Alexander Somervail—On the Lizard Distinct 365 



common magma, than as an altered portion of the hornblende-schist 

 as I then described it — my object at the time being to show that it 

 was not a dyke ; a view I am still convinced of. I now pass on to 

 recent observations. 



These latter observations were made over the whole area, that is, 

 from Polurrian on the west to Porthallow on the east, both of these 

 localities marking the boundary between the killas or slates on the 

 north, and the igneous rocks of the Lizard district to the south. 



I shall briefly localize these observations, beginning with : — 



Porthallow. — In the killas or slates here, at a few feet from their 

 junction with the hornblende and serpentine series, there is a 

 quartzose-like-rock at the base of the cliff, immediately below the 

 iron-lode described by Mr. J. H. Collins, F.G.S.^ The relations of 

 the quartzose-like-rock to the surrounding slates is exactly like 

 that of an intruded igneous mass, which also in its upper portion 

 contains included fragments of the slate. On breaking well into 

 this intrusive like rock, its interior is found to present a very 

 different aspect from its exterior ; the former is chloritic-like, re- 

 sembling a diabase that has undergone much alteration. It contains 

 a large amount of iron-pyrites, and recalled to my mind the chloritio 

 rocks of the Start area in Devonshire, especially those near, or at 

 the junction with, the slates in the Bickerton Valley. 



Following the cliff towards the Inn, at about 36 paces from which 

 (near a building with outside stair and thatched roof) a gabbro 

 (much decomposed) most unexpectedly occurs, where only the 

 ordinary slates were hitherto supposed to exist. This gabbro has 

 all the appearance of cutting through the slates and the arenaceous 

 beds associated therewith ; but on this point I could not arrive at 

 absolute certainty. In the cliff behind the Inn there is also diorite 

 of the greenstone type, but much decomposed. Higher up the road, 

 behind and forming the foundations of the cottages, there is more 

 gabbro and diorite, the former rock cropping out on the roadway 

 and continuing southwards in the direction of the road. 



The occurrence of these rocks, where only slates were supposed to 

 exist, is of considerable importance, especially as they seem to rise 

 through the slates, as does also the adjoining serpentine at Pen- 

 garrock, which, if the case, when connected with the gabbro and 

 serpentine, and perhaps even the greenstone occurring at Nare 

 Head, Gerran Bay, would clearly prove these Lizard eruptive rocks 

 to be subsequent to the killas. However this may be, the occurrence 

 of the gabbro and diorite or greenstone, forming the cliff described, 

 will necessitate a very considerable alteration of the boundary on 

 the present map. 



Eeturning again to the junction on the coast between the eruptives 

 and slates, I examined the hoi'nblende-schist which immediately 

 succeeds the serpentine, tracing it along the entire length of the 

 top of the cliff where it has been quarried. It certainly is a very 

 variable rock, in some portions schistose, in others quite massive; 

 in one portion a diorite, in another passing into a variety of green- 

 1 Quart. Journ. Geol. Soc. vol. 40, 1884, p. 463. 



