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the great majority of crystalline rocks. Iron is removed in solution 
in the water of springs and precipitated as a hydrous peroxide. 
Manganese is commonly associated with iron in minute proportions in — 
igneous rocks, and being similarly removed in solution in water, is 
thrown down as bog manganese or wad. ae 
Silicie Acid, Carbonic Acid, and Sulphuric Acid are the three 
acids with which most of the bases that compose the earth’s crust 
have been combined. With these we may connect the water 
which, besides merely percolating through rocks, or existing as water 
of crystallization in minerals, has been chemically absorbed in the 
process of hydration, and which thus constitutes more than 10 or 
even 20 per cent. of some rocks (gypsum). 
Although every mineral may be made to yield data of more or 
less geological significance, it will be needful to bring under the 
notice of the student here only those minerals which enter as chief 
ingredients into the composition of rock-masses, or which are of . 
frequent occurrence as accessories. Of the species thus introduced, it 
will be proper to dwell more particularly on those of their characters — 
which are of chief interest from a geological point of view, suchas 
their modes of occurrence in relation to the genesis of rocks, and their | 
weathering as indicative of the nature of rock-decomposition. It 
will thus be unavoidable that subjects must be referred to by antici- 
pation which will find fuller treatment in the sequel. But the cross 
references will, it is hoped, enable the reader to pass with ease from 
the enumeration of the facts, which is what is chiefly intended in the 
present section, to the discussion of the meaning of these facts as given 
in subsequent pages. | 
$i. Rock-forming Minerals. 
Minerals as constituents of rocks occur in four conditions, accord- 
ing to the circumstances under which they have been produced. : 
1, Crystalline, as (a) more or less regularly defined crystals; (0) 
amorphous granules or aggregations having an internal crystalline 
structure in most cases easily recognizable with polarized light; (c) 
“crystallites” or “microliths,’ incipient forms of crystallization, 
which are described on p. 100. The crystalline condition may arise 
either from igneous fusion or from aqueous solution. 
2. Glassy or vitreous, as a natural glass usually including either 
crystals or crystallites, or both. Minerals have assumed this condition 
from a state of fusion. ‘The glass may consist of several minerals 
fused into one homogeneous substance. Where it has been 
*‘devitrified,’ that is, has assumed a lithoid or stony structure, these 
component minerals crystallize out of the glassy magma, and may be 
recognised in various stages of growth. 
3. Colloid, as a jelly-like though stony substance, of which | 
' For the microscopic characters of minerals and rocks, see p. 94. 




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