24 ELEMENTARY GEOLOGY 
matter. Gold is an element, and no skill has been 
able to simplify it further; nor has any one succeeded 
in making this substance by any combination. On the 
other hand, water can be separated into two parts, 
oxygen and hydrogen, each of which is a gas and an 
element incapable of further reduction; and not only 
may these two gases be produced from water, but they 
can be made to combine and form water. 
So far, chemists have detected about seventy ele- _ 
ments in the earth’s crust, and there are probably 
others not yet found. While there are about seventy 
elements now known, the great majority of them are 
so rare that they may be neglected in an elementary 
study of geology. Fourteen are of decided importance, 
and nearly the entire earth’s crust is composed of com- 
binations of these. Named approximately in the order 
of their consequence, these are: oxygen, silicon, alumi- 
num, iron, calcium, magnesium, potassium, sodium, 
carbon, hydrogen, phosphorus, sulphur, chlorine, and 
manganese. To these might be added nitrogen, which, 
though not conspicuous in the rocks, is an element of 
prime importance in the air, of which it forms nearly 
79 %, as an inert, tasteless, and invisible gas. 
The elements are variable in habit. Many are metals, 
like iron and gold; some are non-metallic, like sulphur ; 
others are solids, like iron, gold, and sulphur; and a 
few are liquid or gaseous, like mercury and oxygen. 
