Other sheets of film arose from out the southwest, placing layer after layer over the fast fading moon, until finally it was obliterated altogether.
The desert was working out another of its mysterious phases, but none in the camp of the Pony Riders were awake to observe it.
A dense pall of blackness now hovered over the southwest.
All at once a squirming streak of lightning wriggled along the horizon, like a golden serpent, losing itself by a downward plunge into the black abyss beyond the desert.
The air grew suddenly hot and depressing, while a gentle breeze stirred the sage brush on the higher places. The ponies moved restlessly in their sleep, kicking out a foot now and then, as if in protest at some disturbing presence.
Tad Butler, ever on the alert, roused himself, and stepping out in his pajamas took a survey of the heavens.
"I guess we're going to have a storm," he muttered. "I wonder if I ought to wake Mr. Parry? He thought, this afternoon, that there was a storm brewing. Still, there's nothing he can do. The tents are staked down as securely as is possible. No, I guess I'll go back to bed."
The lad did so, and after a few moments of wakefulness, dropped off into a sound sleep.
A few moments later the breeze increased, picking up little patches of sand, which it hurled into the air, scattering the particles over a wide area. Far down to the southwest a low roar might have been heard, and from the blackness there a funnel-shaped cloud detached itself, starting slantingly over the desert. It appeared to be following a northerly course, more or less irregularly, and from its direction, should pass some miles to the westward of the sleeping camp.
Whirling, diving, swooping here and there, lifting great patches of sand and hurling them far up into the clouds, the funnel swept on.