craters.
The oldest volcanic rock appears to be rhyolite, which peers up in two
small hills almost smothered beneath the lake deposits. Its eruption was
long anterior to the lake period. On the other hand, the cessation of
the eruptions of the later basaltic sheets is evidently an event of such
recent date that Mr. Gilbert is led to look forward to their resumption
at some future, but not distant, epoch. As he truly observes, we are not
to infer that, because the outward manifestations of volcanic action
have ceased, the internal causes of those manifestations have passed
away. These are still in operation, and must make themselves felt when
the internal forces have recovered their exhausted energies; but perhaps
not to the same extent as before.

(_f._) _Region of the Snake River._--The tract of country bordering the
Snake River in Idaho and Washington is remarkable for the vast sheets of
plateau-basalt with which it is overspread, extending sometimes in one
great flood farther than the eye can reach, and what is still more
remarkable, they are often unaccompanied by any visible craters or vents
of eruption. In Oregon the plateau-basalt is at least 2,000 feet in
thickness, and where traversed by the Columbia River it reaches a
thickness of about 3,000 feet. The Snake and Columbia rivers are lined
by walls of volcanic rock, basaltic above, trachytic below, for a
distance of, in the former, one hundred, in the latter, two hundred,
miles. Captain Dutton, in describing the High Plateau of Utah, observes
that the lavas appear to have welled up in mighty floods without any of
that explosive violence generally characteristic of volcanic action.
This extravasated matter has spread over wide fields, deluging the
surrounding country like a tide in a bay, and overflowing all
inequalities. Here also we have evidence of older volcanic cones buried
beneath seas of lava subsequently extruded.

(_g._) _Fissures of Eruption._--The absence, or rarity, of volcanic
craters or cones of eruption in the neighbourhood of these great sheets
has led American geologists to the conclusion that the lavas were in
many cases extruded from fissures in the earth's crust rather than from
ordinary craters.[7] This view is also urged by Sir A. Geikie, who
visited the Utah region of the Snake River in 1880, and has vividly
described the impression produced by the sight of these vast fields of
basaltic lava. He says, "We found that the older trachytic la