398 PEOE. T. G. BONNEY ON THE SO-CALLED ' GNEISS ' 



Carboniferous rock ; for instance, in that from Yorsaas (p. 393). 

 This is finer grained and more definitely banded with biotite than 

 is usual ; still, under the microscope, the distinction noted in the 

 field becomes yet more manifest, and the rock is evidently very 

 difi'erent from its neighbours of clastic origin. The quartzes and 

 felspars are granular in outline, but they sho-w obvious signs of 

 having been structurally affected by subsequent pressure. The 

 biotite, however, is still fairly idiomorphic, and is very different 

 from the streaky groups of flakes in the ' Carboniferous gneiss.' 

 In the coarser varieties also of the true crystalline rocks, the 

 distinction is not less marked ; these exactly resemble the ordinary 

 types of the granitoid gneisses and gneissoid granites of the great 

 massif of the Oberland. 



To sum up : my study of these Guttannen rocks, both in the field 

 and with the microscope, leads me to the following conclusion. As 

 is the case with other rocks of Carboniferous age elsewhere in the 

 Alps, they are composed exclusively, or almost so, of the debris of 

 the crystalline rocks of the neighbourhood. Hence they often, like 

 the Torridon Sandstone of Scotland or the Gres Feldspathique of 

 Normandy, are mineralogically identical with a granite or a granitoid 

 gneiss, and occasionally cannot be readily distinguished even struc- 

 turally. But as they are really composed of fragments, water has 

 found a rather easy passage through them, and the pressure to 

 which they have been exposed has facilitated micro-mineral change. 

 The latter agency, by squeezing together and slightly distorting the 

 original fragments, renders the clastic structure less easily recog- 

 nizable. To some extent a new structure has been impressed upon 

 the rock, such as is very conspicuous in the Gres Fddspatliique at 

 Tourlaville, near Cherbourg, when this is compared with the same 

 rock from Omonville (about fifteen miles away to the west), or in a 

 specimen of Torridon Sandstone, which has been taken from the 

 region of the thrust-faults. But the secondary change in the 

 Guttannen rocks is not greater than in either of these instances ; 

 indeed, it is sometimes surpassed in them. It is very similar to 

 that which occurs in rocks of the same age in the district between 

 the Tete Noire and Yernayaz.^ So far as I know, then, this is the 

 only peculiarity : that, in most parts of the Alps, these curious 

 imitations of crystalline rocks are limited in extent, and the fraud 

 is quickly revealed by the incoming of beds of conglomerate or of 

 slate, while at Guttannen I did not happen to discover either the 

 one or the other. So, if we are prepared to call the Torridon 

 Sandstone a granite, and the Gres FeldspatJiique a gneiss, simply 

 because here and there it would be difficult to point out a distinction 

 which would appeal at once to an inexperienced eye — simply because 

 they are rather clever imitations — then we may call the Guttannen 



1 See G-eol. Mag. for 1883, p. 507 : also A. Favre, ' Eecherches Geol. dans 

 les Parties de la Savoie . . . voisines du Mont Blanc,' § 522 ; and Sterry Hunt, 

 ' Chem. and Geol. Essays,' p. 339 (ed. 1875), for a history of Hke confusion as to 

 these imitation-schists of Carboniferous age. 



