ich, by the action of denuding forces, have been
carved into series of ridges and summits. At many points, however, along
the sides of these great chains we find that fissures have been opened
and lines of volcanoes formed, from which enormous quantities of lava
have flowed and covered great tracts of country.

This is especially marked in the Snake River plain of Idaho, in the
western United States. In this, and the adjoining regions of Oregon and
Washington, an enormous tract of country has been overflowed by lava in
a late geological period, the surface covered being estimated to have a
larger area than France and Great Britain combined. The Snake River cuts
through it in a series of picturesque gorges and rapids, enabling us to
estimate its thickness, which is considered to average 4000 feet. Looked
at from any point on its surface, one of these lava-plains appears as a
vast level surface, like that of a lake bottom. This uniformity has been
produced either by the lava rolling over a plain or lake bottom, or by
the complete effacement of an original, undulating contour of the ground
under hundreds or thousands of feet of lava in successive sheets.
The lava, rolling up to the base of the mountains, has followed
the sinuosities of their margin, as the waters of a lake follow its
promontories and bays. Similar conditions exist along the Sierra Nevada
range of California, and to some extent placer mining has gone on under
immense beds of lava, by a process of tunneling beneath the volcanic
rock.

In some localities the volcanoes are of such height and dimensions as
to overlook and dwarf the mountain-ranges by the side of which they lie.
Some of the volcanoes lying parallel to the great American axis appear
to be quite extinct, while others are in full activity. In the Eastern
continent we find still more striking examples of parallelism between
great mountain-chains and the lands along which volcanic activity is
exhibited--volcanoes, active or extinct, following the line of the great
east and west chains which extend through southern Europe and Asia.
There are some other volcanic bands which exhibit a similar parallelism
with mountain chains; but, on the other hand, there are volcanoes
between which and the nearest mountain-axis no such connection can be
traced.

AREAS OF UPHEAVAL AND SUBSIDENCE

There is one other fact concerning the mode of distribution of volcanoes
upon the surface of the globe, to which we m