Vol. 62,] FOLDS PRODUCED AS A RESULT OF FLOW. 717 



some of the structures brought to light by Prof. Lugeon in the pre- 

 Alps. 



In the example that we may consider first (fig. 1, p. '718), the 

 pitch-glacier was built up in the way already described in my 

 previous paper. The three layers, of which the upper surface is in- 

 dicated by the lines a, 6, c, were placed in the experimental trough on 

 April 3rd, 1895; it will be seen that they lie wholly behind the 

 obstacle marked 0. A fourth layer (d) was added on April 9th, its 

 anterior termination lying just upon the summit of the obstacle. 

 The experiment was brought to an end on June 12th, and, on cutting 

 the ' poissier ' or pitch-glacier longitudinally through the middle, 

 the layers «, 6, c were found to have assumed the forms shown by 

 the lines a, b', c. The general resemblance between these folded 

 lines and some of the folded sheets in the Alps is sufficiently obvious ; 

 the second, marked c\ with its ' carapace ' of folds is not unlike the 

 Morcles fold behind the Diablerets ; and my friend Prof. Lapworth 

 compares the third (b') with the Pilatus and Sentis, and the 

 fourth («') with the overslide of the Bavarian front. 



The roots of the experimental folds lie on the other side of the 

 obstacle 0, which may be imagined to stand for Mont Blanc. In this 

 respect they recall the views of Prof. Haug, who, to give one instance 

 only, brings the zone of the Aiguilles d'Arve over the summit of 

 Mont Blanc to form the recumbent fold of the Diablerets. 



The whole of the four lines exhibit the phenomenon of deferle- 

 ment, and may be compared in this respect with Prof. Lugeon's 

 illustration of the three folds of Morcles, the Diablerets, and Mont 

 Gond. 1 In the case of the pitch-glacier the cleferlement is clearly 

 a necessary consequence of the conditions of the experiment. 



According to our present conceptions, there is one very marked 

 difference which distinguishes the folds exhibited by this pitch- 

 glacier from those of the Alps. In the former, the several folds 

 originate from sheets which were superposed on each other at the 

 commencement of the experiment, and in consequence the lower 

 limb of each fold is adjacent to the similar limb of its neighbours, 

 that is, the folds are ' emboite's ' one in the other. In the mountains, 

 on the other hand, the lower limb of a superior fold reposes on the 

 upper limb of the one immediately beneath it, that is, the folds are 

 superposed, and not fitted one into the other. 



In another experiment, however, made between March 24th 

 and June 7th, 1S95, even this difference has disappeared, or at least 

 become greatly reduced. In this instance (fig. 2, p. 718), two 

 obstacles O x and 2 were placed in the path of the pitch : restricting 

 our attention to the second layer, the original position of which is 

 shown by the straight line a a', it will be seen to have formed 

 three folds one behind the other, 2 all lying on the foreland beyond 



1 ' Les Grandes Nappes de Kecouyrement des Alpes du Chablais & de la 

 Suisse ' Bull. Soc. Geol. France, ser. 4, vol. i (1901) fig. 3, p. 731. 



2 Indicated by the thickened lines, the front fold lies above the words 

 ' 24 March ' in fig. 2. 



