380 M. Agassiz oti Glaciers, Moraines, and Erratic Blocks. 



it, afterwards upon the most saliant parts of the plain ; valleys 

 of drainage would then be excavated at the bottom of these 

 clefts, in localities where no current of water could flow without 

 being inclosed within congealed walls ; and when the ice had 

 completely disappeared, the great angular blocks which had 

 covered its surface, or had fallen into the clefts, would be found 

 upon a bed of small rounded pebbles, under which is usually 

 found a laver of sand. In melting from the surface, the ice must 

 necessarily have continued longest in the depressions of the 

 country, in the little longitudinal valleys which are formed by 

 the different zones of the strata of the Jura, and at the bot- 

 tom of the lakes ; and it is undoubtedly to this circumstance we 

 are to attribute the extraordinary position occupied by so many 

 of these blocks, which are perched up, scarcely in equilibrium, 

 upon the highest points of rocks ; and also their constant ab- 

 sence in the hollows, where they are not found, except at least 

 where fresh momentary expansions of the ice were able to pre- 

 cipitate them. 



So long as the level of the ice on the Jura had not fallen be- 

 low the line of Pierre-a-Bot, the blocks which were yet spread 

 over its whole surface, might continue their descent towards the 

 Jura ; but so soon as the ice became thin over the plain of 

 Switzerland, it must have very speedily disappeared, and have 

 only left portions in the deep valleys, and in the basins of the 

 lakes, that is to say, it must have been soon confined to the lower 

 valleys of the Alps. 



In reflecting upon what must necessarily have occurred upon 

 this disappearance of the ice, we are naturally led to think that 

 the transport of the rolled pebbles of the valley of the Rhine, 

 and the deposition of Loss, must have been among its first ef- 

 fects ; and this is confirmed by the facts, that these pebbles are 

 the same with those which we found along with our blocks, and 

 that the Loss is evidently the result of the detritus of the mo- 

 lasse. The frequent debacles of the ice could only at that time 

 convey blocks upon the masses of ice to great distances, or carry 

 them farther in their curren,. 



The melting and maceration of the ice and its repeated con- 

 gelation in cold weather have produced many other geological 

 effects, which it is difficult to account for as produced by any 



