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 of gap or canon in the rock formation, easily forming it into a reservoir. Then if the barrier between the two upper ones were to be weakened, and a sudden weight of water were to be thrown on the lower wall; suppose such wall were to partially collapse, and bring down, say, a clay bank, which would make a temporary barrier loftier than any yet existing, but only temporary; suppose that the quick accumulation of waters behind this barrier lifted the whole mass of water and slime and bog to its utmost height. Then, when, such obstruction had been reached, the whole lower barrier, weakened by infiltration and attacked with sudden and new force, would give way at once, and the stream, kept down from above by the floating bog, would force its way along the bed rock and lift the whole spongy mass resting on it. Then with this new extent of bog suddenly saturated and weakened—demoralized as it were—and devoid of resisting power, the whole floating mass of the upper bog might descend on it, mingle with it, become incorporated with its semi-fluid substance, and form a new and dangerous quagmire incapable of sustaining solid weight, but leaving behind on the higher level only the refuse and sediment of its former existence—all the rubble and grit too heavy to float, and which would gradually settle down on the upper bed rock."

"Really, Dick, you put it most graphically. What a terrible thing it would be to live on the line of such a change."

"Terrible, indeed! At such a moment a house in the