SCANDINAVIAN DETRITUS SPREAD OVER RUSSIA. 
509 
which they contained were floated away in debacles of icebergs and deposited at 
great distances from the source of their origin. Still more recently, Professor 
James Forbes, extending the views of De Saussure by an assiduous personal survey 
of the Alpine glaciers, has demonstrated by exact experiments on the nature of 
their ice and its movements, that glaciers never can advance except by their own 
gravitation and upon inclined surfaces. 
But apart from the Alpine theories and observations, Mr. Lyell and others had 
previously shown how, under former relations of sea and land, icebergs wafted by 
prevailing currents may have carried foreign blocks to great distances, and one of 
us had applied this view to explain the transport of the great foreign boulders 
which are distributed in the central counties of England 1 . Our own view had, we 
think, this advantage, in reference to tracts like this under consideration, that in 
showing the presence of sea-shells of modern characters in mounds of far-borne 
detritus, it completely established, that the surface of such tracts was beneath the 
sea when the blocks were distributed. Hence we subsequently inferred, that the 
glacial Alpine theory, which is constructed upon the belief that such surface was 
sub-aerial, was in such cases entirely inapplicable ; subaqueous action being alone 
admissible. 
After this slight introduction, and referring our readers to the ingenious and 
able works of the writers alluded to, we now proceed to throw together our own 
observations upon the transported matter of the great northern regions. 
The superficial detritus of Russia, Poland and Prussia, like that of other regions 
which we have examined, is referable to the great mountain-chain in its vicinity. 
The chief, if not the only, distinction between it and all other far-borne drift, con- 
sists in the great breadth and length of the dispersed detritus, in reference to the 
low mountains from whence it has been derived ; for whilst in other parts of 
Europe various local centres of elevation have shed their detritus in different di- 
rections (England, France and the Alps offer sufficient examples) , the vast regions 
1 Silurian System, pp. 522 to 547. By consulting these two chapters of a former work by Mr. Mur- 
chison, the reader will find a full development of his views respecting the transport of some drift by 
water. and of great foreign erratics by icebergs. These chapters were written (1838 and 1839) before 
the appearance of the works of MM. Agassiz and Charpentier on the agency of glaciers in transporting 
erratic blocks, a question which Mr. Murchison subsequently considered at some length in an Anniver- 
sary Discourse addressed to the Geological Society of London, 1842, in which will be found some of the 
same ideas developed in this chapter. (Proceedings of Geol. Soc. vol, iii. p. 671.) 
3 u 
