38 STRUCTURAL AND FIELD GEOLOGY 
action of percolating water, fail to show some trace of alteration. They 
may appear to be fresh to the unassisted eye, but thin slices viewed under 
the microscope will almost invariably show that one or other of their 
mineral constituents has undergone change. The felspar of a granite, for 
example, is a mineral which, when unaltered, appears quite clear and 
transparent in thin slices. When alteration has commenced, this is 
shown by a clouded or turbid aspect, affecting the whole or a portion 
only of the mineral. Increasing turbidness marks increasing chemical 
alteration; until eventually all trace of the original felspar disappears, 
and its place is occupied by a white or greyish homogeneous substance, 
This substance is the hydrous silicate of alumina, known as faolinite, and 
is obviously the result of the complete decomposition of the felspar which 
it replaces. As felspar is an anhydrous silicate of alumina and alkali or 
alkaline earth, the chief change brought about has been the removal of 
the soluble bases—the more resisting silicate of alumina being left behind 
as a hydrate. It is quite common, in this way, for certain minerals of 
igneous rocks to become changed into other mineral species, either by 
the gain or loss of some ingredient, or by the gain of one ingredient and 
the loss of another. The new mineral thus formed is known as an 
alteration-pseudomorph;* and all such products of alteration are termed 
secondary minerals—they are thus of later origin than the rock of which 
they form a portion. 
It will be understood now that secondary minerals are simply 
the products of the chemical alteration of essential and accessory 
minerals, They not only replace in whole or in part these primary 
or original constituents, but are frequently met with lining or filling 
cracks and fissures, or occupying the vesicular cavities of igneous 
rocks. 
Chief Minerals of Igneous Rocks.—A large number of minerals enter 
into the composition of igneous rocks—the more important of which 
have been described in preceding chapters. These, as we have seen, 
naturally fall into two groups: (1) PRIMARY or ORIGINAL, and (2) 
SECONDARY minerals. The former group includes two kinds, namely, 
Essential and Accessory, and may be tabulated as follows :— 

PRIMARY OR ORIGINAL MINERALS 
In list I. we include the most important, namely, those which have 
the widest distribution and occur most abundantly—those, in short, 
which are the chief ingredients of the commonest igneous rocks. The 
minerals given in italics are of less importance than the others. All the 
* A pseudomorph is simply a crystalline or amorphous body which 
has assumed the crystalline form of another mineral. There are several 
kinds of pseudomorphs. In certain cases a mineral may be dissolved out 
of a rock and a cavity or mould left ; subsequently mineral matter of a 
different kind may be introduced by infiltration into the cavity, in which 
case we have a substitution-pseudomorph, 
