Or the same earthquake may have heated and hardened the limestones
simply by grinding and squeezing them; or they may have been heated and
hardened in the course of long ages simply by the weight of the thousands
of feet of other rock which lay upon them. For pressure, you must
remember, produces heat. When you strike flint and steel together, the
pressure of the blow not only makes bits of steel fly off, but makes them
fly off in red-hot sparks. When you hammer a piece of iron with a
hammer, you will soon find it get quite warm. When you squeeze the air
together in your pop-gun, you actually make the air inside warmer, till
the pellet flies out, and the air expands and cools again. Nay, I
believe you cannot hold up a stone on the palm of your hand without that
stone after a while warming your hand, because it presses against you in
trying to fall, and you press against it in trying to hold it up. And
recollect above all the great and beautiful example of that law which you
were lucky enough to see on the night of the 14th of November 1867, how
those falling stars, as I told you then, were coming out of boundless
space, colder than any ice on earth, and yet, simply by pressing against
the air above our heads, they had their motion turned into heat, till
they burned themselves up into trains of fiery dust. So remember that
wherever you have pressure you have heat, and that the pressure of the
upper rocks upon the lower is quite enough, some think, to account for
the older and lower rocks being harder than the upper and newer ones.
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