IMPORTANT ELEMENTS AND MINERALS 31 
This gas combines with other substances to form the 
group of carbonates, of which calcite (pp. 29 and 42) is 
a typical example. While present in many rocks, the 
greater part of the carbon of the crust appears to have 
been placed there by the life and death of animals 
or plants, which have taken the carbonic acid gas 
from the air, or the carbonate of lime from the water. 
It is estimated that the percentage of this element in 
the crust is only .22 of the whole.' 
Hydrogen (H).— Hydrogen is one of the two ele- 
ments which constitute water, and in this form it 
is everywhere present in the crust. Many minerals 
enter into combinations with water, and as a result 
of this, the group of hydrous minerals is formed. The 
quantity of water present in the rocks in chemical com- 
bination, or held mechanically in the crevices, is equal 
to many times the bulk of all the water in the oceans. 
Hydrogen, combined with carbon, produces the great 
group called the hydrocarbons. These are present in 
many of the rocks which contain animal or plant 
remains, but they are particularly noticeable when 
they form extensive accumulations of natural gas or 
petroleum (p. 418). Combined with the minerals, hydro- 
gen forms only .21% of the crust; but, including that 
1 Titanium, always supposed to be a rare element, is found by analysis to 
be even more abundant than carbon. It is not conspicuous, and hence its 
importance has long been underestimated, 
