"GREENSTONES OF WESTERN CORNWALL. lOi 



granite, rather hornblende-slate and greenstone than clay-slate ; in 

 the Lizard district we have seen the greenstone intimately mixed 

 with granite, both occurring in the same vein. The transition from 

 clay-slate into hornblende-slate and greenstone is commonly so 

 gradual that we have not been able to trace anywhere a lino of 

 junction between both rocks "*. 



Dr. Boase, in his ' Geology of Cornwall/ observes, that " the 

 compact and schistose greenstones repeatedly pass into each other, so 

 that in traversing them from the granite they appear to alternate ; 

 but this does not hold good to any extent in the length of their beds, 

 as may be seen on the shores of Mount's Bay, where the massive 

 rocks are insulated in the body of the slate " f. 



The same author, in his ' Primary Geology,' makes the following 

 observations : — " The most abundant species of actynolite rock is 

 hard and compact, known provincially by the names of blue-stone or 

 blue elvan. This variety is accompanied by, and gradually passes into 

 blue slate, which is fissile in various degrees, and may also be easily 

 separated into small rhomboidal pieces. The massive kind occurs 

 in elevated ridges on the coast, and is very durable, whilst the slate 

 is decayed to a considerable depth, still retaining its form, but pro- 

 gressively diminishing in tenacity from tho perfect rock to the sur- 

 face" %' He further states that each of the schistose rocks of this 

 district, but particularly the " greenstone," passes into thick lamellar 

 slates, which exhibit various shades of blue and green. 



These rocks, as well as all the others forming the subject of this 

 investigation, are laid down on the Map of the Geological Survey as 

 " greenstones ; " and this classification appears to have been gene- 

 rally accoptcd. In 1867, however, Mr. David Forbes, F.R.S., 

 writing in the ' Geological Magazine,' makes the following observa- 

 tions : — " The writer of these remarks, finding, from an examination 

 of the sheets of the Geological Survey, that large masses of green- 

 stone were represented as occurring in Cornwall, near Penzance, 

 and at the Botallack Mines, immediately imagined that he would 

 there find the same relations of this greenstone to the metallic 

 lodes occurring as he had found to bo the case in South and North 

 America, Spain, Norway, Sweden, &c, and made a journey expressly 

 for this examination ; on arrival he at once found that the rocks 

 had evidently been metamorphosed in situ ; and they no doubt ori- 

 ginally had only been the ordinary sedimentary clay-slates. Had 

 he now been content with the decision of the Geological Survey, that 

 the rocks in question really were greenstones, then he must at once 

 have come to the conclusion that greenstones could be formed by the 

 alteration of clay-slates in situ. It did not, however, require a long 

 examination to prove that the rocks were neither petrologically, 



* Messrs. Von Oeynhausen and Von Decben " On the Junction of tbe Granite 

 and tbe KillasEocks in Cornwall" (Phil. Mag. vol. v. p. 246. London, 1829). 



t ' Contributions towards a Knowledge of tbe Geology of Cornwall,' by 

 Henry S. Boase, M.D., p. 235. Penzance, 1832. 



t • A Treatise on Primary Geology,' by Henry S. Boase, M.D.. p. 44. Lon- 

 don, 1834. 



