One day, after a night camp on the edge of a rather high table land,
they started across a fertile plain that was covered with a rich
growth of grass.

"Good grazing ground here," commented Ned.

"Yes," put in San Pedro. "Plenty much horse here pretty soon."

"Do the natives graze their herds of horses here?" asked Tom.

"No natives--wild horses," explained Pedro. "Plenty much, sometimes
too many they come. You see, maybe."

It was nearly noon, and Tom was considering stopping for dinner if
they could come to a good watering place, when Ned, who had ridden
slightly in advance, came galloping back as fast as his steed would
carry him.

"Look out! Look out!" he cried. "There's a stampede of 'em, and
they're headed right this way!"

"Stampede of what? Who's headed this way?" cried Tom. "A lot of
monkeys?"

"No, wild horses! Thousands of 'em! Hear 'em coming?"

In the silence that followed Ned's warning there could be heard a
dull, roaring, thundering sound, and the earth seemed to tremble.