; in fact, in local usage the term mesa is usually applied
to all of these tables which do not carry volcanic mountains. The mesas
are carved out of platforms of horizontal or nearly horizontal rocks by
perennial or intermittent streams, and as the climate is exceedingly
arid most of the streams flow only during seasons of rain, and for the
greater part of the year they are dry arroyos. Many of the longer
channels are dry for long periods. Some of them are opened only by
floods that come ten or twenty years apart.

The region is also characterized by many buttes. These are plateaus or
mesas of still smaller dimensions in horizontal distance, though their
altitude may be hundreds or thousands of feet. Like the mesas and
plateaus, they sometimes form very conspicuous features of a landscape
and are of marvelous beauty by reason of their sculptured escarpments.
Below they are often buttressed on a magnificent scale. Softer beds give
rise to a vertical structure of buttresses and columns, while the harder
strata appear in great horizontal lines, suggesting architectural
entablature. Then the strata of which these buttes are composed are of
many vivid colors; so color and form unite in producing architectural
effects, and the buttes often appear like Cyclopean temples.

There is yet one other peculiarity of this landscape deserving mention
here. Before the present valleys and canyons were carved and the mesas
lifted in relief, the region was one of great volcanic activity. In
various places vents were formed and floods of lava poured in sheets
over the land. Then for a time volcanic action ceased, and rains and
rivers carved out the valleys and left the mesas and mountains standing.
These same agencies carried away the lava beds that spread over the
lands. But wherever there was a lava vent it was filled with molten
matter, which on cooling was harder than the sandstones and marls
through which it penetrated. The chimney to the region of fire below was
thus filled with a black rock which yielded more slowly to the
disintegrating agencies of weather, and so black rocks rise up from
mesas on every hand. These are known as volcanic necks, and, being of a
somber color, in great contrast with the vividly colored rocks from
which they rise and by which they are surrounded, they lend a strange
aspect to the landscape. Besides these necks, there are a few volcanic
mountains that tower over all the landscape and gather about themselv