MANCHESTER GEOLOGICAL SOCIETY. 
123 
" After this description, he commences the theoretical part of 
this portion of the essay, by supposing that, during a certain 
geological epoch, the ice extended as far from the North Pole 
as it does at the present time from the South, or that its extreme 
limit was about 50 degrees of north latitude. To this epoch he 
refers the scratching and polishing which is so common in 
mountainous countries, and also the transportation and polish- 
ing of erratic boulders. This epoch was succeeded by another, 
during which the ice retreated to a more northerly limit, leaving 
only the mountains and their valleys covered with glaciers. 
' There would thus,^ to use the words of the author, ^ be two 
very distinct periods to be particularized in the epoch of the ex- 
istence of ice in the north of Europe — that during which the 
general covering enveloped the region, and that when glaciers 
existed only in high valleys.^ After discussing the action and 
depth of the ice during these two epochs, he proceeds to consider 
the relation which these masses of ice bear, as to time, and to 
the present race of organized beings of our epoch being created 
successively after the commencement of the retreat of the ice.' 
This opinion necessarily supposes the destruction and re-creation 
of organisms of the same species ; for we find that the following 
shells are common to the three eras of the tertiary formation, 
and are still in existence at the present time, viz. — Dentalium en- 
talis, Dentalium strangulatum, Fissurella Grcsca, Bulla liffnaria, 
Rissoa cochlearella, Murex fistulosus, Murex tubifer, Polymor-' 
phus ffibba, Tubiculina oblongata, Lucina gibbosulla, Isocardia Cor 
and Nucula Margaritacea. This supposition of destruction and 
re-creation is, as far as I am aware, contradictory to any analogy 
which preceding formations afford. 
There is another circumstance in the essay to which I more 
particularly refer, when I state that one of the deductions ap- 
pears to me not to be legitimate. This deduction is based on a 
statement of Mr. Smith, of Jordanhill, who, to use the words of 
Agassiz, ' was the first to point out in the post-tertiary clays, 
o 2 
