d certain vein deposits. Pegmatites,
referred to above, are in a broad sense in this class of "igneous
after-effects," in that they are late developments in igneous intrusions
and often grade into veins clearly formed by aqueous or gaseous
solutions. Among the valuable minerals of the igneous after-effect class
are ores of gold, silver, copper, iron, antimony, mercury, zinc, lead,
and others. While mineral products of much value have this origin, most
of them have needed enrichment by weathering to give them the value they
now have.

WEATHERING OF IGNEOUS ROCKS AND VEINS

No sooner do igneous rocks appear at or near the earth's surface, either
by extrusion or as a result of removal by erosion of the overlying
cover, than they are attacked vigorously by the gases and waters of the
atmosphere and hydrosphere as well as by various organisms,--with
maximum effect at the surface, but with notable effects extending as far
down as these agents penetrate. The effectiveness of these agents is
also governed by the climatic and topographic conditions. Under
conditions of extreme cold or extreme aridity, weathering takes the form
mainly of mechanical disintegration, and chemical change is less
conspicuous. Under ordinary conditions, however, processes of chemical
decomposition are very apparent. The result is definitely known. The
rocks become softened, loose, and incoherent. Voids and openings appear.
The volume tends to increase, if all end products are taken into
account. The original minerals, largely feldspar, ferro-magnesian
minerals, and quartz, become changed to clay, mixed with quartz or sand,
calcite or dolomite, and iron oxide, together with residual particles of
the original feldspars and ferro-magnesian minerals which have only
partly decomposed. In terms of elements or chemical composition, water,
oxygen, and carbon dioxide, all common constituents of the atmosphere
and hydrosphere, have been added; and certain substances such as soda,
potassa, lime, magnesia, and silica have in part been carried away by
circulating waters, to be redeposited elsewhere as sediments, vein
fillings, and cements. Figure 1 illustrates the actual mineral and
volume changes in the weathering of a granite--one of the most common
rocks. The minerals anorthite, albite, and orthoclase named in this
figure are all feldspars; sylvite and halite are chlorides of potash
and soda. The weathering processes tend to destroy the original
minerals, te