He looked to discover the cause.
As his eye glanced along the ground, he perceived that it was uneven-- covered, as far as he could see, with little hillocks.

The mustang was among them.

It was this, then, that was causing him to run so strangely.

Basil had hardly made the observation, when he felt his horse sink suddenly under him, and tumble headlong upon the prairie! The rider was flung from his seat, though not much hurt.

He rose at once to his feet.