The earth, then, is either solid, or has an exceedingly
thick crust, becoming harder as we descend, harder than any known rock.
At least, that is the conclusion pointed at in a paper presented to the British Association by the Tidal Committee for the year. They have ascertained that if the earth had only a shell of solid rock, say fifty miles in thickness, enclosing melted matter, it would yield under the tide almost as freely as a liquid. A globe of glass of the same size as the earth would yield like indiarubber, and it is probable from a series of experiments that the earth as a whole is vastly more rigid than any rock upon her surface, a fact which, it is suggested, may be due to the excessive pressure in the interior. Well, but volcanoes and geysers ? If there is not fire down there, not many miles deep, how do they come ?
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