walls and pillars of volcanic rocks. The entire formation of the chain,
as far as it faces the Rio Grande, is volcanic, the walls of the gorges
consisting generally of a friable white or yellowish tufa containing
nodules of black, translucent obsidian. The rock is so soft that in many
places it can be scooped out or detached with the most primitive tools,
or even with the fingers alone. Owing to this peculiarity the slopes
exposed to the south and east, whence most of the heavy rains strike
them, are invariably abrupt, and often even perpendicular; whereas the
opposite declivities, though steep, still afford room for scanty
vegetation. The gorges run from west to east,--that is, they descend
from the mountain crests to the Rio Grande, cutting the long and narrow
pedestal on which the high summits are resting.

Through some but not all of these gorges run never-failing streams of
clear water. In a few instances the gorge expands and takes the
proportions of a narrow vale. Then the high timber that usually skirts
the rivulets shrinks to detached groves, and patches of clear land
appear, which, if cultivated, would afford scanty support to one or two
modern families. To the village Indian such tillable spots were of the
greatest value. The deep ravine afforded shelter not only against the
climate but against roving enemies, and the land was sufficient for his
modest crops; since his wants were limited, and game was abundant.

The material of which the walls of these cañons are composed, suggested
in times past to the house-building Indian the idea of using them as a
home. The tufa and pumice-stone are so friable that, as we have said,
the rock can be dug or burrowed with the most primitive implements. It
was easier, in fact, to excavate dwellings than to pile up walls in the
open air.