vas of the
hills had been deeply trenched by the lateral valleys, and that all
these valleys had a floor of black basalt that had been poured out as
the last of the molten materials from the now extinct volcanoes. There
were no visible cones or vents from which these floods of basalt could
have proceeded. We rode for hours by the margin of a vast plain of
basalt stretching southward and westward as far as the eye could
reach.... I realised the truth of an assertion made first by
Richthofen,[8] that our modern volcanoes, such as Vesuvius and Etna,
present us with by no means the grandest type of volcanic action, but
rather belong to a time of failing activity. There have been periods of
tremendous volcanic energy, when instead of escaping from a local vent,
like a Vesuvian cone, the lava has found its way to the surface by
innumerable fissures opened for it in the solid crust of the globe over
thousands of square miles."[9]

(_h._) _Volcanic History of Western America._--The general succession of
volcanic events throughout the region of Western America appears to have
been somewhat as follows:--[10]

The earliest volcanic eruptions occurred in the later Eocene epoch and
were continued into the succeeding Miocene stage. These consisted of
rocks moderately rich in silica, and are grouped under the heads of
propylite and andesite. To these succeeded during the Pliocene epoch
still more highly silicated rocks of trachytic type, consisting of
sanidine and oligoclase trachytes. Then came eruptions of rhyolite
during the later Pliocene and Pleistocene epochs; and lastly, after a
period of cessation, during which the rocks just described were greatly
eroded, came the great eruptions of basaltic lava, deluging the plains,
winding round the cones or plateaux of the older lavas, descending into
the river valleys and flooding the lake beds, issuing forth from both
vents and fissures, and continuing intermittently down almost into the
present day--certainly into the period of man's appearance on the scene.
Thus the volcanic history of Western America corresponds remarkably to
that of the European regions with which we have previously dealt, both
as regards the succession of the various lavas and the epochs of their
eruption.

(_i._) _The Yellowstone Park._--The geysers and hot springs of the
Yellowstone Park, like those in Iceland and New Zealand, are special
manifestations of volcanic action, generally in its secondary or
moribu