dy of the
problem has shown us that while parts of each continent may at any
time be depressed beneath the sea, the whole of its surface rarely if
ever goes below the water level. Thus, in the case of North America,
we can readily note very great changes in its form since the land
began to rise above the water. But always, from that ancient day to
our own, some portion of the area has been above the level of the sea,
thus providing an ark of refuge for the land life when it was
disturbed by inundations. The strongest evidence in favour of the
opinion that the existing continents have endured for many million
years is found in the fact that each of the great lands preserves many
distinct groups of animals and plants which have descended from
ancient forms dwelling upon the same territory. If at any time the
relatively small continent of Australia had gone beneath the sea, all
of the curious pouched animals akin to the opossum and kangaroo which
abound in that country--creatures belonging in the ancient life of the
world--would have been overwhelmed.

We have already noted the fact that the uplifting of mountains and of
the table-lands about them, which appears to have been the basis of
continental growth, has been due to strains in the rocks sufficiently
strong to disturb the beds. At each stage of the mountain-building
movement these compressive strains have had to contend with the very
great weight of the rocks which they had to move. These lands are not
to be regarded as firm set or rigid arches, but as highly elastic
structures, the shapes of which may be determined by any actions which
put on or take off burden. We see a proof of this fact from numerous
observations which geologists are now engaged in making. Thus during
the last ice epoch, when almost all the northern part of this
continent, as well as the northern part of Europe, was covered by an
ice sheet several thousand feet thick, the lands sank down under their
load, and to an extent roughly proportional to the depth of the icy
covering. While the northern regions were thus tilted down by the
weight which was upon them, the southern section of this land, the
region about the Gulf of Mexico, was elevated much above its present
level; it seems likely, indeed, that the peninsula of Florida rose to
the height of several hundred feet above its present shore line. After
the ice passed away the movements were reversed, the northern region
rising and the southern s