Once more the tower of smoke, which they had noted the day before, was before them, but now it seemed blacker and more ominous than on the previous day.
It was not far from midday when, away to the westward, they heard rumbling sounds, like distant thunder.
"Vot id vas, ain'd id?" asked Hans, in alarm. "I don'd seen no dunder shower coming up somevere, do I?"
"It did not seem like thunder," said Frank, soberly. "It was more like a rumbling beneath the ground, and I fancied the earth quivered a bit."
"Perhaps it is an earthquake," put in the professor, apprehensively. "I believe they have such convulsions of nature in this part of the world."
Bushnell said nothing, but there was a troubled look on his face, and he urged them all forward at a still swifter pace.
The smoke tower was now looming near at hand, and they could see it shift and sway, grow thin, and roll up in a dense, black mass. It cast a gloom over their spirits, and made them all feel as if some frightful disaster was impending.
Again and again, at irregular intervals, they heard the sullen rumbling, and once all were positive the earth shook.
It was noticed that directly after each rumbling the smoke rolled up in a thick, black mass that shut out the light of the sun and overcast the heavens.
The professor was for turning back, but Bushnell was determined to go forward, and Frank was equally resolute. Hans had very little to say, but his nerves were badly shaken.