That the mountain was of solid rock could not be doubted after a single glance at those sturdy, unflinching walls, black and sheer.
"Well, what do you make of it ?" inquired a voice at his elbow.

He turned to find Mr.Mott standing beside him.
"Earthquake," he replied.

"Thousands of years ago, of course.

Split the island completely in two." "Sounds plausible," mused the First Officer.

"But if that is the case, how do you account for the shallowness of the water in the passage and out here in the basin?
An earthquake violent enough to split that hill would make a crack in the earth a thousand fathoms deep." "I have an idea that if we took soundings in this basin we'd find a section twenty or thirty feet wide in the centre of it where we couldn't touch bottom.