1855.] 



SHARPS ELEVATION OF THE ALPS. 



.115 



Fig. 4. — Section of two Terraces of Alluvium in the Faltelline, cut 

 through by the River Adda. 



Terrace 



Bed of the Adda. 



ought to be greater ; the River Adda has cut through the rubble 

 forming the terraces for about 300 feet, down to the rock below. We 

 learn from this that a body of water stood for a very long period at 

 the level of a, where Sandalo now stands, 2940 feet above the sea ; 

 and that it stood at a later time, and for a shorter period, about 200 

 feet lower, at the level of b. 



The terraces are so uniform in their principal features that it is 

 useless to give other representations of them. 



I noted the heights of the principal terraces in many of the valleys, 

 with the view of comparing them together to determine whether .we 

 ought to refer them to lakes or to the sea. There is usually a village 

 on the broader terraces, of which the height above the sea is pub- 

 lished ; where this is not the case my aneroid gave me a tolerably 

 accurate measure of their elevation. But there is an inevitable source 

 of error caused by the slope of the terraces themselves, which often 

 amounts to 100 feet ; and thus, if we happen to compare the upper 

 end of a terrace in one valley with the lower end of one in another 

 valley, we may think the one 100 feet higher than the other, when 

 they are really at the same altitude : nor can this be obviated by 

 limiting our observations to the upper end of each terrace, as fre- 

 quently that has been washed away, and its traces only remain on 

 the sides of the valleys. I think, however, that 100 feet may be 

 taken as the limit of the errors in my statements of altitude. 



In tolerably level valleys, instead of terraces we find long sloping 

 meadows, in which we cannot separate the deposits which may have 

 been formed under ancient waters from those now forming by the 

 action of the present rivers and of the weather. I have therefore 

 taken no account of these, and only mention them to show that our 

 not finding terraces in such valleys is no proof that waters may not 

 have stood in them to the same height as in those valleys in which 

 terraces are most frequent. 



Terraces are also less evident in broad than in narrow valleys ; 

 perhaps because a given amount of materials would make less show 



