34 ELEMENTARY GEOLOGY 
So we find some minerals enduring, and others un- 
stable. Therefore, while the chemical composition is 
definite, it is subject to change whenever the conditions 
are favorable. By these changes, rocks are often made 
to crumble and decay (see Chapter VI.). 
Another feature of importance among the minerals, 
is the difference in 
| hardness. Some, 
like graphite, or 
black lead, are so 
soft that they will 
mark paper, while 
others, like quartz 
and diamond, are 
harder than glass. 
Moreover, someare 
brittle, and others 
plastic. So there 
is great variety in 
minerals, and upon 
these differences 
Fig. 2. 
Semi-opal, an amorphous mineral. many geological 
changes depend. 
Although the great majority of minerals are erystal- 
line (Fig. 4), some are not built up in definite forms, 
but take irregular shapes. These are then said to be 
amorphous (Fig. 2). If we allow a solution of salt to 
