et es 
HOPKINS ON THE LAKE DISTRICT. 89 
of the gravel from the original sites of its component materials was 
posterior to the same epoch. 
There is another consideration, however, which must be admitted 
as imposing a further limitation on the age of erratic blocks, espe- 
cially when of considerable magnitude. When such a block is found 
reposing on a surface which we believe to have once been rough and 
uneven, but of which all the asperities have been worn down till the 
surface has become comparatively smooth and even, by the operation 
of denuding agencies, we may conclude that the transport of the 
block to such locality was posterior, not only to the deposition of the 
beds on which it rests, but also to any dislocation by which their 
surface might have been rendered rugged, and to the subsequent 
process of denudation, by which such ruggedness must have been 
again destroyed. For it must be admitted that the same action 
which should wear down the asperities of the surface on which the 
block reposes, would also reduce the block itself to comminuted 
pebbles, unless the block were much harder than the beds it rests 
upon, in which case the above conclusion would manifestly not be 
necessarily true. ‘Thus, for imstance, a block of hard crystalline 
granite might originally repose on a non-crystalline mass of sand or 
clay, the surface of which might be modified m almost any degree 
by a gradual and gentle operation of denuding agencies, insufficient 
to produce any appreciable effect on the block itself. 
The most recent formation on which the erratic blocks repose in 
the immediate neighbourhood of the Lakes is the new red sandstone. 
Consequently their transport could not commence till after the depo- 
sition of that formation, which defines the earliest possible limit to 
the time of transport. Again, since numerous blocks have been con- 
veyed over Shap Fell, their transport, supposing it to have been 
subaqueous, must have taken place before the emergence of the fell 
from the ocean, which emergence, on the above supposition, defines 
the latest limit to the period of transport. During what part of the 
interval thus defined, the operation was commenced or principally 
carried on in the neighbourhood of the Lakes, it is impossible, I con- 
ceive, to demonstrate. The transport of the blocks now resting on 
the Wolds of Yorkshire must have been concluded subsequently to the 
deposition of the oolites ; and other blocks, for a similar reason, may 
not have completed their course till after the tertiary period ; but to 
conclude, therefore, that every block from the central mountains of 
Cumberland, which now reposes, for instance, on the mountain lime- 
stone of Kendal Fell, was not transported there till the post-tertiary 
period, is to form an opinion which, I believe, has not the slightest 
foundation to rest upon. We are unquestionably at liberty to sup- 
pose them removed at a much earlier period, should any well-founded 
theory require such supposition. 
20. Surface over which the Blocks have been transported.—This 
is also a pomt of great importance in our discussions of theories of 
erratic blocks. It seems to have been generally assumed in the dis- 
cussions on the efficacy of currents, that the subaqueous surface over 
which blocks have been transported by that agency had the same 
