THE ERRATIC PHENOMENA. 409 



rubbed against underlying rocks. But the setting here being simply 

 ice, these loose materials, fast at one time and movable another, and 

 fixed and loosened again, have rubbed against the rock below in all 

 possible positions ; and hence not only their rounded form, but also 

 their rectilinear grooving. How such grooves could be produced 

 under the action of currents, I leave to the advocates of such a 

 theory to show, as soon as they shall be prepared for it. 



I should not omit here to mention a fact which, in my opinion, has 

 a great theoretical importance, namely, that in the northern erratics, 

 even the largest boulders, as far as I know, are rounded, and 

 scratched and polished, at least, all those which are found beyond 

 the immediate vicinity of the higher mountain ranges ; showing that 

 the accumulations of ice which moved the northern erratics covered 

 the whole country ; and this view is sustained by another set of facts 

 equally important, namely, that the highest ridges, the highest 

 rugged mountains, at least, in this continent and north of the Alps in 

 Europe, are as completely polished and smoothed as the lower lands, 

 and only a very few peaks seem to have risen above the sheet of ice ; 

 whilst, in the Alps, the summits of the mountains stand generally 

 above these accumulations of ice, and have supplied the surface of 

 the glaciers with large numbers of angular boulders, which have been 

 carried upon the back of glaciers to the lower valleys and adjacent 

 plains without losing their angular forms. 



With respect to the irregular accumulation of drift-materials in the 

 north, I may add that there is not only no indication of stratification 

 among them, such unquestionably as water would have left, but 

 that the very nature of these materials shows plainly that they are of 

 terrestrial origin ; for the mud which sticks between them adheres to 

 all the httle roughnesses of the pebbles, fills them out, and has the 

 peculiar adhesive character of the mud ground under the glaciers, 

 and difiering entirely in that respect from the gravels and pebbles 

 and sands washed by water currents, which leave each pebble 

 clean, and never form adhering masses, unless penetrated by an 

 infiltration of limestone. 



Another important fact respecting this glacial diift consists in 

 the universal absence of marine as well as freshwater fossils in its 

 interior, a fact which strengthens the view that they have been 



