6G M. Chavpentier on fie Erratic Phenomena of the North. 



plain the dispersion of the erratic debris of the north in as 

 satisfactory a manner as we can that of the Alps and of the 

 Pyrenees. It will be the task of the geologist to assign ap- 

 proximately the share which each of these agents has had in 

 the production of this great phenomenon. 



The deposits of erratic debris, properly so called, the abra- 

 sion of the rocks, the marks of attrition, the striie, the fur- 

 rows, and the erosions in the form of caldrons, are to be at- 

 tributed to glacier action. Erratic deposits can always be 

 distinguished from the diluvium by the frequency of well- 

 preserved angular debris. The iisars serve not only to prove 

 the existence of the erratic formation in any particular region, 

 but are also of great assistance in determining its limits. 

 For this purpose it would be necessary to delineate on a map 

 the osars the farthest removed from the north, or, in their 

 absence, to indicate the localities where the debris cease to be 

 mixed pell-mell as regards their volume, and where, con- 

 sequently, a selection, according to relative weight, begins to 

 be perceptible.* The line joining all these localities would 

 indicate the limit of the erratic formation properly so called, 

 that is to say, the limit of the debris dispersed by the glacier. 

 It would also exhibit the form of the glacier at the period of 

 its greatest development. Consequently, the regions com- 

 prised between this line and the north, must have been covered 

 by ice at the epoch of its maximum of extent. 



The sedimentary deposits, whether stratified deposits of 

 pebbles, of sand, or of clay, situated within or without that 

 line, are, in my opinion, not the erratic formation, but dilu- 

 vium, that is to say, a sediment whose materials have been 

 conveyed and deposited by water. In the countries which 

 were not submerged by the sea, this transport must have been 

 eflPected by the streams which issued from the glacier, and 

 which, during the period of its melting, doubtless acquired a 

 considerable volume. But in regions covered by the sea, this 



* It will be found that there is rarely an opportunity of obser^-ing marks 

 of attrition in the vicinity of the limit of the erratic formation, because the 

 regions where it terminates being in the plains, the rock constituting the 

 surface is generally covered and masked by the ililuviuiu. 



