of pre-existing minerals in situ: take
Serpentine as an example ; it is a silicate of magnesium (often with
some iron replacing part of the magnesium) with some water, which water
is only driven off at a red heat, and is, therefore, in chemical
combination. Now, there is in nature a very large number of
ferro-magnesian silicates, and several of these, when acted on by
downward percolating water (probably containing small quantities of
alkaline carbonates in solution, and certainly acting over long periods
of time), are hydrated, and one of the substances that may be so formed
is Serpentine.
The
next group is much more important, as it includes some forms of
Calcite, all the true Agates, and the minerals Prehnite, Opal,
Dioptase, Turquois, and many of the forms of Quartz and Chalcedony.
Consider
these same waters referred to above, charged with traces of alkaline
carbonates, and percolating downward through, say, a mass of ancient
lava. First we may ask, " Whence came the carbonate in solution ? "
Eain falling on the earth contains a certain (small) amount of carbon
dioxide or carbonic acid: as the water percolates through the layers of
the soil it becomes further charged with the humus acids—allied to
carbonic acid, and generated largely by the action of bacteria upon the
organic matter which is everywhere present on the surface. This weak
acid solution in the course of time acts on some of the constituents of
the lava, and slowly dissolves, for instance, one of the complex
silicates containing sodium. Thus a weak solution of sodium carbonate
is formed. Such a solution would probably have no perceptible action on
a piece of lava in a laboratory, because we, relatively speaking,
neglect the
