ing,
Montana, and Washington. To the south it passes into the mountainous
region of Mexico, also highly volcanic; and thence into the ridge of
Panama and the Andes. It cannot be questioned but that the volcanic
nature of the Great Basin is due to the same causes which have
originated the volcanic outbursts of the Andes; but, from whatever
cause, the volcanic forces have here entered upon their secondary or
moribund stage. In the Yellowstone Valley, geysers, hot springs, and
fumaroles give evidence of this condition. In other districts the
lava-streams are so fresh and unweathered as to suggest that they had
been erupted only a few hundred years ago; but no active vent or crater
is to be found over the whole of this wide region. A few special
districts only can here be selected by way of illustration of its
special features in connection with its volcanic history.

(_b._) _The Plateau Country of Utah and Arizona._--This tract, which is
drained by the Colorado River and its tributaries, is bounded on the
north by the Wahsatch range, and extends eastwards to the base of the
Sierra Nevada. Round its margin extensive volcanic tracts are to be
found, with numerous peaks and truncated cones--the ancient craters of
eruption--of which Mount San Francisco is the culminating eminence.
South of the Wahsatch, and occupying the high plateaux of Utah, enormous
masses of volcanic products have been spread over an area of 9000 square
miles, attaining a thickness of between 3000 and 4000 feet. The earlier
of these great lava-floods appear to have been trachytic, but the later
basaltic; and in the opinion of Captain Dutton, who has described them,
they range in point of time from the Middle Tertiary (Miocene) down to
comparatively recent times.

(_c._) _The Grand Canyon._--To the south of the high plateaux of Utah
are many minor volcanic mountains, now extinct; and as we descend
towards the Grand Canyon of Colorado we find numerous cinder-cones
scattered about at intervals near the cliffs.[1] Extensive lava-fields,
surmounted by cinder-cones, occupy the plateau on the western side of
the Grand Canyon; and, according to Dutton, the great sheets of basaltic
lava, of very recent age, which occupy many hundred square miles of
desert, have had their sources in these cones of eruption.[2] Crossing
to the east of the Grand Canyon, we find other lava-floods poured over
the country at intervals, surmounted by San Francisco--a volcanic
mountain o