ROCKS 37 
mutual interference, have not assumed the geometrical forms which 
would have distinguished them had they crystallised under more 
favourable conditions. Minerals thus devoid of their proper crystalline 
form are described as anhedral or allotriomorphic—their shape has been 
determined by their surroundings. 
Inclusions in Minerals.—\When examined in thin slices under the 
microscope, the minerals of igneous rocks are often seen to include 
minute crystals or crystalline granules of other minerals. Not infre- 
quently, also, cavities, containing gas or liquid, or, it may be, glass or 
stony matter, appear in less or greater abundance (see Plates 1.2; IV. 4; 
VIII. 4). These inclusions are termed exdomorphs—the minerals which 
contain them being termed ~erimorphs. Obviously, all these foreign 
bodies must have been caught up and enclosed while the perimorphs 
were separating out from the original molten magma. 
Primary or Original Minerals.—Those rock-constituents which crystal- 
lised out from the magma are termed primary or original, to distinguish 
them from another group of minerals which are of later origin than the 
rocks in which they occur. Two kinds of primary minerals are recog- 
nised—namely, (a) Essential and (4) Accessory minerals. Essential 
minerals are those which determine the species of a rock, while accessory 
minerals are, as it were, mere accidental ingredients, the presence or 
absence of which does not affect the general character of a rock. 
Granite, for example, is composed of three essential minerals—felspar, 
quartz, and mica. Take away any one of those, and the rock ceases to be 
a granite. One or more non-essential ingredients, however, may be 
present, and yet the rock remains a granite. Should one of these 
accessory minerals be very abundant or conspicuous, it may give rise toa 
variety. If a granite, for example, contains conspicuous crystals of 
hornblende or of tourmaline, it is termed a hornblendic or a tourmaline 
granite, as the case may be. 
Secondary Minerals.—All rocks are subject to alteration, due especi- 
ally to the action of water percolating through them. This water finds 
its way along fissures and other planes of division, and soaks into 
the rock itself through the minute cracks, capillaries, and interstitial 
pores, which are never wanting in even the most compact and homo- 
geneous kinds. The percolating water contains carbon-dioxide or other 
acid in solution, which has been taken up from the atmosphere by rain, 
or absorbed from the soil. Thus armed, the water attacks the various 
mineral constituents of rocks, which in this way may be more or less pro- 
foundly altered. Some yield much more readily than others, but sooner 
or later the several silicate minerals, of which igneous rocks are so largely 
composed, tend to be chemically broken up—such bases as the alkalies 
and alkaline earths being removed in solution as bicarbonates. Some 
crystalline igneous rocks have been so much affected by the chemical 
action of water, that they have been changed from hard, resisting masses, 
showing a sparkling lustre on freshly fractured surfaces, to dull, soft, 
earthy, or clay-like substances, which may be dug with a spade. Few 
igneous rocks, indeed, which have been long exposed to the insidious 


