal 

 Society, 1868-69, p. 209. [Phil. Mag. April, 1869, p. 306.] 



