50 ELEMENTARY GEOLOGY 
that of some micas; but the cleavage plates are not 
elastic like mica. It is usually white or transparent 
(Fig. 11). This mineral is present all through the 
crust, but is not abundant. Only rarely may it be 
found in large quantities, when it is sometimes mined 
for plaster of paris. 
Salt or Halite (NaCl).—Salt is one of the most 
soluble of minerals, and is constantly formed by the 
decay of compounds containing sodium. All streams 
carry it; and, as we know, some lakes, and all the 
oceans, contain a considerable percentage (the ocean 
about three and a half per cent). In the ocean there 
are many million tons of dissolved salt. Excepting 
when deeply buried, or mm arid regions, this very 
soluble mineral does not form an important part of 
the rocks, for ordinarily the percolating water removes 
it in solution. 
Salt crystallizes in cubes, and is commonly either 
white or colorless, though it 18 often stained red, brown, 
or some other shade, by iron or other impurities. 
Ice.'— A final mineral, well known and common, 
1 Jt was stated on p. 33 that minerals are solids, and the only recognized 
exception to this is the liquid mineral, mercury. Of course the distinction is 
purely artificial ; for if the temperature of the earth were lower, even the 
gas oxygen might become a mineral. Ice is truly a mineral under our defini- 
tion; but it is only in a few parts of the earth that water commonly exists 
in the solid form. Since the liquid condition is the most common, and since 
all the water would become solid with a slight decrease in temperature, we 
may include it under this section, 
