580 Geological Society. — Mr. Lyell on the 



bee, was far more intensely cold at one period than it is now*, yet, 

 that his thoughts had been diverted from the consideration of a long- 

 continued covering of snow on the Scottish mountains, hy the know- 

 ledge that the climate of Great Britain during the several tertiary 

 epochs was warmer than it is at present. He is of opinion that, 

 during a period immediately antecedent to the existing, several os- 

 cillations of temperature may have occurred in the northern hemi- 

 sphere. 



Forfarshire, Mr. Lyell divides geologically into three principal 

 districts : 1st, tlie Grampians, composed of granite, gneiss, mica- 

 slate, and clay-slate, flanked by a lower range of vertical beds of 

 old red sandstone, associated M'ith trap ; 2ndly, the great syncli- 

 nal trough of Strathmore, occupied by the middle and newer mem- 

 bers of the old red sandstone ; and 3rdly, the anticlinal chain of 

 the Sidlaw Hills, consisting of the inferior or grey beds of the old 

 red sandstone, usually accompanied by trap. He further states, 

 that it represents, on a small scale, both geologically and physically, 

 the portion of Switzerland where erratic blocks are most abundant, 

 the Grampians with their crystalline rocks being comparable to the 

 Alps, the secondary chain of the Sidlaw Hills to the Jura, and 

 Strathmore to the great valley of Switzerland ; and that the resem- 

 blance is increased by the occurrence in Strathmore and on the 

 Sidlaw Hills of angular and rounded blocks of Grampian rocks. 



The superficial detritus of Forfarshire, Mr. Lyell divides into 

 three deposits : 1st, the thin unstratified covering on the Grampians, 

 derived from the disintegration of the subjacent strata, with a slight 

 intermixture of pebbles traceable to rocks at a higher level, not far 

 distant ; 2ndly, the unstratified materials enclosing boulders which 

 occur at the base of the hills on both sides of every glen, and not 

 due to taluses formed by landslips, but constituting terraces of 

 transported debris, with a nearly flat top, and sometimes with two 

 steep sides, one towards the river, and the other of less height to- 

 wards the mountain ; and 3rdly, the stratified gravels, sands and 

 clays which overlie the unstratified detritus. Mr. Lyell confines 

 his observations principally to the second and third divisions. 



The terraces or lateral mounds very generally increase in width 

 and depth as they descend from the higher to the lower glens, attain- 

 ing in the latter sometimes a thickness of 100 feet, and occa- 

 sionally so great a breadth as to leave only sufficient room for the 

 river to pass. The inferior part is always unsti'atified, consisting of 

 mud and sand, in which large angular and rounded fi-.^'ments of 

 rocks are imbedded. These boulders are more and more rounded 

 as their distance increases from the hills whence they could have 

 been detached ; but they are more frequently flat-sided than pebbles 

 which have been rounded by water ; and they become more diversi- 

 fied in character by the junction of every tributary glen. In the 

 upper part the mounds often consist of 40 to 80 feet of the same 

 materials as the lower, but regularly stratified. Mr. Lyell then 



* See Proceedings, vol. iii, p. 1 19 [or L. & E. Pliil. Mag. vol. xv. p. ;599]. 



