TRANSACTIONS OF SECTION C. 707 



Within the zone, folia of quartz-felspar (the compressed granite-veins) alternated 

 ■with the dioritic material. By further loss of bases the ultimate product was 

 sometimes a quartzose gneiss or even a gneissoid quartzite. 



The same principles were found to be on the whole applicable to the Gneissic 

 Series of Anglesey ; but in that area the earth-pressures were greater and more 

 uniformly distributed, so that contortion was excessive and the bands of non- 

 schistose rock were in smaller proportion. Diorite was modified into hornblendic 

 and chloritic schists ; or, in the vicinity of granite-veins, into mica-gneiss. Felsite 

 passed into mica-schist. Other changes were not yet worked out. 



No limestone was known in the Malverniau rocks, but calcite-veins, when 

 abundant, were associated with rotten ferruginous schists. In Anglesey the 

 crystalline limestones were in lenticular masses, and were overlain by rotten 

 schists intermixed with quartzose bands. It seemed probable that these limestones 

 were endogenous deposits derived from the decomposition of the adj acent rocks. 



In the transformation of the igneous rocks into schists, the hornblende and 

 felspars were converted into black and white micas, quartz, chlorite, epidote, 

 sphene, garnets, and iron-ores. Such profound chemical changes suggested that the 

 view of metamorphism here advocated should be called the Chemico-mechanical 

 theory. 



8. The Origin of Banded Gneisses. Bij J. J. H. Teall, M.A., F.G.S.^ 



The author first discussed the meaning of the term gneiss. This term was 

 generally understood to connote a more or less foliated rock of granitic composi- 

 tion. Dr. Lehmaun had proposed, however, that it should be used in a structural 

 sense only, as meaning a more or less foliated plutonic rock. He would thus speak 

 of granite-gneiss, diorite-gneiss, and gabbro-gueiss. The author called attention 

 to specimens illustrating gneissic structures in acid and basic plutonic rocks. 

 AVhen various examples of gneissic rocks — that is, rocks of the composition of 

 plutonic igneous rocks but possessing parallel structures — were compared, two 

 types of parallel structure might be recognised ; the one characterised by a parallel 

 arrangement of the constituents, the other by an arrangement of the constituents 

 in bands of varying mineralogical composition ; thus, bands having the mmeralo- 

 gical composition of a diorite frequently alternated with others having the 

 composition of granite. He proposed to discuss a possible mode of origin for the 

 banded gneisses of the latter type. It was now admitted that those of the former 

 type were largely due to the plastic deformation of masses of plutonic rock either 

 ■during or subsequent to the final stages of consolidation. 



Many observers were, however, still inclined to believe that those of the latter 

 type could only be accounted for by supposing that the original materials had ac- 

 cumulated by some process akin to sedimentary deposition. Now a possible mode of 

 origin for these could be found if we could show : (1) that plutonic masses are 

 liable to vary in composition, and (2) that such masses are occasionally deformed 

 either during or subsequent to their consolidation. Scrope long ago proved that 

 the laminated structure of certain volcanic rocks (liparites) is due to the plastic 

 deformation of heterogeneous masses of acid lava. Any heterogeneous lump if 

 deformed into a flat sheet will show laminated or banded structures, because each 

 individual portion must of necessity take the form of the entire mass. Scrope not 

 only proved this, but also called attention to the similarity between the structures 

 of acid lavas and those of gneisses and schists. (Geology of Ponza Isles, ' Trans. 

 Geol. Soc' 2nd ser. vol. ii. p. 228.) 



The author then proceeded to refer to illustrations of the fact that plutonic 

 masses do vary in composition. He referred to the so-called contemporaneous 

 veins, which are often more acid, and to the concretionary (?) patches which are 

 often more basic in composition than the main mass of the rock with which they 

 are associated. He also referred to cases in which granite and diorite may be 

 seen to vein each other in the most intricate manner, and especially drew attention 

 to photographs taken at the Lizard last year illustrating this feature. If complex 



' Printed in full, with illustrations, in Geol. Mag. for 1887, p. 484. 



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