J. J. H. Teall — Origin of Banded Gneisses. 487 



of crystalline rocks running E. and W. a few miles south of Haver- 

 fordwest, in Pembrokeshire, and similar rocks show similar relations 

 in the Lizard peninsula. The last-mentioned case will shortly be 

 described at greater length. 



We have now to consider the question of deformation. No one 

 will doubt that deformation necessary to produce banded structures 

 in heterogeneous plutonic magmas may take place in connexion 

 with intrusion. The only question that can arise is whether many 

 banded gneisses have originated in this way. At present I have no 

 definite opinion on this point. There is, howevei", another way in 

 which the necessaiy deformation may be produced, viz. by the 

 operation of the earth-stresses of which we have such striking 

 evidence in mountain-regions. With reference to this mode of 

 deformation I will content myself by referring to the work of Heim 

 and Baltzer in the Alps, of Reusch in the Bergen peninsula, of Leh- 

 mann in the gi-anulitic region of Saxony, and of Lapworth and the 

 Geological Survey (Messrs. Peach and Home) in the north-west of 

 Scotland. It will not be doubted by any one who has studied the 

 writings of these authors, and is at all familiar with the phenomena 

 they describe, that lai'ge masses of country have been profoundly 

 modified by the earth-stresses. It may be a long time before we 

 become acquainted with the precise nature of the movements and 

 deformations, but that they ai'e adequate to convert a heterogeneous 

 plutonic mass into a banded gneissic series is to say the least highly 

 probable. It must of course be remembered that sedimentary rocks 

 may also be involved in the movements, so that the occasional 

 presence in a gneissic series of bands having the composition of 

 sedimentary rocks is no proof that the whole series is of sedimentary 

 origin. 



Having by these general remarks established, as I believe, the 

 a priori probability of the theory I am endeavouring to illustrate, I 

 will now refer to what appears to me to be a case in point. In the 

 Lizard district of Cornwall we find two or three rock-types occurring 

 as portions of an igneous complex and as portions of a banded 

 gneissic series. Where they exhibit the relations of igneous rocks 

 (Fig. 1, Plate XIV.), they may be termed granite, diorite andgabbro; 

 where they occur as portions of a banded gneissic series they possess 

 the same specific gravity, but usually show a more or less marked 

 parallel structure and in certain cases differ in mineralogical com- 

 position. Consider first of all the petrographical characters of the 

 different rock-types. 



Granite. — This rock never occurs in large independent masses, but 

 only as comparatively small veins and dykes. It is found in gabbro 

 (north of Pen Yoose), serpentine (Kynance) and diorite (Pen 

 Voose). It is present also, in a somewhat modified form, in the 

 banded gneissic series — the "granulitic series" of Prof. Bonney.'' 

 The least modified rock is a fine-grained granular aggregate of 

 quartz, felspar and dark mica. The felspar is mostly unstriated, 

 but a striated variety also occurs. Both felspar and quartz are 

 1 Quart. Jourii. Geol. Soc. vol. xxxix. (1883), p. 1. 



