M. Agassiz on the Erratic Blocks of the Jura. 177 

 left on their edges these concentric dikes of rounded blocks 

 termed moraines, I remembered that the northern slope of the 

 Jura, which faces the Alps, also presents polished surfaces, 

 known to us by the name of laves, and to which I had hither- 

 to paid but little attention. On my return to Neufchatel, I 

 hastened to examine more attentively these polished surfaces, 

 and I have ascertained that they are completely independent 

 of the stratification of the beds, and of the direction of the 

 chain of the Jura ; that they extend along the whole surface, 

 following its undulations, passing equally over the Neocomian 

 and the Jurassic formations, and presenting a polish as unin- 

 terrupted as the surface of a mirror, wherever the rock has been 

 recently exposed. These surfaces are sometimes even, some- 

 times undulated, often traversed by furrows more or less deep 

 and sinuous, but which never occur in the direction of the slope 

 of the mountain. On the contrary, these furrows are oblique 

 and longitudinal ; in short, they have a direction which at once 

 excludes the idea of a current of water having been the cause 

 of the erosions. Besides, we remark, on the surfaces that are 

 well preserved, fine lines, similar to the markings that would 

 be produced by a diamond point on glass ; and which follow, in 

 general, the direction of the obhque furrows. To any one who 

 has seen the Alps, it is evident that it is the ice which has 

 produced this polishing. It is over an extent of more than 

 twenty leagues to the east and west of Neuchatel, that the 

 northern slope of the Jura presents this aspect, that is where- 

 ever it has been examined in this point of view, and neverthe- 

 less these facts have remained unknown to the present time. 



As we find the erratic blocks of the Jura reposing on these 

 polished surfaces, the question naturally presents itself, Whe- 

 ther, as M. Charpentier had admitted, the glaciers had really 

 extended to the Jura range, pushing before them blocks of al- 

 pine rocks, and polishing the surface on which they moved ? 

 A constant fact opposes this idea, an idea, indeed, which M. 

 Charpentier has already abandoned, viz. that the erratic blocks 

 of the Jura are angular, while the blocks of the vwraines, con- 

 stantly pushed by the movements of the glaciers and rubbed 

 aganist one another, are always rounded. This is also the case 

 wUh blocks of the most recent moraines, with those which ac- 



Vor.. XXIV. NO. XLVII. .FANUAKV 1838. M 



