Erratic Blocks in Plains. 233 



In the exterior chains of the Alps, the polishing does not 

 reach to a greater height than 6000 or 7000 feet. It cannot be 

 doubted, that this limit, which is so well marked, indicates the 

 level of the bed of ice at the epoch of its greatest thickness. The 

 rugged peaks, which exceed that height, thus rose like islets 

 in the midst of this sea of ice, and the blocks which were de- 

 tached from them fell on the surface. Not being confined in 

 narrow valleys, but the whole vast sea of ice being open to 

 them, these blocks were not liable to be knocked against one 

 another in their progress towards the lower districts, and it 

 is thus that they could be transported as far as the Jura, with 

 their surfaces rough and their angles prominent; whereas, the 

 matters which were beneath the ice, were triturated, polished, 

 rounded, and scratched. Now, if in Switzerland, the limit of 

 the great mass of ice extended as high as 9000 feet in the Alps, 

 and if it oscillated between 4000 and 5000 feet in the Jura 

 which no longer presents glaciers, what is more natural than 

 to admit, taking into account the geographical portion of the 

 localities, that, in Scotland, the great proportion, if not the 

 whole, of the surface, was entirely under ice during the whole 

 duration of the glacial epoch. Hence the majority of the de- 

 tached blocks of the Scotch mountains must have been trans- 

 ported under the ice, and consequently rubbed, rounded, 

 polished, and scratched. I say the majority, for it is probable 

 that some were detached when the ridges were free from ice, 

 and when the valleys alone were occupied by glaciers ; and 

 these latter have necessarily remained more or less angular, 

 and have retained their rough surfaces, just like the blocks of 

 the morames of the glaciers of the present day. Foreign 

 blocks, whose origin is not British, and which were doubtless 

 transported on the surface of the great sheet of ice, or on rafts 

 of ice at the period of its dissolution, ought to be angular, 

 and, for the most part, are so in reality. In this way, the form 

 of erratic blocks implies, in some degree, at first sight, their mode 

 of transport. I am able to add, as a confirmation of what I have 

 said as to the form of the erratic blocks of Scotland, that the 

 blocks of the Jurassic rocks, which we meet with in the dilu- 

 vium of the interior valleys of the Jura, are all rounded ; a 



VOL. XXXIII. NO. LXVI. OCTOBER 1842. Q 



