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.
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.
[METAMORPHIC ROCKS]
[Metamorphic rocks] come from earlier-formed rocks that have undergone a change or a metamorphosis. All [igneous] and [sedimentary rocks], and earlier-formed metamorphic rocks too, can be changed, without being moved to some other place, into new and different rocks. As they are changed, they may become harder, new minerals may form, and they may look entirely different. For example, [granite], an igneous rock, can be changed into the [metamorphic rock] known as [gneiss]; [limestone], a [sedimentary] rock, can be changed into [marble]; [shale], a sedimentary rock, can be changed into slate. These changes occur because the earth is a big and complex chemical [system]. The agents that bring about these changes, which always occur below the surface of the earth, are heat, pressure, and [fluids]—both liquids and gases. Several different kinds of change or metamorphism can take place.