Yol. 52.] ROCKS OF THE LIZARD DISTRICT. 45 



map as coming down to the shore on the northern side of the cove ; 

 but if there be an outcrop here, it was concealed on this occasion. 

 At any rate, I think it must be isolated from the main mass of the 

 serpentine ; this crops out in many places on the moorland which 

 rises on the northern side of the little valley, being seen over an 

 area measuring about 200 yards from east to west, and not more 

 than this in a transverse direction, hornblende-schist cropping out 

 south, east, and north of it. Whether this mass of serpentine is 

 continuous with the one at Porthallow we could not determine, the 

 two being separated by rather more than a furlong of covered ground. 

 Macroscopically it agrees fairly well with one variety of that rock, 

 but it is perhaps a little smoother in aspect and brighter in colour. 

 It has been much cracked and then cemented by yellowish 

 steatite, so that it is apt to break up under the hammer. The two 

 rocks agree fairly well in microscopic structure, each resulting from 

 the alteration of a fine-grained peridotite, which appears to have 

 consisted simply of olivine and iron oxide (i. e. a kind of dunite). 1 

 We ascertained that at Porthallow the serpentine not only runs 

 up the cliff in a dyke-like fashion, but also does this at each end, so 

 that the shape of the mass might be compared — very roughly — to a 

 flattish crescent with its back to the water. 2 The quarry afforded 

 better sections than it had done for some years. I can understand 

 doubts arising about the interpretation of particular sections, but 

 not as to the general fact that the serpentine is intrusive in the 

 hornblende-schist. 



We again paid much attention to the structure in the serpentine 

 resembling a foliation, which may be observed in many places, 

 more distinctly on the western than on the eastern coast, though on 

 the latter also, as from Kildown Point to Kennack Cove, it is often 

 to be found. Frequently it is barely perceptible on a perfectly fresh 

 face of the rock, becomes visible on wave-worn surfaces, and is 

 quite distinct on weathered crags. It obviously proceeds from a 

 slight alignment of the constituents in the original peridotite, and in 

 the case of bastite-serpentine seems more especially determined by 

 that mineral. While it is not seldom fairly persistent in orientation 

 over considerable areas, marked changes in direction may be 

 observed ; and while in the neighbourhood of included masses of 

 granulite it is sometimes parallel with the junction-surfaces, cases 

 not unfrequently occur where it makes a high angle with them. 

 In other words, we have failed to discover any necessary relation 

 between the structures in the two rocks. The serpentines in which 

 this 'foliation' exists do not exhibit, macroscopically or micro- 

 scopically, any indications of strain or crushing, and I adhere to the 

 opinion expressed by Gen. M c Mahon and myself, 3 that the structure 

 was produced while the original peridotite was consolidating. 



1 I refer to the ordinary serpentine of Porthallow, regarding the banded 

 variety as exceptional. 



2 The slopes here are steep, rough, and much overgrown with brambles and 

 coarse herbage, so that it is not easy to map the outcrops, and we have not 

 attempted to do this very precisely. 



3 Quart. Journ. Geol. Soc. vol. xivii. (1891) pp. 447, 476. 



