HIGHEST LIMIT OF THE ERRATIC ZONE. 



357 



formed rocks, or by terraces formed of materials of erratic 

 origin. 



Within a district of small extent only, this highest limit 

 oftentimes appears defineable in an horizontal line ; but in 

 this case it is caused by an illusion, owing to the small degree 

 of inclination in that line. Although but slightly inclined, 

 the highest limit of the erratic zone is nevertheless so cir- 

 cumstanced ; this limit is formed by a surface which inclines 

 considerably from the centre of the mountainous region to- 

 wards its boundaries, cutting the sides of the mountains in 

 very different lines from that of the niveau. 



One of the most important, and indeed, essential elements 

 of the problem arising from erratic phenomena, is a know- 

 ledge of the inclination of the highest level. At this present 

 moment various opinions exist as to the absolute height at 

 which the traces of erratic phenomena cease ; but these 

 heights have never been considered in connection with the 

 horizontal distances of the points at which they were deter- 

 mined, in such a manner at least, as to enable us to deter- 

 mine the superficial limit 1 have made this calculation for the 

 valley of the Rhone ; for that of the Drause, St. Bernard to 

 Martigny, and for a portion of the lower Swiss basin, over 

 which the erratic phenomena of Valais are distributed. I 

 have also made it for some portions of the valley of the 

 Aar. Probably the publication of these results may occasion 

 similar researches in the other vaUies of the Alps, Pyrenees, 

 and Vosges. 



Elevation of the highest limit of the Erratic Zone. 



Near the summit of the Grimsel. . 2,300 m. 

 Near d'Aernen in Valais (Charpentier) . 1,813 

 Near the basin of Brieg. . . . 1,520 

 In the environs of Martigny. . . 1,450 



2 c 2 



