In many places the mile-high walls were without a single ledge to break their vertical faces. When they came to the first such place they saw that the ground near the base was riddled with queer little pits, like tiny craters of the moon. As they looked there was a crack like a cannon shot and the ground beside them erupted into an explosion of sand and gravel. When the dust had cleared away there was a new crater where none had been before.

Humbolt wiped the blood from his face where a flying fragment had cut it and said, “The heat of the sun loosens rocks up on the rim. When one falls a mile in a one point five gravity, it’s traveling like a meteor.”

They went on, through the danger zone. As with the peril of the chasm’s heat, there was no choice. Only by observing the material that littered the base of the cliffs could they know what minerals, if any, might be above them.