2 STRUCTURAL AND FIELD GEOLOGY 
acquire sufficient skill to diagnose microscopically all the 
more commonly-occurring rocks—those, namely, which are 
likely to come under the notice of architects, civil engineers, 
agriculturists, and others. 
The rock-forming minerals are not a numerous class, and 
only a few are of pre-eminent importance. For example, 
the essential mineral constituents of the most abundant and 
widely distributed igneous rocks may be counted on the 
fingers. The components of common schistose rocks, and of 
the great class of derivative rocks, are even fewer in number. 
When the student is able to determine some twenty minerals 
under the microscope, he should have little difficulty in 
diagnosing most of the fine-grained rocks that he is likely to 
meet with. Slight though this knowledge may be, it will yet 
enable him to appreciate what petrographers have to say as 
to the genesis of crystalline igneous and schistose rocks, and 
will undoubtedly aid him in his own field-observations. 
For convenience of description, the common rock-forming 
minerals have been grouped under the following heads: 
Oxides, Silicates, Haloids, Sulphides, Carbonates, Sulphates, 
Phosphates, and Elements. As the minerals included under 
these several heads are of very unequal importance, the 
descriptions of the less significant species are given in small 
type. 
Rock-Forming Minerals 
L OXIDES 
_ By far the most important rock-forming oxide is silica, 
next to which come various oxides of iron. The other oxides 
here described are of less frequent occurrence—some two or 
three being hardly entitled to rank as true rock-formers. 
Quartz is chemically pure silica (SiO,). It is harder than 
any other common rock-former, being 7 in the scale of hard- 
ness.* The minerals which are much harder than quartz 
_ play a very subordinate part in rocks, the only species that 
need be mentioned here being spinel and corundum. Quartz 
has a specific gravity of 2:65, and when it assumes a crystalline 
form, appears most frequently as hexagonal prisms terminated 
by corresponding pyramids (see Fig. 1). Most minerals can 
* See Appendix B. 

