298 



PROE. T. G. BONNET ON THE SOUTHERN ORIGIN [Aug. I907, 



illustration of how readily, if Prof. Lugeon's interpretation be 

 correct, rock-masses can adapt themselves to their environment. 

 It is a portion of a long section through the Prealpes medianes 

 on the right bank of the valley of the Drance du Biot. Here, on 

 the north-west side of the Col de Brion, we find that an advancing 

 nappe, consisting of a bed of Trias overlain by two breccias 

 (parted by a ' schiste ardoisier '),* has slid over Upper Cretaceous 

 strata, and has thrust itself, together with a captive piece of 

 quartzite and of Permian, like the end of a bent ringer into a mass 

 of FTysch (fig. 2), which, to allow of such an intrusion, must have 

 been as plastic as putty. I shall no doubt be told that masses of 

 rock may « flow ' when subjected to great pressure. Perhaps they 

 may, but it must be shown that this general statement is appli- 

 cable here, and we must be careful not to let a phrase beguile us. 



Pig. 2. — Section in the Chablais Alps {part of a long section, 

 fig. 2, pi. wiv, of Prof. Lugeon's paper). 



A.__. 



A = Col de Brion. 



B — Tavaneuse. 



C = Pointe de Nantaux. 



1 = Malm. 



2 = Cretaceous. 



2 a = Upper Cretaceous. 



3 = Flvsch. 



4 = Trias. 



5 = Quartzite (Trias). 



6 = Permian. 



7 = Breche inferieure. 



8 = Schistes ardoisiers. 



9 = Breche superieure. 



In lead, under ordinary pressures ; in iron, under exceptional, as 

 Tresca has shown, 2 deformation may be produced without rupture 

 of continuity, to which the term ' flow ' may be applied. Under 

 the latter conditions, as Prof. F. D. Adams 3 and others have shown, 

 similar deformation can be produced in marble, but here the move- 

 ment is mainly the result of a kind of fracture and regelation, to 



1 This triplet apparently represents the upper part of the Trias, with 

 perhaps the lowest part of the Jurassic. 



2 Comptes-rendus Acad. Sci. Paris, yol. lix (1864) p. 754, & vol. Ixiv (1867) 

 p. 809. 



3 Geol. Mag. 1897, p. 513, & Phil. Trans. Eoy. Soc. ser. A, vol. cxcv (1901) 

 pp. 363-401. 



