Suddenly he straightened up with a flushed face. His countenance bore an angry look.
"Come here, fellows," he said, "and tell me what you make of this mark at the side of the rock."
He indicated a queer abrasion on one side of the stone. The living stone showed whitely where the lichen and moss had been scraped aside.
"Looks like some cuss had put a lever under it," pronounced Cal, after a careful inspection.
"That's what. Fellows, this rock was deliberately tilted so that it would come down on us and crush us. Now there's only one bunch of men that we know of mean enough to do such a thing and that's----"
Something whistled past Nat's ear with a noise somewhat like the humming of a drowsy bee, only the sound lasted but for a fraction of a second.
It struck the rock behind him, and not half an inch from a direct line with his head, with a dull spatter.
The boy could not help turning a trifle pale as he realized what an exceedingly narrow escape he had had. Cal's countenance blazed with fury.
"The--the dern--skunks!" he burst out, unlimbering his well polished old revolver.
"No good, Cal," he said, "the best thing we can do is to get out of here as quickly as possible. One man up there behind those rocks could wipe out an army down in here."
Cal nodded grimly, as he recognized the truth of the lad's words. Truly they were in no position to do anything but, as Nat had suggested, get out as quickly as possible.
As they reached this determination another bullet whizzed by and struck a rock behind them, doubly convincing them of the wisdom of this course. Fortunately, as has been said, the boulder had rolled clear across the floor of the narrow canyon, such had been its velocity. This was lucky for the lads, for if it had obstructed the way they would have been in a nasty trap. With no room to turn round and no chance of going ahead their invisible enemies would have had them at their mercy.
But if they could not see the shooters on the hillside, those marksmen appeared to have their range pretty accurately. Bullets came pattering about them now in pretty lively fashion. Suddenly Herr Muller gave an exclamation and a cry of mingled pain and alarm. A red streak appeared at the same instant on the back of his hand where the bullet had nicked him. But this was not the cause of his outcry. The missile had ended its career in the case in which he carried his photographic plates.