Next to solid silicic acid we must reckon carbonate of lime, and then
the combinations of silicic acid with alumina, potash, and soda, with lime,
magnesia, and oxyd of iron.
The substances which we designate as 'rocks' are determinate associations of
a small number of minerals, in which some combine parasitically, as it were,
with others, but only under definite relations; thus, for instance, although
quartz (silica), feldspar, and mica are the principal constituents of
granite, these minerals also occur, either individually or collectively, in
many other formations. By way of illustrating how the quantitative
relations of one feldspathic rock differ from another, richer in mica than
the former, I would mention that, according to Mitscherlich, three times
more alumina and one third more silica than that ossessed by feldspar, give
the constituents that enter into the composition of mica. Potash is
contained in both -- a substance whose existence in many kinds of rocks is
probably antecedent to the dawn of vegetation on the earth's surface.
The order of succession, and the relative age of the different formations,
may be recognized by the superposition of the sedimentary, metamorphic, and
conglomerate strata; by the nature of the formations traversed by the
erupted masses, and -- with the greatest certainty -- by the presence of
organic remains and the differences of their structure.
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