oling of molten rock material. The ultimate source of
this molten material does not here concern us. It may come from deep
within the earth or from comparatively few miles down. It may include
preexisting rock of any kind which has been locally fused within the
earth. Wherever and however formed, its tendency is to travel upward
toward the surface. It may stop far below the surface and cool slowly,
forming coarsely crystallized rocks of the granite and gabbro types.
Igneous rocks so formed are called _plutonic_ intrusive rocks. Or the
molten mass may come well toward the surface and crystallize more
rapidly into rocks of less coarse, and often porphyritic, textures. Such
intrusive rocks are porphyries, diabases, etc. Or the molten mass may
actually overflow at the surface or be thrown out from volcanoes with
explosive force. It then cools quickly and forms finely crystalline
rocks of the rhyolite and basalt types. These are called effusives or
extrusives, or lavas or volcanics, to distinguish them from intrusives
formed below the surface. The intrusive masses may take various forms,
called stocks, batholiths, laccoliths, sills, sheets and dikes,
definitions and illustrations of which are given in any geological
textbook. The effusives or volcanics at the surface take the form of
sheets, flows, tuffs, agglomerates, etc.

Some of the igneous rocks are themselves "mineral" products, as for
instance building stones and road materials. Certain basic intrusive
igneous rocks contain titaniferous magnetites or iron ores as original
constituents. Others carry diamonds as original constituents. Certain
special varieties of igneous rocks, known as pegmatites, carry coarsely
crystallized mica and feldspar of commercial value, as well as a
considerable variety of precious gems and other commercial minerals.
Pegmatites are closely related to igneous after-effects, discussed under
the next heading. As a whole, the mineral products formed directly in
igneous rocks constitute a much less important class than mineral
products formed in other ways, as described below.

=Igneous after-effects.= The later stages in the formation of igneous
rocks are frequently accompanied by the expulsion of hot waters and
gases which carry with them mineral substances. These become deposited
in openings in adjacent rocks, or replace them, or are deposited in
previously hardened portions of the parent igneous mass itself. They
form "contact-metamorphic" an