210 Prof. Favre on the Origin of 



These pebbles mast consequently have passed over the de- 

 pressions of the lake. But how could they do so, since their 

 transport appears to be anterior to the development of the 

 glacier ? That is the difficulty, and it is this which has given 

 rise to the notion of the theory of excavation, in which it is 

 supposed that the pebbles of the older drift have been heaped 

 up by pre-glacial currents in the depths of the lake, and that 

 when the glacier reached them it excavated that portion of the 

 lake which had been filled up. It is supposed, then, that it has 

 produced a great excavation, and that it then spread before it 

 all this enormous mass of pebbles which it drew from this great 

 depression. This idea, generalized and applied to other localities, 

 has produced the hypothesis which is known by the name of 

 the theory of excavation. I think I have been impartial in this 

 explanation. 



To this theory I believe I am able to offer objections which 

 seem to me to be very serious. In the first place, when the 

 glacier originally began to carry away from the depths of 

 the Lake of Geneva all the enormous mass of pebbles which 

 is now deposited lower down, how did it effect it ? Did 

 the glacier slide over the solid rock without leaving any interve- 

 ning mass of these pebbles between the two ? For if it left 

 beneath it no pebbles, it ought to push before it an enormous 

 mass of this debris, such a mass as can with difficulty be repre- 

 sented — a mode of action which would be the more singular, 

 because nothing amongst existing causes countenances this sup- 

 position, no part of the glacier being seen to push before it an 

 accumulation of rolled pebbles. If, on the contrary, the glacier 

 covered these rolled pebbles again, the excavation seems to me 

 to be very difficult, because the glacier moulds itself upon its 

 under surface, and causes the pebbles of the underlying bed of 

 mud to advance very slightly. 



Moreover, according to one or the other of these suppositions, 

 [ am unable to comprehend how the deposit of older drift could 

 be accumulated below Geneva without any admixture of clay or 

 glacial mud having been produced. 



But there is an objection which appears to me to be still 

 more opposed to the theory of excavation. 



The supporters of this theory assign to the glacier which 

 formerly invaded our lake a force sufficiently great to enable it 

 to remove, from a depth of 800 metres near Meillerie, all the 

 pebbles of the older drift*. Nearer Geneva the lake is not so 

 deep, and the glacier had still, at this point, the necessary 

 power to scour out of the bed of the lake all those pebbles ; for 

 we know that this glacier has extended several leagues further, 

 * Mortillet's Theory. 



