IMPORTANT ELEMENTS AND MINERALS 27 
The great majority of minerals contain this active 
element, and 47 % of the solid crust is oxygen; it is one 
of the two substances which compose water, forming 
nearly 86% of the ocean; it forms 21% of the atmos- 
phere; and animals and plants contain stores of it in 
their tissues. The fact that there is so much oxygen 
in the crust of the earth, points to the conclusion that 
in the early geological ages there was much more of 
this gas in the air than now; and although some of it 
is constantly being given back to the atmosphere, it 1s 
more than probable that each year witnesses a slight 
decrease in the percentage of oxygen in the air. Per- 
haps the time may come when there will not be enough 
of this precious gas to support the life of the globe. 
Silicon (Si).—In the earth’s crust the element 
silicon is never found uncombined; but it is known 
to chemists as a dull brown powder. In importance 
it ranks second to oxygen, which is its constant com- 
panion. ‘Twenty-seven per cent of the crust is made 
of this element, so that 74% of the substance of the 
known rocks is oxygen and silicon. Its simplest com- 
bmation is the oxide, silica (Si0,), which is so common 
as rock crystal or quartz (p. 37) and as grains of sand. 
Besides this simple combination, there are others which 
to the mineralogist are known as the silicates. In 
these the silicon and oxygen are combined with other 
elements (see Feldspar, p. 40). 
