} 
1848.] DARWIN ON THE TRANSPORTAL OF ERRATIC BOULDERS. 321 
fathoms. As far then as these considerations can be at all trusted, 
we are, according to the view given in this paper, in a position to 
explain the transportal of the boulders from a lower to a higher level, 
in Great Britain as well as in the United States. I will make only 
one other remark on this head: though I believe that Great Britain 
subsided during the glacial period, yet I conceive it must also have 
subsequently attamed during this same prolonged period a consider- 
able portion of its present height. I infer this from the plain marks 
of true glacier action, low down the valleys in North Wales, within 
300 feet of the present level of the sea*. 
A second objection of apparently considerable weight has been 
- advanced against the theory of floating ice; namely, that in some 
instances the blocks decrease very regularly in size in proceeding from 
their source. Prof. H. D. Rogers+ says that this is markedly the 
case in going southward in the United States. According to Mr. Hop- 
kinsf it is also the case in the Lake district ; “the blocks becoming 
smaller as we approach the coast of Yorkshire, till they degenerate 
into pebbles in the more remote localities, in which the Cumbrian 
rocks can be identified.” He adds, ‘“‘'These facts are strongly in 
favour of those views which would refer the transport of these masses 
to diluvial currents.’ This sorting of the boulders does not always 
hold good: on the plains of Patagonia the two largest boulders 
which I saw were near the outskirts of the deposit. Sir R. Mur- 
chison also remarks on the vast size of the many boulders in the 
south-east parts of Shropshire, near the southern limit of his 
northern drift, though he elsewhere states that the boulders generally 
decrease in size in going from north to south. In these cases, if we 
look at the boulders as having all been transported on icebergs, there 
certainly appears no reason why they should have been dropped 
from such immense masses of ice, with any approach to order ac- 
cording to their size and to their distance from their source. But 
this does not hold good with boulders transported in sheets and 
fragments of coast-ice: here the buoying agent is not of dispropor- 
tionate power to its burthen; as the ice decays, the heaviest frag- 
ments would naturally be apt to drop out first ; and it would appear 
from the accounts given to us, that the largest boulders durmg some 
winters escape being moved at all, whilst the smaller ones are drifting 
onwards. Moreover, the boulders (and great stress may probably be 
laid on this point) which had travelled furthest, would, from having 
been repeatedly stranded, and necessarily so every summer, be most 
worn, and therefore would be smaller than those which had travelled 
to a shorter distance. 
I have shown, in my volume on South America, that the sea has 
* Since the above was written, I have found that Mr. Trimmer, in his inter- 
esting paper on the Geology of Norfolk (Journal of the Royal Agricultural Society, 
vol. vii. part 2), has shown that that district subsided at least 600 feet, and was 
likewise upraised during the boulder or glacial period. 
t+ Address to the Association of American Geologists, 1844, p. 45. 
t¢ Journal of the Geological Society, vol. iv. p. 98. 
