172 



[June 19, 



higher hills, which seem to have formed harriers to its further spread. 

 It seems, therefore, that although the connexion of these several and 

 distinct deposits is, owing to their irregular and wide spread, not 

 always apparent, it is probable that they are related to the same 

 phenomena, and that they present two phases of causes having a 

 common and contemporaneous origin. 



In all rivers subject to floods, three forms of sediment will be de- 

 posited: first, gravel and shingle in the more direct channel; secondly, 

 sand in the more sheltered places ; thirdly, fine silt where the flood- 

 waters are at rest out of the direct channel. 



In such manner the author conceives the high- and low-level 

 gravels and the loess of all the levels to have been formed. 



b' I' I". Representing the Loess. 

 d. A high-level gravel. 

 c' c' . Lower-level gravel. 



vn, n, o. The levels to which the river rose during inundations at different 

 periods. 



If, therefore, the flood-water origin of the loess be admitted, it follows 

 that, as it is found rising from 50 to 100 feet above the highest bed 

 of the fluviatile gravels which mark the channels of the old rivers, 

 it gives a measure of the magnitude of the floods of that period, 

 showing that they rose at times 50 to 100 feet above their summer 

 low levels ; like, in fact, the rivers in arctic regions, but to a greater 

 extent. Such conditions show their great erosive power, and 

 furnish the evidence wanting on the former occasion to prove that 

 such greater power had existed. Though a greater rainfall was 

 inferred from other causes, this more direct evidence was wanting. 



The author mentions his discovery, on the occasion of his last visit 

 to Paris, of freshwater shells of the genera Limneus and Valvata at 

 two places in the low-level gravels of Paris, and again at Rouen. He 

 also gives a section of some remarkable contortions, which he refers 

 to ice-action, in the high-level gravel of Charonne. 



