82 ELEMENTARY GEOLOGY 
Chemically precipitated Rocks. — Solvent Power of 
Water. As water flows or seeps through the ground, 
and even as it runs over the surface, it finds materials 
that it can carry away in solution. Even minerals 
(such as quartz) which do not appear to be soluble, may 
be taken up in minute quantities; but it is not to be — 
understood that this power of solution is usually due 
to the water itself. Pure rain water has little power 
of dissolving most minerals. 
From the decaying leaves, and indeed from the air, 
the rain absorbs impurities, such as carbonic acid gas. 
In passing through the decaying leaves, water may 
unite with the humic acids, or it may encounter alkaline 
substances which are easily dissolved; and in going 
through the soil, it may find readily soluble substances 
which have been introduced by the decay of the rock- 
forming minerals. These impurities transform water 
in some cases into a weak acid, in others a weak 
alkali; and in this condition it can do work of solu- 
tion, for it may either be able to attack the minerals 
directly, or substances may have been prepared for it 
by the previous decay of the minerals. 
If water is warmed, it will dissolve more salt or 
sugar than it could if cold; and so, also, as the tem- 
perature of water increases in the earth, its power of 
solution is correspondingly increased. Therefore, in 
some places, as at the outlet of a hot sprmg, the water 
