'16 PROF. W. J. SOLLAS ON RECUMBENT [NOV. I906, 



32. Recumbent Folds produced as a Result of Flow. By "William 

 Johnson Sollas, Sc.D., LL.D., F.R.S., F.G.S., Professor of 

 Geology in the University of Oxford. (Read June 13th, 1906.) 



Our views as to the various kinds of deformation which affect the 

 earth's crust received a remarkable extension with the memorable 

 discoveries of Lapworth, Peach, and Home in the north of Suther- 

 land. These have now culminated in the long series of revelations 

 which we owe to Bertrand, Rothpletz, Schardt, Kilian, Haug, 

 Lugeon, Termier, Suess, and other workers in various regions of the 

 Alps, who seem to have accomplished a veritable revolution in this 

 branch of enquiry. 



The long recumbent folds, which are perhaps the most surprising 

 of the new forms of disturbance lately brought to light, can be more 

 readily demonstrated than explained. 



If we turn to one of the most complete and consistent accounts 

 of these phenomena, Lugeon's description of the pre-Alps of Chablais, 

 we perceive a series of recumbent folds, so gieatly exceeding in 

 horizontal extension their thickness vertically, that they are 

 commonly spoken of as sheets rather than folds : they lie with 

 remarkable flatness one on the other ; and as a rule those higher in 

 the series extend farther to the front than those below, a feature 

 referred to as 'deferlement' by the French, or 'leap-frog' as 

 translated by my friend, M. Allorge. 



The roots or origin of several of the lower of these folds are 

 visible in the high Alps adjacent, but the roots of the higher folds, 

 which form the pre-Alps, must be sought in the zone of Mont Blanc 

 and the Brianconnais. Thus, some of the uppermost folds may have 

 surmounted the obstacle presented by Mont Blanc on their way to 

 the front in the pre-Alps. 



It is no doubt true that overfolds may be traced into normal 

 anticlines, and if long recumbent folds may be regarded as merely 

 exaggerated overfolds, we may admit that their origin is not beyond 

 our powers of comprehension ; but, even in this case, their subsequent 

 history presents many difficulties to the imagination. 



Many of the features presented by recumbent folds are more 

 suggestive of flowing than bending, and long ago this led me to 

 offer a comparison between some of the features presented by the 

 now-lines in pitch-glaciers with those made familiar to us by the 

 sections of M. Marcel Bertrand. 1 An account of my first experi- 

 ments with pitch -glaciers was brought before the Geological Society 

 in 1895 2 ; but, as no detailed description has been published of others 

 made subsequently, I take this opportunity to present the results of 

 one or two of them, which recall in several striking peculiarities 



1 Eep. Brit. Assoc. 1895 (Ipswich) p. 680. 



2 Quart. Journ. Geol. Soc. vol. li, p. 361 



