Man's physical universe

xanabras

180 THE EARTH AS MAN'S ABODE

mineral is a pure material or nearly pure material that occurs in nature),

quartz {i.e., silicon dioxide), feldspars, and white micas {i.e., complex

silicates of sodium, potassium, aluminum, calcium, magnesium, and

iron). When liquid solutions are cooled slowly, the crystals formed

grow to a large size, while small crystals are formed from rapidly

cooled solutions.

Granites which have been formed slowly have large

quartz and feldspar crystals that give a coarse texture, while other,

more rapidly cooled granites have small quartz and feldspar crystals

and consequently a fine texture.

Basalt contains crystals which are microscopic in size and therefore

presents a homogeneous appearance; it contains less quartz and more

calcium, magnesium, and iron compounds than the granitic rocks.

The chief constituents of the basalts are hornblendes and black mica.

The lower melting-point of basalt results in liquid magma, which flows

more rapidly and cools more quickly. This is the magma of which the

cone type of volcanic mountains is generally made. The basalts are

denser than the granites and are found chiefly under the oceans.

Mountains like the Rocky Mountains, which have been pushed up

from below, are largely granitic in nature. Granite is widely used as a

building-stone.

Sedimentary Rock Is a Product of Erosion.

Sedimentary rock can be identified by characteristic strata formed

by deposition of eroded material from water. The layers are produced

by the uneven rate of sedimentation; thus a full, rushing stream of the

spring carries more materials and carries larger particles farther than

the shallow stream of late fall. Again a stream is constantly eating its

way into the hills, moving so that the mud and silt eventually are

dropped where the sand was deposited before, while the sand is dropped

farther upstream where the gravel was formerly deposited before the

rapid-flowing portion of the water had moved upstream.

Layers of sediment are cemented together by minerals removed

chemically from the water; and as these layers become buried by layer

after layer of rock up to a thickness of thousands of feet, the pressure

makes them more compact.

Conglomerates are sedimentary rocks made from the coarser gravel

and sand. Sand, which is nearly pure quartz produced from the disintegration

of granite, is cemented together to form sandstone. The smaller

sedimentary particles {i.e., the silt and clay) form the characteristic

laminated shales.

Some of the black-shale deposits contain vast quantities of petroleum,

which can be separated, but not at a cost that will compete with