From the point of view of their formation, therefore, precious stones may be naturally divided into two classes.
The
first comprehends stones produced by direct fusion, by crystallization
in an excess of their melted substance, by volatilization of their
elements; in a word, by the direct intervention of heat.
The second includes stones which have been formed in the midst of a solution of which water has been generally one of the constituent elements.
Hence
some precious stones are met with in those portions of our globe which
have been subjected to a high temperature, while others are found in
those that have never supported such a temperature, or what comes to
the same thing, that have been perfectly cooled at the period when they
furnished to the water the elements of the stones of which we are
speaking.
It
is therefore of some importance to distinguish those portions of our
globe which have been subjected to the action of fire from those that
have not; and this is easily done.
GEOLOGICAL POSITION.
When we consider the solid part of our globe we recognize immediately its division into earth,— using that word in its agricultural sense; and stones
