A. Somervail — Banded Rocks of the Lizard District. 509 



The origin of the banded structure in the " granulitic " and 

 "hornblende" group of rocks in this district has been accounted 

 for in three separate ways by sedimentation, deformation, and by 

 injection. 



Sedimentation. — This explanation was very naturally and con- 

 sistently given by Prof. Bonney on the ground of the rocks being 

 thought by him to be of sedimentary origin. There are, however, 

 certain arrangements in the banded structure of the "granulitic" 

 group which no false bedding could produce, or even simulate. 

 These arrangements of structure I have already given as quoted 

 from General McMahon, and I have myself also noted other appear- 

 ances such as the numerous fine thread-like processes and broader 

 ones, which cross at various angles from the one band to the other. 



If, on the other hand, these rocks are of igneous origin— as I 

 believe them to be — then we must seek for some other cause to 

 explain their banding, even allowing that some of them were 

 originally ashes, altered and deformed by subsequent dynamical 

 causes. 



Deformation. — Mr. J. J. H. Teall, M.A., F.R.S., F.G.S., has 

 advanced this theory and has applied it both by observation and 

 experiment in a clear and able manner. He regards a large portion 

 of the rocks at least forming the ' granulitic ' group as a complex of 

 igneous rocks partly dioritic and granitic, deformed by dynamic 

 causes, during, or subsequent to their consolidation. This theory 

 (if I mistake not) not only pre-supposes, but points to the existence 

 of heterogeneous masses of these rocks in the field which were the 

 raw materials out of which the banded structure was wrought by 

 subsequent dynamic agencies, which kneaded them more completely 

 together, and produced the secondary results of banding as now 

 exhibited. If this theory is confined to the mere mechanical 

 movement of the rocks without the aid of their partial re-fusion 

 and the consequent effect of chemical agency, it seems to me 

 very hard to explain much of the phenomena presented by the 

 banding as we see it in the field. The subsequent movements 

 might, and I believe do account for much of the eccentricities 

 exhibited by the course of the bands, such as the flexures and 

 zig-zag features occasionally observed, along with other signs of 

 deformation, which has modified but not produced this structure. 

 This theory also seems to account for the quasi-banding in portions 

 of the " hornblende " group, such as the thin deposits of felspar, 

 epidote, etc., developed between the cleavage-planes of the schistose 

 varieties of these rocks, but, to account for the normal or true baud- 

 ing when it does occur, it seems almost powerless. 



The principal objections to this theory presenting themselves to 

 my own mind, as far as the "granulitic group " is concerned, are as 

 follows : — 



1. The seeming inability of deformation to produce out of an 

 igneous complex the symmetry frequently prevailing in the banded 

 structure. This symmetry consists of the very parallel arrangement 

 of the various bands exhibited in some localities ; one band of diorite 



