38 ELEMENTARY GEOLOGY 
it is amorphous, but usually crystalline, and very often 
is found in perfect crystals, — six-sided prisms, termi- 
nating in six-sided pyramids (Fig. 5). Among the 
varieties are agate, amethyst, Jasper, etc. 
Quartz is light in weight and very hard, so hard 
indeed that it cannot be scratched with a knife, but 
will cut glass. It is brittle, and when it breaks, has 
a rough fracture like that of broken glass, this being 
known as the conchoidal or shelly fracture. Being hard, 
it is, of common minerals, the most resistant to mechan- 
ical wear. It is also chemically strong or difficult 
to change; and silica once formed, remains as silica 
throughout all the changes to which a are ordi- 
narily subjected. 
Quartz is slightly soluble in percolating water which 
bears alkaline substances; and therefore many plants 
are able to build it into their skeletons by absorbing it 
from the soil-water, and carrying it upward through 
their roots. The saw-like edges of some of the grasses 
and sedges, which cut the hand when drawn over 
them, are studded with particles of silica thus derived. 
There are also water plants, the Diatoms, and water 
animals, particularly certain Infusoria and sponges, 
which build skeletons of silica taken from the water. 
Because of its hardness and chemical strength, quartz 
may give great durability to a rock. There is no com- 
mon mineral which resists destruction so well. This 
