The words died on the Professor's lips, as the tough stick, which had been raised above him, was brought down with a resounding whack, squarely on the top of his uncovered head.
Sudden darkness overwhelmed Professor Zepplin. He sank down with a moan, into utter oblivion.
When finally his heavy eyelids had struggled apart, night had fallen. At first, he could not imagine where he was nor what had happened. Shooting pains throbbed through his head and down into his arms and body.
The Professor uttered a suppressed moan, closed his eyes and lay back, vainly groping about in his disordered mind for a solution of the mystery.
Step by step he went back over the occurrences of the afternoon, which gradually became clearer, until at last he reached the point where he had finished his examination of the specimens of ore, in front of the cave entrance.
"And that's where I am now," decided Professor Zepplin, sitting up. "But, what happened then? I have it. Something hit me."
His hand instinctively went to his injured head. Then, with trembling fingers he began searching for the bag of minerals.
It was nowhere to be found. The Professor marveled at this for some minutes.
Like a blow, the answer came to him.
"Robbed!" he exclaimed.