On the Physical Features of British Norih America. 213 
region is severe, but steady. From the commencement of 
November till May the whole country is ice-bound, so that 
the vegetation is perfectly dormant. The spring is very 
lingering, owing to the great extent of surface occupied by 
‘water, and the neighbourhood of the large lakes on the one 
hand, and of Hudson Bay on the other, the slow melting of 
the ice which accumulates on these sheets of water keeping the 
temperature depressed till far on in the season. Thus, in cross- 
ing Lake Superior, on the 9th of June, the expedition encoun- 
tered much cold weather, and got entangled in the ice floes that 
were, even so late in the season, drifting about the lake. The 
summer temperature is high, and for the same reason that 
renders the spring late, the autumn is prolonged beyond its 
normal extent, the influence of the large internal masses of 
water not having the effect of producing an equalised climate 
like that of a sea-coast, but merely prolonging the force exer- 
cised by the half-yearly extremes of heat and cold. 
To the west of the Laurentian Axis commences the region 
of plains that extends to the eastern base of the Rocky 
Mountains, and north and south throughout the whole central 
district of the North American continent. In the neighbour- 
hood of Lake Winipeg, the primitive rocks are overlaid by 
nearly horizontal strata of Silurian and Devonian age, consist- 
ing of limestones sometimes containing magnesia to such an ex- 
tent that the soil derived from their decomposition must be of 
inferior quality. Excepting along the margins of the group 
of lakes that lie close to the axis, outcrops of these limestones 
are, however, rarely met with, the floor of the plateau being 
almost everywhere concealed by superficial deposits, consist- 
ing of sands, gravels, and marls, the bulk of which have 
been derived by denudation from the cretaceous strata that at 
one time must have overlaid the area now occupied by the 
chain of lakes that extends from Lake Winipeg to Great 
Bear Lake. A succession of steps, composed of these super- 
ficial deposits, and covered with a great profusion of erratic 
blocks, raises the level as we proceed westward, until, at an 
altitude of 1600 to 2000 feet, the finely assorted and well- 
mixed soils of. the drift deposits cease, and the surface of 
the plain is occupied by strata of the age of the chalk, but 
NEW SERIES.—VOL. XIv. No. 11.—ocrT. 1861. 25 
