72 =. H. Howorth—Traces of a Great Post-Glacial Flood. 
by the dropping of moraines or the ploughing up of bergs, would 
be unfavourable for life” (id.). It is, indeed, quite obvious 
that no icebergs could pass over ground containing shells without 
reducing them to powder; yet without passing along the bottom 
or grounding, how are they to collect the shells? Again, speaking 
of the shells from various zones found together, Mr. Mellard Reade 
well says, ‘‘ Had the confusion (7.e. the confusion of the shells) been 
due to the ploughing up of icebergs, as suggested by Forbes, the dis- 
turbance at each stage of subsidence would not have reached beyond 
a certain depth below the surface of the sea, and all below that 
depth would have been free”’ (Journ. Geol. Soc. vol. xxx. p. 382). It 
must be remembered that the shells found in the high-level marine 
drifts are not in situ as they were when living, but are, as we shall 
show presently, a mixed assemblage of littoral and deep-water shells. 
They would need, therefore, to be collected together ; how could 
icebergs perform this task ? Why, again, should icebergs leave 
them in such isolated positions as we now find them? What special 
reason why Vale Royal and Moel Tryfaen should be selected for 
them to discharge their loads? The fact is, the position is not 
arguable. Let us now turn from icebergs to ice in another form, 
to the creeping of the foot of a vast ice-cap filling the British 
seas, which it has been gravely urged might have pushed these 
marine beds into their present position. This theory was, I believe, 
first pressed by Dr. Croll, and the view is concisely stated by one 
of its supporters, Mr. Tiddeman, who says: ‘It is a point insisted 
on by some geologists, that wherever rolled stones or marine shells 
are found in the Boulder-clay, it must be of marine origin. 1 do not 
think either of these characters is infallible. Mr. Croll has shown 
that the Caithness Till, which contains shells, need not necessarily 
be marine, but may have been formed by the ice-sheet working 
over a previous sea-bed, and pushing the shells on to the land. 
Jn this way shells scratched and broken may be found at very 
much higher levels than the sea in which they lived and died. They 
are as much boulders as the scratched stones alongside of them, and 
are no more evidence of the drift in which they lie having been 
formed under the sea than Spirifers and Producti found in limestone _ 
river gravel would be proof of its being marine. In very many 
places the ice-sheet must have passed over what had previously been 
the sea-bed, and if its course took it thence inland, we should be 
surprised not to find sea-shells mixed with the drift formed by it” 
(Journ. Geol. Soc. vol. xxviii. p. 471.) 
Mr. Belt has adopted the same view. Thus he says of the shells 
at Moel Tryfaen: ‘ They are just where they ought to be found, on 
the supposition that an immense body of ice coming down from 
Northern Ireland, from Scotland, and from Cumberland and West- 
moreland, filled the basin of the Irish Sea, scooped out the sand with 
the shells that had lived and died there, and thrust them far up 
amongst the Welsh hills that opposed its course southward and 
around the great height of which Liverpool forms the apex ” (Nature, 
vol. x. p. 26). 
