We have already compared the chemical elements to the alphabet and the
minerals to words. We can now go a step further and compare rocks to
sentences. We put words together to make sentences; nature puts minerals
together to make rocks. A sentence does not have to be made up of a
definite number of words, nor does a rock have to be made up of a
definite number of minerals. Some rocks, such as granite, may be
composed of several minerals. Others, such as dolomite and rock gypsum,
consist of only one mineral.
Minerals do not lose their identities when they make up a rock. Instead,
they are merely associated together in varying proportions. Some rocks,
as we will find later, instead of being composed of the minerals
themselves, are made up of fragments of earlier-formed rocks.
Ordinarily, we think of rocks as hard and solid substances, such as
limestone and granite, but some geologists consider loose and uncemented
materials, such as sand, gravel, or volcanic ash, to be rocks also. The
words _sediments_ or _deposits_ are often used to describe this
uncemented or loose material.
Rocks are commonly grouped, according to how they formed, into three
great classes known as _igneous_, _metamorphic_, and _sedimentary_.
[Illustration: A rock is made up of minerals. The igneous rock
_granite_, for example, consists chiefly of quartz and feldspar; other
minerals such as mica and hornblende are commonly present.]