NECK SIX MILES NORTHEAST OF JUANTAFOYA.
tains. 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
MESAS AND BUTTES.
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