78 M. Renoir on the Traces of Ancient Glaciers. 



breadth of this extensive valley. Moreover, erratic blocks, 

 even admitting that they had been rubbed against the rocks 

 by the action of a great current of water, could produce no- 

 thing like the appearances observed here, for reasons which I 

 have had occasion already to explain in a notice inserted in 

 the Bulletin, t. xi. p. 53. They could not draw furrows with 

 rounded edges like those in question, nor mamelonate the sur- 

 faces of rocks by polishing them equally in every direction, 

 nor above all could they trace fine striae in them, rectilinear 

 and parallel, and always in the same direction as the valley, 

 that is to say, in the direction which the moving glacier must 

 necessarily have taken. Besides, it may be asked, by what 

 accident iSi^facies of these surfaces is found to be identically 

 the same as that of rocks undergoing the process of polishing 

 by glaciers at the present time, and which we have an oppor- 

 tunity of witnessing with our own eyes ? I am therefore of 

 opinion that the cause which the Society has assigned for the 

 polish of the rocks at Fontenil is not the one that is generally 

 adopted in the present day. 



The best preserved parts of these rocks, and on the surface 

 of which the polish is most perfect, are those which have been 

 recently exposed by the workmen who have removed, in 

 quarrying, the sand or soil which covered them. These only 

 shew in perfection the fine striae, which have disappeared 

 from all those long exposed to the action of atmospheric agents, 

 and in which the polish has already undergone considerable 

 alteration. This deterioration is observed in all polished sur- 

 faces, but it may easily be conceived to be more or less rapid 

 as the rock, from its composition, is more or less fitted to re- 

 sist meteoric influences. Here the polished surfaces belong 

 to a neocomien formation. 



Polished rocks are not observed solely in the valley of the 

 Isere ; they are also found in many parts of that of Romanche : 

 and M. Gras, who intends to occupy himself with researches 

 of this nature, will doubtless discover them in all the other 

 great valleys of the department. 



The glacier which, at the commencement of the general 

 melting in these latitudes, has taken its direction along the 

 bason in which the town of Grenoble is built, must have been 

 of immense size, for it was composed by the union of all those 



