Mr. Murphy on the Glacial Climate and the Polar Ice-cap. 395 



of ice are piled up and jammed together, damming up the river ; 

 and destructive floods ensue when the icy barrier gives way. Several 

 examples of these phenomena which came under his notice are re- 

 corded by the author. 



At Presqu'isle Point, Lake Ontario, " banks " of ice are formed by 

 the dashing up of the water. These banks exercise an abrading 

 influence on the shore. In the winter of 1874-75 these ice "banks " 

 were greater than ever known previously, extending 300 feet out 

 into the lake, whilst some of the " hummocks " of ice were 20 feet 

 high. Some sixteen years ago a portion of the shore 40 feet wide 

 was laid bare in one winter. The rocks thus exposed were finely 

 polished, grooved, and scratched. These marks have now been 

 obliterated by the very agent that formerly produced them. The 

 author also gives several instances of the formation and cutting 

 through of sandbanks and headlands. The cutting back of the 

 coast-line of the lake is proceeding at the rate of from 30 to 40 feet 

 in twenty years. 



Fringing the basin of Lake Ontario are terraced highlands of 

 loose gravel, with here and there apparent stratification of a partial 

 character, called Artemisia-gravel by the Canadian Survey. A 

 peculiar feature of these gravel ridges is the presence in them of 

 numerous small lakes. The author records the bursting of the 

 natural dam of one of these lakes, and mentions the occurrence of a 

 peculiar hollow, probably the basin of a lake that burst in a similar 

 way in former times. 



Freshwater-shell deposits occur here and there ; further down the 

 river a brackish-water shell is found in them ; and they are replaced 

 lower down by marine deposits. The author quotes various opinions 

 relative to the formation of these terraces and shell-deposits by sup- 

 posing the former existence of a barrier of ice or rock lower down 

 the river, and concludes that by these phenomena noteworthy altera- 

 tions are year by year being made on the surface of the land. 



4. " The Glacial Climate and the Polar Ice-cap." By Joseph 

 John Murphy, Esq., F.G.S. 



The author agrees with Mr. Croll in thinking that a glacial epoch 

 must be one of maximum eccentricity of the earth's orbit, and 

 that the northern and southern hemispheres during such an epoch 

 must be glaciated alternately; but he maintains, in opposition to that 

 writer, that the glaciated hemisphere must have its summer in aphe- 

 lion. He intends this paper to be a reply to Mr. Croll's objections to 

 this theory as put forth in his work on Climate and Time. The 

 author holds that some of Mr. Croll's conclusions are erroneous, and 

 concludes with some remarks on Mr. Tylor's paper read before the 

 Society in April 1875. 



12. " On the Mechanism of Production of Volcanic Dvkes, and on 

 those of Monte Somma." By II, Mallet, Esq., F.P.S., F.G.S. 



The author stated that in 18G4 he made a careful trigonometrical 

 survey of the escarpment of Monte Somma, esjDecially with reference 

 to the numerous dykes by which the rocks composing it are inter- 

 sected. He described in detail the phenomena of direction of the 



