30 ELEMENTARY GEOLOGY 
reef-building corals, are chiefly made of this sub- ~ 
stance. | 
Combined with sulphur and oxygen, calcium forms. 
another common mineral, gypsum (CaSO,+ 2 H,0) | 
(p. 49). Besides this, the element enters into many 
of the complex silicates, so that we find it as a com- 
mon constituent of many rocks. Nearly 4% of the 
crust is calclum. | 
Magnesium (Mg), Potassium (K), and Sodium (Na). — 
Magnesium, potassium, and sodium never exist in a 
free state, but when obtained by artificial means they 
are found to be metals. They enter into the compo- 
sition of many silicates, but are less abundant than the 
preceding elements, each one forming about 2.5% of the 
crust. Combined with chlorine, thus forming chlorides, 
each of these elements produces a soluble salt, which 
is present in nearly all waters, but is particularly 
noticeable in sea-water. By far the most abundant of 
these is the common salt (pp. 32 and 50) of the ocean, 
sodium chloride (NaCl). 
Carbon (C).—Carbon exists in the pure state in the 
form of diamond, and graphite or the black lead of 
pencils; but its most common condition is in com- 
bination with oxygen. It is present in the tissue of 
every animal and plant, and when they decay or burn, 
the oxygen of the air combines with the carbon to 
form the important carbonic acid gas (CO,), (p. 118). 
