1854.] HORNER — IGNEOUS ROCKS IN CAWSAND BAY. 359 



In the vicinity of the Aar glacier, in the Bernese Alps, the rocks 

 of granite and gneiss offer a curious and instructive exemplification 

 of these phsenomena. 



On the mountains on the left side of this glacier are seen numerous 

 small systems of these concentric foliations ; the mountains being 

 covered with little rounded bosses {roches moutonnees). In this 

 instance these roches moutonnees cannot have been produced, as 

 is indeed the case in other places, by the action of glaciers ; for, 

 by investigating their structure more closely, and by following the 

 different ravines which disclose the interior of the rocks, we clearly 

 see that these rounded bosses are produced by concentric exfolia- 

 tions, and that the lowest visible laminae are equally well polished 

 as those on the very surface of the rocks. An additional proof 

 of this conclusion is seen in the higher parts of the Aar glacier. 

 We have there the limits between the granite and gneiss rocks 

 and the stratified chlorite and mica slates, which latter do not show 

 any of the concentric exfoliations. As soon as the granite appears, 

 we find these very remarkable rounded bosses, which are wanting 

 farther east in the schists. Had the glacier produced these rounded 

 rock-surfaces, both formations would equally have been rounded and 

 smoothed. 



In comparing well-preserved polished rock-surfaces resulting from 

 the action of glaciers, with the polished surfaces due merely to the 

 process of the concentric exfoliation of gneiss and granite, it is quite 

 possible to find some distinctive characters. On the polished surfaces 

 produced by glaciers the strise run generally pretty parallel to the 

 longitudinal axis of the valley ; whilst on the surface of the concentric 

 layers the strise follow generally the line of the maximum of the 

 inclination of the laminae. It is also very often to be observed that the 

 polished concentric surfaces have a somewhat greater lustre, which 

 may probably have been produced by small exudations of siliceous 

 matter during the process of concentric exfoliation. These few 

 remarks will be sufficient to show how desirable it is that the 

 interior structure of rocks should be examined with the greatest 

 care, and at different places ; so that we may not confound, in all the 

 extensive gneiss and granite districts of the Alps, the striae and 

 polishings of glaciers with other phsenornena, very similar at first 

 sight, but which have been produced by quite different causes. 



May 3, 1854. 



John Petherick, Esq., and John Coode, Esq., C.E., were elected 

 Fellows ; and M. Joachim Barrande was elected a Foreign Member. 



The following communications were read : — 



] . On some Intrusive Igneous Rocks in Cawsand Bay, near 

 Plymouth. By Leonard Horner, Esq., F.R.S., F.G.S, 



Several of the principal features in this locality are described by 

 Sir Henry De la Beche, in his " Report on the Geology of Cornwall, 



VOL. X. — PART I. 2 C 



