IMPORTANT ELEMENTS AND MINERALS 43 
One may easily tell calcite from any other common 
mineral, by placing upon it a drop of hydrochloric acid, 
when there is a boiling of effervescent gas, produced 
by the reaction between the calcite and the acid, which 
liberates carbonic acid gas. 
Calcite, which is the main constituent of limestone, 
is one of the most abundant of the common minerals. 
It is present in nearly all waters on or in the earth, 
for it is constantly being formed by the destruction 
of minerals which contain calcium. Its abundance in 
the water makes it possible for many animals and some 
plants (corals, shell-fishes, some seaweeds, etc.) to take 
it from solution and build it into their skeletons; and 
they do this so commonly that great beds of limestone 
are deposited in the sea by the accumulation of the 
remains of these organisms. While abundant in the 
beds of limestone and marble, and while scattered 
through nearly all the rocks, calcite is not a common 
mineral. Being soft, capable of ready decomposition 
in contact with weak acids, and soluble in the water 
of the crust, this mineral is both mechanically and 
chemically weak, and hence the rocks made of it 
do not resist the weather well. 
In connection with calcite, we may consider one of 
the least common minerals, dolomite ((CaMg)Co,), which 
is chemically like calcite, excepting that it also contains 
some magnesium. In most respects it resembles cal- 
