44 PROF. T. G. BONNET ON THE [Feb. 1896, 



intrusive finer-grained rock seems to have softened, bent or drawn 

 out some fragments of the coarser gabbro, and even to have partly 

 melted down others, destroying the pyroxenic rather than the 

 felspathic constituents, so that the latter are scattered sporadically 

 in the newer rock. 1 Prof. Sollas, it will be remembered, has- 

 described a somewhat similar occurrence in the Carlingford district. 2 

 This intrusive rock is less abundant than the ordinary gabbro, but 

 more so than the ' greenstone ' 3 dykes, which cut them both, and 

 usually do not exceed 4 or 5 feet in thickness. 



A low spring tide enabled us to make a more complete examina- 

 tion of the southern side of Porthoustock Cove. As mentioned in our 

 paper (p. 491), General M c Mahon and myself felt uncertain as to 

 the nature of the rocks forming the shore and adjacent cliffs. They 

 presented, as we remarked, some resemblance to the more dioritic 

 members of the ' granulitic group,' and were cut by ' greenstone ' 

 dykes, like those mentioned above, the gabbro being near at hand ; 

 for we traced it on the slopes above the cliff almost to the head of 

 the cove. 4 This time we managed to get along the shore eastward, 

 to a point where the ordinary gabbro was exposed in the face of the 

 cliff. Here the rocks on which we stood consisted beyond question 

 of the fine-grained gabbro or dolerite, becoming, as usual, sporadic- 

 ally porphyritic in the neighbourhood of the coarser rock, and from 

 this point we succeeded in following it westward, and convincing 

 ourselves that, though somewhat disguised by the effects of faulting 

 and weathering, it is the dominant rock on this side of the cove. 

 It is cat, as has been said, by ' greenstone ' dykes, and traversed 

 not unfrequently by reddish veins. Some of these appeared due to- 

 infiltration, but others resembled a rather felspathic gabbro, by no 

 means identical with that forming the principal massif. Thus 

 Porthoustock Cove marks the position of a fault or group of faults, 3 

 which brings together the Crousa Down gabbro, with its associated 

 ini;rusives, and the hornblende-schist. 



Serpentine. — We examined the mass at Porthkerris 6 more care- 

 fully than on former occasions. It is represented in the geological 



1 Microscopic examination confirms the statements made above, and 

 discloses one or two other facts of general interest. In the granular ground- 

 mass the felspars are generally fairly well preserved ; the pyroxenic constituent 

 was originally an augite, very pale brown in thin sections, which now, without 

 change of external form, is replaced by a brown, strongly pleochroic hornblende 

 (brownish straw-colour to rich brown). The felspar of the included fragments 

 is almost wholly replaced by secondary and mostly earthy material ; the augite 

 by fibrous, often felted, aggregates of a greenish actinolitic hornblende. Here 

 and there the included fragments are so minute that one can hardly identify 

 them with certainty. 



2 Trans. Roy. Irish Acad. vol. xxx. (1894) p. 477. The changes, however, in 

 this instance are different — probably owing to local circumstances — from those 

 observed by him. 



3 I use this vague term designedly, for they may be anything from a fine- 

 grained dolerite or basalt to a hornblendic diabase. 



4 Still cut by the fine-grained gabbro or dolerite and by the ' greenstone.' 



5 The coves in the Lizard almost invariably correspond with one or more 

 dislocations. 



6 Polkerris in earlier papers. In this I have followed the nomenclature of 

 the 6-inch Ordnance map. 



