132 



THE GEOLOGIST. 



rightly from the portions of his paper read, he seemed to retain 

 Macculloch'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 Jifting-^o^ev 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 l'OOO, it is evident 

 that the height of the water must be at least T 6 oths 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 



