e,
by which the relative age of mountains may be estimated. The oldest
mountains are the lowest, while the younger and more recent ones tower
above their elders, and are usually more torn and dislocated also.
This is easily understood, when we remember that all mountains and
mountain-chains are the result of upheavals, and that the violence of
the outbreak must have been in proportion to the strength of the
resistance. When the crust of the earth was so thin that the heated
masses within easily broke through it, they were not thrown to so
great a height, and formed comparatively low elevations, such as the
Canadian hills or the mountains of Bretagne and Wales. But in later
times, when young, vigorous giants, such as the Alps, the Himalayas,
or, later still, the Rocky Mountains, forced their way out from their
fiery prison-house, the crust of the earth was much thicker, and
fearful indeed must have been the convulsions which attended their
exit.

[Illustration: A PHYSICAL MAP OF THE UNITED STATES.]

The Laurentian Hills form, then, a granite range, stretching from
Eastern Canada to the Upper Mississippi, and immediately along its
base are gathered the Azoic deposits, the first stratified beds, in
which the absence of life need not surprise us, since they were
formed beneath a heated ocean. As well might we expect to find the
remains of fish or shells or crabs at the bottom of geysers or of
boiling springs, as on those early shores bathed by an ocean of which
the heat must have been so intense. Although, from the condition in
which we find it, this first granite range has evidently never been
disturbed by any violent convulsion since its first upheaval, yet
there has been a gradual rising of that part of the continent; for the
Azoic beds do not lie horizontally along the base of the Laurentian
Hills in the position in which they must originally have been
deposited, but are lifted and rest against their slopes. They have
been more or less dislocated in this process, and are greatly
metamorphized by the intense heat to which they must have been
exposed. Indeed, all the oldest stratified rocks have been baked by
the prolonged action of heat.

It may be asked how the materials for those first stratified deposits
were provided. In later times, when an abundant and various soil
covered the earth, when every river brought down to the ocean, not
only its yearly tribute of mud or clay or lime, but the debris of
animals and pl