1862.] 49 



between the two periods. The effect of this slow elevation would 

 be to increase the velocity and erosive power of the rivers. This 

 action, with the other agencies before alluded to, operating upon the 

 successive portions of the substrata, has gradually worn even those 

 deep and long valleys, through which so many of the rivers of these 

 districts flow. According to variability in the rate of elevation, to 

 intervals of repose, or to deflections in the current and velocity of 

 the river, there may exist intermediate levels or terraces of gravel, and 

 variations in the inclination of the slopes, which may add much to 

 the complexity of the problem. 



The Fauna of the Low-level Gravel. Of the forty-three species 

 of Mollusca found in the higher gravels, thirty-four occur also in the 

 low levels, together with seven others, making a total of forty- one 

 species. Added to these, there are eight marine species found at Men- 

 checourt, with the Cyrena fluminalis of the Nile and of Grays. With 

 this one exception, they are all common living species of England 

 and France. As with the former group, there is nothing to give a 

 definite clue to the character of the climate of the period. The 

 general absence of southern forms, and the preponderance of such 

 as have a wide northern range, may, however, be noticed. With 

 regard to the Mammalia, the number of determined species is small, 

 and the general argument follows nearly the same line as that re- 

 lating to the Mammalia of the higher gravels. As with the Mol- 

 lusca, most of the species are common to the two series, whence it 

 is inferred that there was no great or sudden break, and that the 

 change both of conditions and of climate was transitional. There is 

 one genus only, viz. the Hippopotamus, about which some diffi- 

 culty has been felt with reference to the condition of climate. Four 

 tusk teeth of this creature have been found at St. Roch, and in this 

 country its remains are found associated with those of the Reindeer. 

 Without pretending to explain the difficulty, the author does not see 

 why, if the other large Pachyderms were fitted, as they are now 

 known to have been, by warm covering and special adaptation to 

 inhabit cold climates, this extinct species of Hippopotamus should 

 not also have been so adapted. 



The physical phenomena point to an increased volume of water in 

 the rivers, and want those marked indications of ice-action seen in the 

 high-level gravels. Still, boulders of considerable size were trans- 



VOL. XII. E 



