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.