foot in ten thousand years; it is most likely, indeed, much to exceed
this amount. From data afforded by the eruptions in Java and in other
fields where the quantity of volcanic dust contributed to the seas can
be estimated, the writer is disposed to believe that the average rate
of sedimentation on the sea floors is twice as great as the estimate
above given.

Accumulating at the average rate of one foot in ten thousand years, it
would require a million years to produce a hundred feet of sediments;
a hundred million to form ten thousand feet, and five hundred million
to create the thickness of about ten miles of bed. At the rate of two
feet in ten thousand years, the thickness accumulated would be about
twenty miles. When we come to consider the duration of the earth's
geologic history, we shall find reasons for believing that the
formation of sediment may have continued for as much as five hundred
million years.

The foregoing inquiries concerning the origin of volcanoes show that
at the present time they are clearly connected with some process which
goes on beneath the sea. An extension of the inquiry indicates that
this relation has existed in earlier geological times; for, although
the living volcanoes are limited to places within three hundred miles
of the sea, we find lava flows, ashes, and other volcanic
accumulations far in the interior of the continents, though the energy
which brought them 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