All crystals, as
quartz or diamonds, have been made by deposits from water. Hot water
can hold in solution much more of a solid than cold water. Therefore,
when hot water comes out of the earth and is cooled, some of the
saturating substance must be deposited as a solid. It is done in
various ways, especially two.
Suppose a little pool with perpendicular sides, say twenty feet across.
It leaps and boils two feet high. It deposits nothing till the water
comes to the cooling edge. Then it builds up a wall where it
overflows, and wherever it flows it builds. The result is that you
walk up the gentle slopes of a broad flat cone, and find the little
lakelet in a gorgeous setting, perfectly full at every point of the
circumference. If there is but little overflow, the result may be to
deposit all the matter where it first cools, and make a perpendicular
wall around the cup two or ten feet high. If the overflow is too much
to be cooled at once, the deposit may still be made fifty or one
hundred feet from the point of issue. If the overflow is sufficient,
it may be building up every inch of a vast cone at once, every foot
being wet.
[Illustration: The Punch Bowl, Yellowstone Geysers.]
Many minerals are held in solution and are deposited at various stages
of evaporation. Let us suppose the lake to have the bottom sloping
toward the abysmal center; the different minerals will be assorted as
if with a sieve.
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