rest on rock pavements which are polished and scored with glacial
markings. Hence toward the close of the Paleozoic the southern
lands of the eastern hemisphere were invaded by great glaciers or
perhaps by ice sheets like that which now shrouds Greenland.

These Permian ground moraines are not the first traces of the work
of glaciers met with in the geological record. Similar deposits
prove glaciation in Norway succeeding the pre-Cambrian stage of
elevation, and Cambrian glacial drift has recently been found in
China.

THE APPALACHIAN DEFORMATION. We have seen that during Paleozoic
times a long, narrow trough of the sea lay off the western coast
of the ancient land of Appalachia, where now are the Appalachian
Mountains. During the long ages of this era the trough gradually
subsided, although with many stillstands and with occasional
slight oscillations upward. Meanwhile the land lying to the east
was gradually uplifted at varying rates and with long pauses. The
waste of the rising land was constantly transferred to the sinking
marginal sea bottom, and on the whole the trough was filled with
sediments as rapidly as it subsided. The sea was thus kept
shallow, and at times, especially toward the close of the era,
much of the area was upbuilt or raised to low, marshy, coastal
plains. When the Carboniferous was ended the waste which had been
removed from the land and laid along its margin in the successive
formations of the Paleozoic had reached a thickness of between
thirty and forty thousand feet.

Both by sedimentation and by subsidence the trough had now become
a belt of weakness in the crust of the earth. Here the crust was