168 A. Somermil — Schists of the Lizard District. 



This statement I must confess is rather fatal to my own views ; 

 but before forming a final opinion, I should like again to revisit this 

 locality, as I cannot but think that the General may possibly have 

 mis-read the section. 



There is another example of , the intrusion of a porphyritic diabase 

 in the talco-micaceous group at Polpeor, first noted by Prof. Bonney, 1 

 and also referred to by General McMahon, 2 as follows : " The 

 eruptive character of this rock is undoubted. At low-water during 

 spring-tides, the reefs outside the island at Polpeor are exposed for 

 about a mile from the shore. The porphyritic diorite may be seen 

 in these reefs cutting right across the strike of the gneiss. Nearer 

 land it is an intruder in the green schists, generally following their 

 bedding, but constantly shifting from one horizon to another ; 

 whilst in the cliffs of the mainland it frequently shows itself at a 

 still higher horizon among the micaceous and hornblendic-schists." 



Prof. Bonney has certainly described this porphyritic diabase, or 

 diorite as a true intrusive dyke, and the above quotation lends it all 

 necessary support. When it was first pointed out to me by Mr. 

 Howard Fox, I could not then and cannot yet separate it from the 

 other hornblendic rocks or schists exposed there, as it seemed to 

 me to form only a portion of these, but. in a much less crushed 

 condition. I afterwards had the pleasure of finding that Mr. Teall 

 had previously expressed the same views in his notes for the " long 

 excursion " of the Geologists' Association for 1887. 



The relations as to the upper and lower position of these two 

 groups is, I think, rather a difficult question. Prof. Bonney, who 

 dealt with this matter on stratigraphical grounds, referred the 

 " granulitic " to the uppermost position ; but there seem to me good 

 reasons for nearly reversing this conclusion, or at least modifying 

 it to a considerable extent. There is the actual evidence of rocks 

 with as good a claim as any to.be regarded as " granulitic," holding 

 the infra-position, which may be seen by boat in sailing round the 

 Lizard Head, where this group is found supporting the hornblende- 

 schists. This infra-position, however, I believe to be somewhat 

 irregular, and to be regulated by certain circumstances, although on 

 the whole I am inclined to regard the hornblende as the upper and 

 outer margins of the same magma out of which both have been 

 formed. 



VI. Concluding Eemarks. 



I cannot conclude this paper without remarking that none of the 

 views therein are held dogmatically, and expressing the hope that 

 fox-mer observers will renew their attention, and fresh ones take up 

 the investigation of this complicated, yet interesting area. The 

 difficulties are great, among which is. the origin of the banded 

 structure, in the rocks herein dealt with, which in my next paper 

 I shall try to explain on the grounds of segregation, arising from, or 

 taking place during the cooling of the common magma out of which 

 all these rocks seem to me to have been formed. 



1 Quart. Journ. Geol. Soc. vol. xxxix. p. 4. 



2 Quart. Journ. Geol. Soc. vol. xlv. p. 534. 



