To M'Nicholl's question:
"Why can't these streaks be simply prolonged mountain crests reflecting the sun's rays more vividly by their superior altitude and comparative smoothness?"
Barbican readily replied:
"These streaks can't be mountain crests, because, if they were, under certain conditions of solar illumination they should project shadows—a thing which they have never been known to do under any circumstances whatever. In fact, it is only during the period of the full Moon that these streaks are seen at all; as soon as the sun's rays become oblique, they disappear altogether—a proof that their appearance is due altogether to peculiar advantages in their surface for the reflection of light."
"Dear boys, will you allow me to give my little guess on the subject?" asked Ardan.
His companions were profuse in expressing their desire to hear it.
"Well then," he resumed, "seeing that these bright streaks invariably start from a certain point to radiate in all directions, why not suppose them to be streams of lava issuing from the crater and flowing down the mountain side until they cooled?"
"Such a supposition or something like it has been put forth by Herschel," replied Barbican; "but your own sense will convince you that it is quite untenable when you consider that lava, however hot and liquid it may be at the commencement of its journey, cannot flow on for hundreds of miles, up hills, across ravines, and over plains, all the time in streams of almost exactly equal width."
"That theory of yours holds no more water than mine, Ardan," observed M'Nicholl.
"Correct, Captain," replied the Frenchman; "Barbican has a trick of knocking the bottom out of every weaker vessel. But let us hear what he has to say on the subject himself. What is your theory. Barbican?"