
40 STRUCTURAL AND FIELD GEOLOGY 
4. Accessory Minerals; Magnetite, Ilmenite, Heematite, Apatite, 
Rutile, Zircon, Sphene, etc. 
The members of these several groups combine according to a some- 
what definite plan, which may be termed the law of mineral combination. 
Thus, in igneous rocks,* one or more members of the first group (Feds- 
pathic Silicates) are associated with one or more members of the second 
group (ferromagnesian Silicates). With these are more sparingly 
associated members of the fourth group (Accessory Minerals); while 
Free Silica may or may not be present. 
Classification of Crystalline Igneous Rocks 
No quite satisfactory classification of these rocks is at 
present possible. From the chemical point of view they 
have been grouped according to the percentage of silica they 
contain, as acid, neutral or intermediate, and basic, but there 
are so many gradations from the one type into the other 
that this arrangement breaks down when we come to apply 
it. We find, for example, that certain rocks of the same 
general character, and which obviously constitute a family, 
are under this chemical classification divided instead of being 
erouped together. Some andesites, for instance, would be 
termed intermediate, while others would be described as basic. 
Perhaps a nearer approach to a satisfactory classification is 
reached by taking into consideration the mineralogical con- 
stitution of the rocks, and arranging them according to the 
character of their dominant ingredients. This arrange- 
ment does not in effect differ much from that whichis 
based on. the silica percentage, but it has at leastagiae 
negative merit of not separating closely allied types of 
rock, 
The most important rock-forming minerals are un- 
doubtedly the felspars. In the great majority of eruptive 
rocks they play a prominent rdéle, since a large number 
have alkali felspar as their chief constituent, while another 
considerable division is characterised by the presence of soda- 
lime felspar as the dominant ingredient. In the remaining 
types of rock, felspar is either absent or plays the subordinate 
part of an occasional accessory mineral. In one division 
of those rocks, felspathoids (nepheline, leucite, etc.) take the 
* With the exception, of course, of the ultra-basic rocks, which contain 
neither felspars nor felspathoids, 
