178 M. Agassiz on the Erratic Blocks of the Jura. 



company existing glaciers ; now, if the erratic blocks of the 

 Jura had been pushed by great glaciers to a distance so consi- 

 derable as that of the Jura, they ought to be much more 

 lounded than those of the moraines. The same ought to have 

 been the case if the erratic blocks had been transported by cur- 

 rents of water, whatever may have been their depth or rapidity. 

 By this last hypothesis, it is impossible to explain how the lakes 

 situated between the Alps and the Jura were not completely 

 filled up, especially as we can demonstrate that they existed prior 

 to the transport of the blocks. Very recently it has been at- 

 tempted to reconcile these various phenomena by admitting, 

 that the transport of the angular blocks was effected on floats 

 of ice moved by currents of water ; but facts which I have ob- 

 served on the Jura, and which had not been previously no- 

 ticed, oppose this explanation. The erratic blocks of the Jura 

 do not repose immediately on the polished surfaces ; wherever 

 the rolled pebbles which accompany the large blocks have not 

 been disturbed by subsequent causes, we find that they form a 

 bed of some inches sometimes of several feet in thickness, on 

 which the angular blocks repose. These pebbles also are much 

 rounded, even polished, and are so arranged that the larger are 

 at the surface, and that the smaller, which often pass into a fine 

 sand, are at the bottom, immediately on the polished surface. 

 This order of superposition, which is constant, is completely 

 opposed to the idea of transport by currents ; for, in this latter 

 case, the order of superposition of the rounded pebbles would 

 be the i-everse. The presence of a fine sand at the surface of 

 the polished rocks also proves, that no powerful cause has acted 

 on the surface of the Jura since the period of the transport of 

 these alpine rocks ; and it is without doubt to the pressure of 

 this sand on the polished surfaces that we are to attribute the 

 fine lines engraved on them, and which would not have existed 

 if that sand had not been moved by a current of water ; for 

 neither our torrents, nor the water of our lakes when violently 

 agitated, produce any thing at all analogous on the same rocks, 

 even when they are charged with sand. I do not doubt that 

 most of the phenomena attributed to great diluvial currents, 

 and especially those recently described by M. Selfstroem, have 

 been produced by ice. 



