ce into Canada. The Mesozoic sediments were
almost entirely laid down to the west and south-west of the protaxis,
upon the flat-lying Palaeozoic rocks, and in the prairie region they are
still almost horizontal; but in the Cordillera they have been thrust up
into the series of mountain chains characterizing the Pacific coast
region. The youngest of these mountain chains is naturally the highest,
and the oldest one in most places no longer rises to heights deserving
the name of mountains. Owing to this unsymmetric development of North
America the main structural watershed is towards its western side, on
the south coinciding with the Rocky Mountains proper, but to the
northward falling back to ranges situated further west in the same
mountain region. The great central area of Canada is drained towards
Hudson Bay, but its two largest rivers have separate watersheds, the
Mackenzie flowing north-west to the Arctic Ocean and the St Lawrence
north-east towards the Atlantic, the one to the south-west and the other
to the south-east of the Archean protaxis. While these ancient events
shaped the topography in a broad way, its final development was
comparatively recent, during the glacial period, when the loose
materials were scoured from some regions and spread out as boulder clay,
or piled up as moraines in others; and the original water-ways were
blocked in many places. The retreat of the ice left Canada much in its
present condition except for certain post-glacial changes of level which
seem to be still in progress. For this reason the region has a very
youthful topography with innumerable lakes and waterfalls as evidence
that the rivers have not long been at work. The uneven carving down of
the older mountain systems, especially that of the Archean pro taxis,
and the disorderly scattering of glacial material provide most of the
lake basins so characteristic of Canada.

_Lakes and Rivers._--As a result of the geological causes just mentioned
many parts of Canada are lavishly strewn with lakes of all sizes and
shapes, from bodies of water hundreds of miles long and a thousand feet
deep to ponds lost to sight in the forest. Thousands of these lakes have
been mapped more or less carefully, and every new survey brings to light
small lakes hitherto unknown to the white man. For numbers they can be
compared only with those of Finland and Scandinavia in Europe, and for
size with those of eastern Africa; but for the great extent of
la