August 25.--_We make 12 miles this morning, when we come to monuments
of lava standing in the river,--low rocks mostly, but some of them
shafts more than a hundred feet high. Going on down three or four miles,
we find them increasing in number. Great quantities of cooled lava and
many cinder cones are seen on either side; and then we come to an abrupt
cataract. Just over the fall on the right wall a cinder cone, or extinct
volcano, with a well-defined crater, stands on the very brink of the
canyon. This, doubtless, is the one we saw two or three days ago. From
this volcano vast floods of lava have been poured down into the river,
and a stream of molten rock has run up the canyon three or four miles
and down we know not how far. Just where it poured over the canyon wall
is the fall. The whole north side as far as we can see is lined with the
black basalt, and high up on the opposite wall are patches of the same
material, resting on the benches and filling old alcoves and caves,
giving the wall a spotted appearance.

The rocks are broken in two along a line which here crosses the river,
and the beds we have seen while coming down the canyon for the last 30
miles have dropped 800 feet on the lower side of the line, forming what
geologists call a "fault." The volcanic cone stands directly over the
fissure thus formed. On the left side of the river, opposite, mammoth
springs burst out of this crevice, 100 or 200 feet above the river,
pouring in a stream quite equal in volume to the Colorado Chiquito.

This stream seems to be loaded with carbonate of lime, and the water,
evaporating, leaves an incrustation on the rocks; and this process has
been continued for a long time, for extensive deposits are noticed in
which are basins with bubbling springs. The water is salty.

We have to make a portage here, which is completed in about three hours;
then on we go.

We have no difficulty as we float along, and I am able to observe the
wonderful phenomena connected with this flood of lava. The canyon was
doubtless filled to a height of 1,200 or 1,500 feet, perhaps by more
than one flood. This would dam the water back; and in cutting through
this great lava bed, a new channel has been formed, sometimes on one
side, sometimes on the other. The cooled lava, being of firmer texture
than the rocks of which the walls are composed, remains in some places;
in others a narrow channel has been cut, leaving a line of basalt on
either side. It i