366 Alexander Somervail — On the Lizard District. 



stone, and other portions with a well-defined porphyritic structure 

 resembling the variety at Porthoustock. In a sentence, there is 

 every grade between a diorite and a greenstone, schistose and mas- 

 sive, non-porphyritic and porphyritic. 



Porthoustock. — Here I examined the greenstone on the extreme 

 south side of the Cove, and found it, as I had done on previous 

 occasions, traceable through many varieties into fine-grained 

 varieties resembling an aphanite, and others into epidiorite, which 

 latter had an apparent passage into the hornblende-schists proper. 



Passing over the great alternating succession of gabbro, and 

 greenstone or epidiorite (which I have already described),^ with the 

 gradual decrease of the latter towards Coverack, there is nothing 

 I have specially to note except a very remarkable and beautiful 

 variety of gabbro occun-ing as a dyke in the serpentine on the 

 immediate west side of Coverack Pier. The diallage in this gabbro 

 has a brilliant metallic silvery lustre, being quite like a mica at first 

 sight ; the only gabbro of the kind that I am acquainted with. 



Black Head area. — At a little distance N.W. of Chynhal Point, 

 near the base of the cliflf, a diorite dyke one foot in thickness cuts 

 through the serpentine ; and at the Black Head, also at the base of 

 cliff, there are two dykes of a similar chai'acter in the serpentine 

 which have a N.N.W. and S.S.E. strike or trend. 



In this area the serpentine shows evidence of much disturbance, 

 and from this cause, or as a result of cooling, is full of structures ; 

 among which is a strongly-marked surface foliation which has the 

 usual N.N.W. and S.S.E. strike which is common to this structure 

 in the serpentine of nearly every locality. 



On the west side of Beagle Hole there are two more diorite dykes 

 in the serpentine, and near the Point there is one of gabbro ; and 

 two or three more of the latter occur (beside those of Downance 

 and Lankidden Coves already known and recorded) on the west 

 side of Lawn Vinoc. Beside these I have no doubt that careful 

 search would discover many more in what are at present regarded as 

 unbroken masses of serpentine. Passing over a large area I have 

 nothing special to record until near — 



Polpeor Cove. — It is here and to the westward that the great 

 development of mica-schist occurs, the origin of which is a problem 

 yet waiting solution. Many appearances would seem to indicate 

 that it is an advanced stage in the metamorphism of the hornblende- 

 schist, for the reasons already given in my former papers.^ I have 

 only now to add that on extracting a number of lenticular or nodular 

 decomposed masses from the heart of the mica-schist on the west 

 side of the road leading down into the Cove, and on breaking them 

 up their centres were found to consist of the green porphyritic 

 diorite or diabase-like rock so common in the reefs of the Cove. To 

 this may be added the fact that the mica-schist west of Polpeor 

 Cove, at and near the Lizard Point, contains similar nodules of the 

 ordinary hornblende-schist. 



1 Geol. Mag. Vol. VI. 1889, p. 425. 



2 Geol. Mag. 1890, Vol. VII. p. 163. 



