As they are weathered, some rocks dissolve and go into solution. For
example, a number of the Texas creeks and rivers carry calcium carbonate
in solution because they flow through areas where limestone rocks, which
consist mostly of calcium carbonate, are being weathered. (Water that
contains a large amount of dissolved rock material is called _hard_
water.)
_Cementing materials and chemical sediments._—
Some of the waters containing dissolved rock material seep through loose
sediments where the dissolved material may come out of solution and form
a _cement_, which binds the sediments together. For example, when loose
sand sediments are cemented, they form sandstone. Three of the most
common cements are iron oxide, calcium carbonate, and silicon dioxide,
although a number of other materials also serve as cements.
Dissolved rock materials come out of solution not only to serve as
cementing agents but to form the chief mineral of some sedimentary rocks
as well. Sedimentary rocks of this kind form mostly in lakes and seas
into which much dissolved material is carried by rivers. When the
dissolved material comes out of solution, it is said to be
_precipitated_ and the mineral sediments it forms are the _chemical_
sediments. Some limestones originate this way. You can see examples of
precipitated materials by noting the crust-like deposits that form
inside some water pipes and teakettles, as dissolved material in the
water comes out of solution.
[Illustration: Precipitated sediments are commonly observed lining a
teakettle.]
_Sedimentary rocks formed by plants and animals._—
The dissolved rock material can come out of solution in another way.
Some plants and animals are able to take dissolved calcium carbonate out
of the sea water and use it to build their shells and other structures.
Some of these organisms, such as corals and algae, can grow upward from
the sea floor in large groups to form reefs that later become reef
limestones. Other limestones are made up of the remains of plants and
animals that collect on the sea floor and become cemented together.