Geological Society* 519 



diameter on the summit of Craigowl, the highest point of the Sid- 

 law Hills, and exceeding 1500 feet above the level of the sea. 



In conclusion, Mr. Lyell offers some remarks on the conditions 

 under which glaciers may have existed in Scotland, and the differ- 

 ences between them and those of the glaciers of Switzerland. He 

 states that the glaciers of the latter country being situated 11° 

 further to the south, they can present but an imperfect analogy with 

 permanent masses of ice in Forfarshire, and that it is to South 

 Georgia, Kerguelen's Land and Sandwich Land that we must look for 

 the nearest approach to that state of things which must have existed 

 in Scotland during the glacial epoch. In those regions of the south- 

 ern hemisphere the ice reaches to the borders of the sea, and the 

 temperature of summer and winter being nearly equalized, the gla- 

 ciers probably remain almost stationary, like those of the Alps in 

 winter, and can be diminished by only the first two of the three 

 causes which tend to check an indefinite accumulation of snow in 

 Switzerland ; viz. 1st, evaporation without melting; and 2ndly, the 

 descent of glaciers by gravitation, considered by M. Agassiz to be 

 not very influential : — the third cause, the descent of glaciers arising 

 from alternate liquefaction and freezing, he conceives must be wholly 

 suspended in these regions. 



As the tertiary strata prove that a warm climate certainly pre- 

 ceded the assumed glacial epoch in the northern hemisphere, and as 

 a mild climate has since prevailed, Mr. Lyell says, there are three 

 distinct phases of action to be considered in studying the supposed 

 glaciers of Scotland : 1st, the coming on of the epoch ; 2nd, its con- 

 tinuance in full intensity; and 3rd, its gradual retreat. At the 

 commencement of the first condition, only the higher mountains 

 would send down glaciers to be melted in the plains below, as at pre- 

 sent in Switzerland, and in Chili between the 40th and 50th degrees 

 of latitude. The ice would therefore thus be constantly advancing 

 and retreating, but progressively, century by century, gaining ground, 

 in consequence of diminishing summer heat; and pushing its terminal 

 moraines forward, it would fill up lakes and other inequalities, till 

 it finally reached the sea. During the second condition, when the 

 motion of the ice would be very small, there would be, Mr. Lyell 

 states, vast accumulations of snow filling the plains and valleys to a 

 great height, and leaving bare only the higher peaks and precipices 

 of the mountains. From these points, he conceives the erratic 

 blocks were detached and conveyed almost imperceptibly along the 

 surface of the frozen snow to great distances. Lastly, at the break- 

 ing up and gradual retreat of the glaciers during the third period, he 

 is of opinion, the boulders were deposited in the various situations 

 in which they are now found, and that moraines, or lateral and trans- 

 verse mounds, were successively deposited, and lakes formed by which 

 stratified materials were accumulated in certain positions. 



