508 
THEORIES TO EXPLAIN ERRATIC DRIFT. 
the Scandinavian chain 1 . When we consider, that throughout this vast space, 
these blocks have all been transported from the same range of mountains and often 
carried to enormous distances, it will readily be admitted, that whilst it is entirely 
different from the regions we have just been considering, no portion of Europe 
affords so fine a field for the discussion of the difficult problem, of how such heavy 
masses were so far transported ? In the earlier days of geological science, this 
great spread of northern detritus was merged with the coarse debris of other parts 
of Europe under the term “ diluvium,” meaning thereby that it was the wreck 
of a general deluge which had passed over our continents. With increased ob- 
servation, however, it was found, that whilst certain tracts of country (like our 
great Siberian case) were entirely exempt from them, each region which contained 
such foreign materials had derived them from contiguous chains and from various 
points of the compass ; and hence it was concluded (at least by many geologists), 
that they were drifted to their relative existing positions by various currents of 
water, set in movement in different directions by elevations and depressions of 
separate masses of land. 
Latterly this subject has attracted more than ordinary attention, through the 
labours of several observers in the Alps, and new theories have arisen. Whilst 
Sefstrom and his followers in the north had been contending, that all the detritus 
of which we are now about to treat resulted from a great northern deluge, Agassiz 
and his predecessors Venetz and Charpentier, showing the transporting force of 
glaciers, endeavoured to demonstrate, that many of the heaps of detritus around 
the flanks of the Alps are nothing more than “ moraines,” the residue of ancient 
and more extensive glaciers. 
Arguing from the phenomena of the Alps, M. Agassiz further attempted to 
establish a general glacial theory, by which he supposed, that all the northern he- 
misphere was, during a long period, covered with ice and snow; that glaciers, 
advancing by expansion from certain centres, and carrying with them, on their lower 
surfaces, the blocks and pebbles which were entangled in them when they first 
moved from the mountain side, scratched and polished the surfaces of the conti- 
nents over which they passed, precisely in the same manner as rocks are now 
affected on a small scale by the existing glaciers of the Alps ; and lastly, that upon 
the melting and breaking up of these great former glaciers many of the large blocks 
1 See the Map on which the southern limit of these blocks is marked. 
