586 Geological Society : — Dr. Buckland on the Evidence of 



the Tay, with the impervious till, containing Grampian boulders and 

 fragments of the subjacent grey sandstone. The finest instances of 

 erratics observed by Mr. Lyell occur on Pitscanly Hill, 700 feet, and 

 the adjacent hill of Turin, 800 feet above the level of the sea. About 

 forty feet below the summit, on the southern side of the former, is 

 a block of mica-slate thirteen feet long, seven broad, and seven in 

 height above the ground. Four smaller and equally angular masses, 

 from three to six feet in diameter, lie close to its north end, as if se- 

 vered from it. One of the nearest points at which this gneiss occurs 

 in situ, is the Craig of Balloch, fifteen miles distant, on the northern 

 extremity of the Creigh Hill, and between these points intervenes 

 the great valley of Strathmore and the hills of Finhaven. Other 

 Grampian boulders, from three to six feet in diameter, occur on the 

 hills between Lumley Den and Lundie, at the height of 1000 feet ; 

 and Mr. Blackadder has found fragments of mica-schist one foot in 

 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 countrj' 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 stationarj', 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 tertian,' 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 uiequalities, till 

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



