rost; this, called _salitre_, being a sort
of impure saltpetre, left after the evaporation and subsidence of the
floods.

They have entered this cheerless waste, and are about midway across it,
when the cry of alarm is heard; he who gave utterance to it being older
than the others, and credited with greater knowledge of things. That
which had caught his attention, eliciting the cry, is but a phenomenon
of Nature, though not one of an ordinary kind; still, not so rare in the
region of the Chaco; since all of them have more than once witnessed it.
But the thing itself is not yet apparent save to him who has shouted,
and this only by the slightest sign giving portent of its approach. For
it is, in truth, a storm.

Even after the alarmist has given out his warning note, and stands on
his horse's hips, gazing off in a certain direction, the others, looking
the same way, can perceive nothing to account for his strange behaviour.
Neither upon the earth, nor in the heavens, does there appear anything
that should not be there. The sun is coursing through a cloudless sky,
and the plain, far as eye can reach, is without animate object upon it;
neither bird nor beast having its home in the _salitre_. Nothing
observable on that wide, cheerless waste, save the shadows of themselves
and their horses, cast in dark _silhouette_ across the hoary expanse,
and greatly elongated; for it is late in the afternoon, and the sun
almost down to the horizon.

"What is it?" asks Aguara, the first to speak, addressing himself to the
Indian who gave out the cry. "You appear to apprehend danger?"

"And danger there is, chief," returns the other. "Look yonder!" He
points to the level line between earth and sky, in the direction towards
which they are travelling. "Do you not see something?"

"No, nothing."

"Not that brown-coloured stripe just showing along the sky's edge, low,
as if it rested on the ground?"

"Ah, yes; I see that. Only a little mist over the river, I should say."

"Not that, chief. It's a cloud, and one of a sort to be dreaded. See!
it's rising higher, and, it I'm not mistaken, will ere long cover the
whole sky."

"But what do you make of it? To me it looks like smoke."

"No; it isn't that either. There's nothing out that way to make fire--
neither grass nor trees; therefore, it can't be smoke."

"What, then? You appear to know!"

"I do. 'Tis _dust_."

"Dust! A drove of wild horses? Or may they be mount