l of the land--a fact which is beyond controversy, the other
agencies are very plain and recognisable consequences of the upheaval.
There are, however, many geologists who do not think this explanation
adequate.

It is pointed out, in the first place, that the glaciation seems to
have come long after the elevation. The difficulty does not seem to
be insurmountable. The reduction of the atmospheric vapour would be
a gradual process, beginning with the later part of the elevation and
culminating long afterwards. The reduction of the carbon-dioxide would
be even more gradual. It is impossible to say how long it would take
these processes to reach a very effective stage, but it is equally
impossible to show that the interval between the upheaval and the
glaciation is greater than the theory demands.

It is also said that we cannot on these principles understand the
repeated advance and retreat of the ice-sheet.

This objection, again, seems to fail. It is an established fact that
the land sank very considerably during the Ice-Age, and has risen again
since the ice disappeared. We find that the crust in places sank so low
that an arctic ocean bathed the slopes of some of the Welsh mountains;
and American geologists say that their land has risen in places from
2000 to 3000 feet (Chamberlin) since the burden of ice was lifted from
it. Here we have the possibility of an explanation of the advances and
retreats of the glaciers. The refrigerating agencies would proceed
until an enormous burden of ice was laid on the land of the northern
hemisphere. The land apparently sank under the burden, the ice and snow
melted at the lower level and there was a temperate interglacial period.
But the land, relieved of its burden, rose once more, the exposed
surface absorbed further quantities of carbon, and a fresh period of
refrigeration opened. This oscillation might continue until the two sets
of opposing forces were adjusted, and the crust reached a condition of
comparative stability.

Finally, and this is the more serious difficulty, it is said that we
cannot in this way explain the localisation of the glacial sheets. Why
should Europe and North America in particular suffer so markedly from
a general thinning of the atmosphere? The simplest answer is to suggest
that they especially shared the rise of the land. Geology is not in
a position either to prove or disprove this, and it remains only a
speculative interpretation of the fact We