882 ELEMENTARY GEOLOGY 
the cavity. Later, with a slight change in conditions, 
calcite is precipitated, while later still a deposit of the 
iron ore is made; so that on each wall of the vein 
there are three bands of minerals. This banded struct- 
ure is common in true veins, and oftentimes there 
are many bands of different minerals in a single vein 
ac cadva (Hig. 230). The valueless minerals 
=| (usually the non-metallic, such as quartz 
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and calcite) are called gangue or vein- 
stone. The metallic mineral (in this 
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case iron) is known as the ore. 
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} Po Not only may decrease of temperature cause 
Wii]  Geposits, but the water of the vein may be sub- 
jected to loss of some of its materials. Or it 
may take up other substances, so that, as a result 
of the change, something is necessarily deposited, 
— just as we may precipitate certain substances 
from solution by the addition of others, as one 
= sees commonly illustrated in the chemical labo- 
ratory. The mineral vein is the site of great 
Fra. 230. chemical change, one of whose results is the 
Cross-section of filling of the cavity with gangue and ore. [ven 
vein, showing ores of gold and silver may be thus gathered 
ee struct into veins. 
REPLACEMENT Deposits. When a buried tree 
is enclosed in rock, the wood fibre, by the action of percolating 
water, may be replaced bit by bit, with particles of silica 
(Fig. 231). We then have a silicified or petrified tree, in which 
the woody structure is perfectly preserved, though the tree is 
changed to stone. Or the shell of an animal, composed of car- 
bonate of lime, like the shell of the oyster or clam, may be re- 
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