14 THE ROCK-FORMING MINERALS 



tion or the removal of certain constituents, or some constituents 

 may be removed and others substituted for them. The entire 

 substance of a mineral may be removed and its place taken, 

 molecule by molecule, by another, retaining the form, sometimes 

 even the cleavage, of the first. The study of pseudomorphs is 

 often of the greatest service, as throwing light upon the history 

 of the rock in which they occur. 



Compound crystals are formed by the joining of simple crystals. 

 When two half- crystals are united along a plane in such a way 

 that their faces and axes do not correspond, they are said to be 

 twinned. When the twinning is repeated along numerous parallel 

 planes, the crystal is a polysyntheiic twin. Two crystals united at 

 the ends to form a right angle, are called geniculate, while two 

 geniculate crystals may be so combined as to form a cross, and 

 then are said to be cruciform. 



Crystals of the same form may vary in length and in the size of 

 their corresponding faces, which gives rise to numerous irregulari- 

 ties of shape. 



Rock-forming Minerals. — The number of known minerals is 

 exceedingly great, and is constantly increasing, but only a few 

 enter in any important way into the constitution of the earth's 

 crust. We now proceed to a consideration of these constituent 

 minerals, which are called rock-forming minerals, because the 

 rocks are aggregations of them. It must be emphasized that the 

 student can gain no real knowledge of minerals or rocks by merely 

 reading about them ; it is necessary that he should familiarize 

 himself with actual specimens. 



A. MINERALS COMPOSED OF SILICA 



Next to oxygen, silicon is by far the most abundant constituent 

 of the earth's crust, though never occurring alone. It is united 

 with oxygen to form silica (Si0 2 ) or enters into the formation of 

 more complex compounds. The oxide, silica, is the commonest 

 mode of occurrence and forms the most abundant of all the 

 minerals. 



