A. Somervail — Banded Rocks of the Lizard District. 5 1 1 



in both (what I may term) the heterogeueous and homogeneous 

 granulitic masses. Although I must admit such an arrangement 

 can (to a certain extent) be produced by pressure exerted on clays, 

 as described by Mr. Teall, by artificial means, but not to meet all 

 the facts as presented in the field. Third, the injection theory, 

 because even in the solitary instance in which it can be applied, 

 the parallel bands of diorite and granite show no decided signs of 

 intrusion into each other. 



III. Due to Segregation. 



The only theory which to my mind seems to explain the banded 

 structure in these Lizard rocks is segregation. (By this term I 

 mean the separation of the unlike, and the union of like mineral 

 constituents taking place during their cooling.) It seems to me to 

 satisfy all the different aspects of their banded structure ; and these, 

 as already stated, present a considerable variety of more or less 

 distinct types, some of them so palpably due to this cause that in 

 my field notes ; made during a number of years, I have again and 

 again the word "segregation " written against the description of these 

 banded rocks. 



If I can succeed in showing — as attempted in my former paper — 

 that the varieties of dioritic and gi'anulitic rocks were differentiated 

 out of a common magma, then it would seem to follow that such 

 a separation on a smaller and less perfect scale, under conditions 

 to which the term segregation might be more properly applied, 

 would produce such a result as the banding in all its varied aspects. 



The proofs already given of such a separation need not here be 

 again recapitulated ; but to those proofs might be added the fact 

 mentioned by Mr. Howard Fox and myself in a joint paper ' that 

 the hornblendic and felspathic lines circle round the porphyritic 

 crystals of felspar in the dark or dioritic bands of the granulitic 

 series. These lines must have so arranged themselves after the 

 cooling of the felspar forming these crystals, which, besides being 

 an excellent proof of the segregation of these mineral constituents 

 from the common magma, is also a convincing one of the igneous 

 origin, and, let me add, also of the plutonic origin of these rocks. 



Besides these facts, I have already drawn attention to others 

 bearing out the important part that segregation has played in these 

 Lizard rocks, in my short paper " On the Greenstone and Associated 

 Bocks of the Manacle Point," 2 which seems to afford the only 

 possible explanation of that area. 



Still further, in my last communication, already referred to (p. 166), 

 I drew attention to a concretionary mass of granulitic rock at Kil- 

 down Cove composed of concentric layers of dioritic and granulitic 

 minerals, which is simply the banded structure in a rudely circular 

 form produced by segregation. This mass is certainly not the 

 product of an original and an intruding member, but clearly the 



1 Geol. Mag. Dec. III. Vol. V. p. 77. 



2 Geol. Mag. Dec. III. Vol. VI. p. 425. 



