22 , PROF. T. G. BONNEY ON THE [Feb. 1 896, 



crushing or of recomposition, whether as serpentine or as peridotite. 

 The breadth and persistency of the bands in this hornblende-schist, 

 as I have more than once pointed out in similar cases, are further 

 difficulties in the dynamo-metamorphic hypothesis. In order to 

 obtain by crushing bands such a3 those described above, the original 

 rock would have had to be almost incredibly coarse ; yet such bands 

 are not infrequent both at the Lizard and in Sark. 



As regards the ' stratification ' hypothesis which I formerly ac- 

 cepted, I may mention a difficulty, the gravity of which has increased 

 with my experience. If the rock originally were a basic tuff 

 (and no other materials would give us the appropriate chemical com- 

 position), the conditions of deposit must have been very exceptional. 

 Undoubtedly, beds of fine-grained ash occasionally exhibit great 

 regularity of structure, but commonly this is speedily interrupted 

 by the setting in of coarse material, the rock becoming ' knubbly' 

 if not agglomeratic. But these hornblende-schists are singularly 

 free from blotching or irregular spotting 1 ; the bands of different 

 mineral composition alternate just as the arenaceous and argillaceous 

 layers at Morlaix in Britanny or Port Erin in the Isle of Man. 

 This difficulty might, indeed, be eluded by supposing the materials 

 to have undergone metamorphism so extreme as to dissociate the 

 constituents of the bits of tachylyte and the lumps of scoria, 

 and that Ito have been followed by an orderly process of segre- 

 gation somewhat analogous to what occurs in the formation of 

 flint, converting ultimately the rather heterogeneous materials into 

 fairly uniform mixtures of minerals not at all minute. This 

 hypothesis, however, though obviously a possible one, obliges us, 

 as I feel more and more strongly, to assume conditions which are so 

 exceptional that they can hardly have prevailed over such large areas. 

 Hence I am forced to adopt the other hypothesis. 2 



III. The Genesis op the ' Granulitic Group/ 



On this subject fewer words are needed. I may content myself 

 with saying that the result of this visit has been to strengthen my 

 confidence in the correctness of the explanation adopted in our last 

 paper, 3 namely, that these banded rocks have been produced, as 

 probably most banded gneisses have been produced, by fluxional 

 movements in a heterogeneous magma ; the cause of heterogeneity 

 in this case being, as in the one described in Sark, 4 the intrusion 

 of an acid into a more basic rock, by which the latter has been 



1 The nearest approach to this is the case near Mullion Cove, described above, 

 and even here there is nothing resembling the irregularity which is usual in 

 volcanic ashes. 



2 It also accords better with that which is now generally adopted for the 

 granulitic group. This is supposed to be almost in the condition which it 

 assumed on consolidation ; but its darker member is sometimes not very 

 different from corresponding parts of the hornblende-schist, so that one can 

 hardly assume the two rocks to have had totally diverse origins. 



3 Op. cit. vol. xlvii. (1891) p. 477. 4 Op. cit. vol. xlviii. (1892) p. 132. 



