No sound is so terrible even to horses.

It threw the leader into an hysteria of panic.

Others of the herd were falling or staggering in the lake; the remnant rushed up the slope and over the sheltering crest of the hill beyond.
Every nerve in the body of Alcatraz urged him to leap away with arrowy speed, passing even the grey mare--she who now shot off across the hills far in the van--but behind him raced weaker and slower horses, the older stallions and the mares with their foals.

Instinct proved greater than fear.

He swept around the rear of his diminished company to round up the laggards, but they were already laboring to the full of their power as five horsemen streamed across the crest with their rifles carried at the ready.