IMPORTANT ELEMENTS AND MINERALS 33 
best and most familiar example. Chlorine forms only 
01% of the crust, but is important in the ocean, of 
which it forms about 27%. 
ComMoNn MINERALS OF THE EARTH’S CRUST 
General Statement. — When they unite, the elements 
follow definite laws and produce definite results. They 
build themselves together in regular proportions, form- 
ing molecules, and if free to act under favorable condi- 
tions, these often construct a regular geometrical form, 
or a crystal. These combinations produce minerals, 
which may be defined as homogeneous solids’ of definite 
chemical composition, occurring in nature, but not of 
apparent organic origin. 
A mineral may be composed of a single element, 
such as gold, copper, or silver; or, as is very common, 
it may be made of two or more elements chemically 
combined. These may be only lightly bound together, 
as in the case of cinnabar (the sulphide of mercury), 
where a slight heat serves to drive the elements apart ; 
or they may be so firmly joined, and their chemical 
affinity so strong, that, like silica, they will resist 
almost any effort to separate them. What the cause 
of this affinity is we do not know, but it is the 
operation of a powerful law of nature. 
1 The single exception is mercury, which is liquid instead of solid. 
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