FORM OF MINERALS 75
feldspar
,to kaolin, quartz or pearl spar into tale, iron pyrites or iron glance
into brown-iron ore, azurite into malachite, augite into green earth.
The true nature of such bodies is shown by the internal structure,
having no relation to the external form or apparent system of
crystallization.
The
process of petrifaction of organic bodies is in reality a species of
pseudomorphic formation, and has been produced in all the above modes.
External and internal casts of organic bodies are not uncommon. In
other cases the original substance has been replaced by some mineral
which has preserved not merely the external form, but even the
"minutest detail of internal structure; so that the different kinds of
wood have been distinguished in their silicified trunks. The most
common petrifying substances are silica and carbonate of lime. In
encrynites, echinites, belemnites, and other fossils, the crystals of
calc-spar often occur in very regular positions. In some varieties of
petrified wood both the ligneous structure and the cleavage of the
calc-spar are observable.
Different
from the above are mineralized bodies, in which the original structure
is still retained, but their chemical nature partially changed. In
these a complete series may be often traded, as from wood or peat,
through the varieties of brown coal, common coal, anthracite, and
graphite, perhaps even to the diamond.
