GREENSTONES " OF WESTERN CORNWALL. 155 



19. On the so-called " Greenstones " of Western Cornwall. 

 By J. Arthur Phillips, Esq., F.G.S. &c. (Read February 23, 1876.) 



I x a paper which I had the honour of reading before this Society in 

 March 1874, I described and gave analyses of two varieties of 

 Cornish Greenstone, both of which were characterized as being pro- 

 bably altered hornblendic slates. In the present communication it 

 is my intention to give the results of a chemical and microscopical 

 examination of some of the more important of the so-called Green- 

 stones of West Cornwall, reserving for a future occasion an account 

 of those found east of the parish of St. Stephens. 



Penzance District. — The most extensive group of these rocks is 

 probably that situated on tho margin of Mount's Bay, in the vicinity 

 of Penzance, and extending, with comparatively little interrup- 

 tion, from Penlee Point, about two and a half miles south-west of 

 that town, to a mile east of the village of Gulval. From the latter 

 locality to Marazion there does not appear to be any extensive ex- 

 posure of rocks of this class, but they again make their appearance 

 in considerable masses in the sea-cliff extending eastward from 

 Marazion to Cuddan Point. 



This district may be regarded as being to a certain extent classic 

 ground, since the rocks which now occupy our attention have fre- 

 quently been the subject of the investigations and speculations of 

 the earlier geologists. 



In a paper read by Prof. Sedgwick before the Cambridge Philoso- 

 phical Society, March 20th, 1820, he says : — " In the cliffs between 

 Penzance and Mouschole, we found the ordinary varieties of clay- 

 slate, much intersected by contemporaneous veins of quartz. These 

 rocks were succeeded by, and alternated with, beds of compact fel- 

 spar, greenstone, hornblende rock, and the purple schist before 

 mentioned, all resting conformably, and, on the great scale, exhibit- 

 ing a slaty texture, although many of the hand specimens derived 

 from them broke into irregular fragments "*. 



Dr. J. Forbes, who wrote about the year 1822, remarks, with 

 regard to what he calls the " slate formation " of Cornwall : — 

 "This formation is much more complicated than the last (the 

 granite), and affords much greater scope for geological research. 

 It comprehends, as far as I have been able to ascertain, five distinct 

 rocks. These are clay-slate, hornblende rock, greenstone, compact 

 felspar, and slaty felspar. The three first of these are so well 

 known, have such distinctive characters, and are of such general oc- 

 currence, that we are not very likely to be mistaken in discriminating 

 and giving them their proper appellation. Of the correctness of appli- 

 cation of the two last terms I am less certain. From the peculiarity 



* " On the Physical Structure of those Formations which are immediately 

 associated with the Primitive Ridge of Devonshire and Cornwall," bv the Rev. 

 Adam Sedgwick, M.A., M.G.S. (Trans. Cambridge Phil. Soc. vol. i. p." 134). 



