" GREENSTONES " OE WESTERN CORNWALL. 163 



what lighter in colour, and has a browner tint. Examined under 

 the microscope, it is seen to have been subjected to a greater amount 

 of alteration ; but it nevertheless retains cloudy outlines of felspar 

 crystals, portions of which afford, in polarized light, the usual stria) of 

 plagioclase. The augite is represented by pseudomorphs after that 

 mineral, coloured by viridite, while magnetite has been partially 

 replaced by a greenish-grey material, and apatite appears to be 

 unchanged. 



III. Fine-grained Crystalline Rock from Tolcam Quarry. — This 

 rock is of a dark sage-green colour, is exceedingly fine-grained, and 

 breaks with a somewhat conchoidal fracture. A microscopical ex- 

 amination of thin sections shows it to consist of a granular base, 

 containing quartz, through which hornblende and magnetite are 

 plentifully disseminated. 



In spite of the large amount of alteration to which this rock 

 has evidently been subjected, its original constitution may still, 

 to some extent, be traced, augite having become replaced by patches 

 of viridite and hornblende, while felspar is either represented by 

 indistinct pseudomorphic forms, or completely merged in the granular 

 translucent base ; magnetite, on the contrary, remains comparatively 

 unchanged, and apatite is apparently absent. 



IV. Chapel Bode, west of causeway leading to St. MicliaeVs 

 Mount. — This rock, which rests upon slate, is crystalline, of a 

 greenish-grey colour, and is traversed by numerous fissures, some 

 of which have become filled with hornblende ; in its general ap- 

 pearance it more nearly resembles a felsite than either of the fore- 

 going. Under the microscope it is seen to be composed of a fel- 

 spathic base, porphyritically enclosing crystals of somewhat altered 

 plagioclase, together with a little granular quartz ; throughout this 

 are disseminated viridite, apatite, crystals of magnetite, and angular 

 patches of hornblende, the latter representing replaced crystals of 

 augite. 



With regard to the petrological character of the rocks above 

 described, I would remark that No. I. is probably a rock resulting 

 from the alteration either of gabbro or dolerite, while No. II. and 

 No. III., which do not differ very materially from it in chemical 

 composition, may be regarded as the results of a still more advanced 

 stage of metamorphism *. In No. IV. the proportion of silica is the 

 same as in Nos. I. and II., but the amount of lime and magnesia is 

 somewhat less, while that of alumina is greater. 



Associated with the before-described rocks are others, possessing a 

 distinctly lamellar structure, but which appear to graduate from 

 ordinary killas, on the one hand, to imperfectly cleavable slates, on 

 the other. 



A slate belonging to the killas class, which comes in contact with 

 highly crystalline rocks in the Tolcarn Quarry, analyzed in dupli- 

 cate, afforded the following results (sp. gr. = 2*76) : — 



* Mr. Allport, when on the ground, suggested that these rocks might be 

 altered gabbros. 



