132 
THE GEOLOGIST. 
rightly from the portions of his paper read, he seemed to retain 
Macciilloch's idea of a deep lake behind this ice-barrier. 
In this we differ in opinion ; hence the reason of our taking up the 
subject in this article. In the discussion which followed the reading 
of Professor Jamieson's paper, a great many valuable comments were 
made. Mr. Grwynn Jeffreys confirmed Mr. Jamieson's opinion of 
the relationship in time of the " parallel roads" to the great submer- 
gence in the Drift period, — namely, that the " parallel roads " were of 
more recent date, — and stated that in the forty-feet beach," out of 
forty species of shells, three quarters of the number were Boreal 
forms, and one quarter Arctic, and that this beach was more recent 
than the Clyde beds, the shells in which were all of Arctic form. 
Mr. Mallet seized, with his characteristic astuteness, upon the diffi- 
culty of a deep lake being retained by a barrier of ice. He made 
some very excellent remarks upon the formation of the roads by 
lake-waves, and on the differences of lake-beaches and sea-beaches in 
respect to the curves and heights of the slopes formed by the shallow 
waves of the one, and deep waves of the other; but he appeared to 
us to go all wrong in his objection to the existence of an ice-barrier, 
by contesting that the ice being of less specific gravity than the water, 
the water of the lake would float and overturn the barrier, — an utter 
misconception, we conceive, of the case, which would have been better 
put by making reference to the absolute weight of the ice-barrier ; for 
it is evident that the liftin^^ov^Qv of the lake water would be in 
proportion to its depth, and that for an object to be floated from one 
side the efficient raising power of the water must be got by doubling 
the height necessary to float the whole mass from both sides. Now, 
as the specific gravity of ice is to water as '920 to 1000, it is evident 
that the height of the water must be at least y^Q-^hs higher than the 
barrier to be floated, and which kept it back. How then was the 
water retained on the one side, at such an elevation above that of the 
lip of the barrier itself ? 
If we conceive the whole region filled with glacier-ice, and that at 
a period when the climatal conditions had changed from the intense 
cold of the glacial era to a much milder temperature, — as is shown 
to have been the case, if the "roads " are subsequent to that epoch, 
by the proofs of amelioration of temperature at the period of the 
" forty-feet beach," shown in the increase of Boreal and the dimi- 
nution of Arctic shells, — it is evident we must have had the formation 
of the roads taking place at a period of thaw, and therefore there 
