158 J. A. PHILLIPS ON THE SO-CALLED 



mineralogically, or chemically greenstones, or even any allied rock, 

 being nothing more than clay-slates altered in situ, and possessing 

 none of the properties of greenstones beyond the greenish tinge 

 which coloured them"*. 



Still more recently (1872) the Eev. W. S. Symonds, F.G.S., says 

 that at Penzance the slates are traversed by greenstones and fel- 

 stones, which occur again at Newlyn, but that what these rocks 

 are it is difficult to determine ; he is, however, decided in his state- 

 ment that there are numerous interbedded traps or elvans every- 

 where associated with these rocks f . 



Having in view the differences of opinion which have at various 

 times been entertained with regard to these rocks, I have endea- 

 voured, by the aid of chemical analysis, and by an examination of 

 thin sections, to obtain such data as may be necessary for their 

 more correct classification, as well as for the approximate determi- 

 nation of the nature of the different changes they may have severally 

 undergone. The various specimens submitted to examination were 

 selected during repeated visits to the locality ; and on one of these 

 occasions I had the advantage of being accompanied by Mr. 8. 

 Allport, who, I am pleased to learn, purposes laying before this 

 Society the results of a careful microscopical examination of the 

 rocks of the Penzance district. 



Their position around the shores of Mount's Bay will be under- 

 stood by reference to Sheet xxxiii. of the Government Geological 

 Map. In the neighbourhood of Penzance these rocks, at first sight, 

 appear to consist of a series of fissile greenish slates dipping from 

 the granite, and consequently in directions varying from south-east 

 to nearly south. A closer examination, however, reveals the fact, 

 not only that some have a much less slaty structure than others, but 

 also that many of them form compact crystalline beds £, in which 

 no distinct trace of lamination can be detected ; from these hard 

 rocks, which sometimes, as at Tolcarn, project boldly above the sur- 

 face, the beds appear to graduate through different varieties of horn- 

 blendic slate until they become ordinary killas. In the rocks ex- 

 posed along the sea-shore these alternations have sometimes the 

 appearance of being very gradual ; but where they have been opened 

 upon by quarrying, as on the left of the road at Tolcarn, nearly 

 opposite St. Peter's church, the divisions are more distinctly 

 marked. In this locality an ordinary grey killas overlies a band, 

 some 4 feet in thickness, of a fine-grained slightly lamellar rock, 

 somewhat resembling certain varieties of basalt, which is again 

 succeeded by a tougher and more distinctly crystalline mass. As 

 there is no exposure across this latter band, its width cannot be 

 accurately determined ; but it probably does not exceed 20 yards. 

 On the opposite side of the valley it projects above the surface of 



* " On the Alleged Hydrothermal Origin of certain Granites arid Metamor- 

 phic Eocks," by David Forbes, F.R.S. (Geological Mag. vol. iv. p. 57). 



t ' Records of tbe Rocks,' by the Rev. W. S. Symonds, F.G.S., Rector of 

 Pendock, p. 297. London, 1872. 



Some of these " beds " are probably intrusive. 



