338 
the theory of drifting ice over a submerged country to facts with 
which he had been long acquainted in Forfarshire, he found great 
difficulty in accounting for the constant subterposition of the till 
with boulders to the stratified deposits of loam and gravel; for the 
till ascending to higher levels than the gravel, and often forming 
mounds which nearly block up the drainage of certain glens and 
straths.; for its constituting, with a capping of stratified matter, 
narrow ridges, which frequently surround lake-swamps and peat- 
mosses; and for the total absence of organic remains in the till. 
Since, however, Professor Agassiz’s extension to Scotland of the 
glacial theory, and its attendant phenomena, Mr. Lyell has re-ex- 
amined a considerable portion of Forfarshire; and having become 
convinced that glaciers existed for a long time in the Grampians, 
and extended into the low country, many of his previous difficulties 
have been removed. ‘There are, nevertheless, facts connected with 
the ridges of stratified materials resting upon till, which he is unable 
to explain. He also states, that though he had for years inferred 
from the evidence of fossil shells sent to him from Canada by Capt. 
Bayfield, that the climate of North America, in the latitude of Que- 
bec, 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, by 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, the Grampians, composed of granite, gneiss, mica- ~ 
slate, and clay-slate, flanked by a lower range of vertical beds of 
old red sandstone, associated with 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 
* See ante, p. 119. 
