164 A. Somervail — Schists of the Lizard District. 



There are also as good evidences to show that these rocks — if 

 they do not pass directly into — form part and parcel with those at 

 the base of the Lizard Head, and the outlying rocks, which may 

 well be claimed as granulitic, the whole having in my opinion a 

 common age and origin, although differing widely in their litho- 

 logical aspects. A transition, however, can be noted between the 

 greenish or actinolitic-schist and the hornblende, and hornblendic, 

 and granulitic gneissic rocks of the reefs. Many such like transi- 

 tions have been noted and described by Messrs. Teall and Fox in 

 their joint paper already referred to. 



To my mind it seems quite clear that these " talco-micaceous " 

 rocks have no separate existence apart by themselves, but are for the 

 most part formed out of the "hornblende" series, by severe mechan- 

 ical pressure and accompanying chemical change. Mr. Teall l has 

 most correctly described this very area as a zone of intense dynamic 

 metamorphism, and I think it is almost impossible for any observer 

 not to note the fact, that here, more than in any other portion of the 

 district, the original structure, texture and mineral constituents of 

 these rocks have been wholly changed and replaced by secondary 

 products to a very high degree. 



The argument of their distinctness as a group drawn from their 

 infra-position to the " hornblendic " and " granulitic " rocks, 2 is, I 

 think, quite fallacious, as this is based on their treatment as a strati- 

 fied series of deposits having an ascending rise from south-west 

 to north-east, which however is only a system of divisional and 

 cleavage planes, from which neither the true thickness, nor yet the 

 true positions of the several masses of rock can be correctly estimated. 



Provisionally, then, we shall take for granted the non-existence of 

 the talco-micaceous group, regarding it meanwhile as but altered 

 portions of the two former, which, right or wrong, will not how- 

 ever materially affect the points at issue in this paper. 



IV. Origin of the Granulitic and Hornblende Groups. 



As to the correct nature and origin of the rocks making up these 

 two groups, there is considerable diversity of opinion. 



Prof. Bonney 3 in his subsequent and modified views is still 

 inclined to regard considerable portions of the rocks in each group 

 as of sedimentary origin. 



Mr. Teall inclines to similar opinions, but seems to restrict those 

 rocks which he believes to be of sedimentary origin to a much 

 smaller compass than is done by the former observer. 



General McMahon, 4 F.G.S., in his recent able contribution to this 

 subject, expresses the opinion that a certain portion, or portions of 

 each group is made up of volcanic ashes and lavas. 



If I may here venture an opinion of my own as to the exact mode 

 of origin of these rocks, it is that there are no true tuffs or lavas, 

 but that the whole are essentially rocks of plutonic origin. This 



1 Notes on the Lizard, from the Long Excursion of the Geologists' Assoc, for 1887. 



2 Quart. Journ. Geol. Soc. vol. xxxix. p. 5. 



3 Q.J.G.S. vol. xlv. p. 543. * Q.J.G.S. vol. xlv. p. 519. 



