hydrated sulphate of lime, and so soft that it may be scratched
with the finger nail, is readily taken up by water, giving to the
water of wells and springs a peculiar hardness difficult to
remove.

The dissolving action of moisture may be noted on marble
tombstones of some age, marble being a limestone altered by heat
and pressure and composed of crystalline grains. By assuming that
the date on each monument marks the year of its erection, one may
estimate how many years on the average it has taken for weathering
to loosen fine grains on the polished surface, so that they may be
rubbed off with the finger, to destroy the polish, to round the
sharp edges of tool marks in the lettering, and at last to open
cracks and seams and break down the stone. We may notice also
whether the gravestones weather more rapidly on the sunny or the
shady side, and on the sides or on the top.

The weathered surface of granular limestone containing shells
shows them standing in relief. As the shells are made of
crystalline carbonate of lime, we may infer whether the carbonate
of lime is less soluble in its granular or in its crystalline
condition.

THE FORMATION OF CARBONATES. In attacking minerals water does more
than merely take them into solution. It decomposes them, forming
new chemical compounds of which the carbonates are among the most
important. Thus feldspar consists of the insoluble silicate of
alumina, together with certain alkaline silicates which are broken
up by the action of water containing carbon dioxide, forming
alkaline carbonates. These carbonates are freely soluble and