“Then you have a theory of your own?”
“Yes,” said the captain; “it seems to me that first of all this was merely a jagged ravine, running from the mountain’s shoulder right down to the sea.”
“That’s what I thought. With a stream at the bottom.”
“No stream,” said the captain. “Nothing but vegetation. Down this a stream of red-hot lava must have flowed and burned the vegetation clean away, leaving a place for the mud to come down and harden as you have it now. It may have been a year after the eruption—twenty, fifty, or a hundred years, but there it is.”
“If you are right, we should see traces of the burning on the trees,” said the major.
“That does not follow. These trees may have sprung up since, right to the very edge of the stream, but no farther.”
“Then under this mud or bitumen there ought to be lava according to your ideas. How shall we prove it?”
“If I am right,” said the captain, “we shall find that this stream ends all at once, just as the lava hardened when the flow ceased, for there was no stream of volcanic matter right down to the shore.”
“And there is no stream of mud any further,” said Mark laughing; “for there’s the end.”
Mark was quite right, for about a couple of hundred yards below them the mighty walls of verdure suddenly came together and blocked out further progress, while, when they reached the spot, it was to find that the bituminous mud spread out here into a pool, further progress being, as it were, stopped by a dam of blackish rock which resembled so much solidified sponge, so full was it of air-holes and bubble-like cells.