"The effects of the great Neapolitan earthquake of 1857 were so studied by Mr. Robert Mallet," continued the professor. "He disabused his mind of all superstition, threw away all the past mysteries, and attacked the problem from its mechanical side only. He believed that an earthquake was a series of shocks, or blows; but what he learned led other and later students to the discovery that an earthquake is not made up of blows at all." "That's all very well to say," grumbled Mr. Roebach. "I'm pretty solid on my feet; but what was it but a shock that threw me down? Tell me that, sir!" "Very easily explained," said the scientist, smiling. "Which will the quicker take you off your feet--a blow from, say, Jack's fist, or your stepping inadvertently upon a piece of glare ice? The ice, because it affords you so insecure a footing, is likely to throw you easier than a pretty solid blow; eh?" "True enough," admitted the oil hunter, smiling at Jack. "Although Darrow looks to be a pretty husky youngster." "My point is this," pursued the professor. "An earthquake is a continuous series of intricate twistings and oscillations in all possible directions, up and down, east and west, north and south, of the greatest irregularity both in intensity and direction. This writhing of the earth--of the very foundations of the ground we walk on--caused our recent overthrow," concluded Mr. Henderson. But the two boys were much more interested in the possibility of there being an active volcano in the neighborhood. The volcanic ash which covered the leaves and grass like road-dust assured them all that some huge "blow-hole" of the earth was near. "I wasn't looking for no such things as volcanoes," said Andy Sudds, seriously, "when I shipped for this voyage. I reckoned volcanoes blowed mostly in the tropics." "Alaska is a mighty field of active volcanoes," declared Professor Henderson. "But they have been mostly active on the Pacific coast, and among the islands which form a barrier between that ocean and Bering Sea. Islands have been thrown up, while others have sunk there because of volcanic disturbances, within the last few years." "And I presume the earthquake and the volcanic eruption are closely connected?" suggested Mark. "We may safely believe that," agreed the professor. "I am sorry my instruments are not at hand. I sincerely hope none was damaged when the _Snowbird_ made such a bad landing." "And I'd like to give the machine an overhauling at once to see just how badly she's damaged," Jack Darrow said, hastily. "What do you say, Mark?" "I'm with you," returned his chum. "Can't we take Andy and Wash, Mr. Henderson, and go right up to that hollow and see what needs to be done to the flying machine? Perhaps we can get off for Aleukan by to-morrow if we hustle."