The total number of rock-forming minerals is very large, but many of
them are very rare, and form but a very small part of the earth’s crust.
The most abundant materials or earths of which rocks are composed are
_silica_, _lime_ and _aluminum_. Silica or flint is very universally
diffused. It is found almost pure in quartz, opal, chalcedony, rock
crystal, and the flinty sand of the sea-shore. Lime is also a very
generally distributed earth, and is usually found in the form of
carbonate. Under the several names of marl, limestone, oolite, and chalk
it constitutes mountains, and even ranges of mountains. Aluminum is
likewise very abundant, and of great importance to mankind. It enters
largely into the clayey or argillaceous earths, and forms part of
various kinds of rock which possess the property of not permitting water
to pass through its substance--a property which renders it of
inestimable value both for natural and artificial reservoirs of water.