forth to the earth's surface has ceased to operate
in those parts of the land. In these cases of continental volcanoes it
generally, if not always, appears that the cessation of the activity
attended the removal of the shore line of the ocean or the
disappearance of great inland seas. Thus the volcanoes of the
Yellowstone district may have owed their activity to the immense
deposits of sediment which were formed in the vast fresh-water lakes
which during the later Cretaceous and early Tertiary times stretched
along the eastern face of the Rocky Mountains, forming a Mediterranean
Sea in North America comparable to that which borders southern Europe.
It thus appears that the arrangement of volcanoes with reference to
sea basins has held for a considerable period in the past. Still
further, when we look backward through the successive formations of
the earth's crust we find here and there evidences in old lava flows,
in volcanic ashes, and sometimes in the ruins of ancient cones which
have been buried in the strata, that igneous activity such as is now
displayed in our volcanoes has been, since the earliest days of which
we have any record, a characteristic feature of the earth. There is no
reason to suppose that this action has in the past been any greater or
any less than in modern days. All these facts point to the conclusion
that volcanic action is due to the escape of rock water which has been
heated to high temperatures, and which drives along with it as it
journeys toward a crevice the rock in which it has been confined.

We will now notice some other explanations of volcanic action which
have obtained a certain credence. First, we may note the view that
these ejections from craters are forced out from a supposed liquid
interior of the earth. One of the difficulties of this view is that we
do not know that the earth's central parts are fluid--in fact, many
considerations indicate that such is not the case. Next, we observe
that we not infrequently find two craters, each containing fluid lava,
with the fluid standing at differences of height of several thousand
feet, although the cones are situated very near each other. If these
lavas came from a common internal reservoir, the principles which
control the action of fluids would cause the lavas to be at the same
elevation. Moreover, this view does not provide any explanation of the
fact that volcanoes are in some way connected with actions which go on
on the floors of