IMPORTANT ELEMENTS AND MINERALS 29 
although widely disseminated throughout the rocks, 
it is not so truly common as are oxygen and silicon. 
Still it is almost constantly present, as is shown by 
the yellow and red colors of the soils and rocks, which 
are tinted by some one of the oxides of iron. Owing 
to its distinguishing colors, it seems more abundant 
than it really is. It forms about 5% of the rocks of 
the surface. 
Manganese resembles iron in many respects, and 
occurs with it in the same combination of elements. 
It is much less common than iron but is still widely 
disseminated, and its presence is often shown by the 
black or purple stains on the surface of rocks. It forms 
only .08 % of the rocks. 
Calcium (Ca). — Calcium, a light yellowish metal, is 
another element that is never found uncombined in 
the earth. United with carbonic acid gas (CO,) (and 
another molecule of oxygen), calcium forms a very 
common mineral, known as calcite (CaCO;) (p. 42), 
which is present in the marbles used for decoration 
and monuments. Limestone also is mostly made of 
this carbonate of lime. 
Being slightly soluble, this mineral is almost every- 
where present in the water that flows over the land, 
as well as that which forms the sea. So animals are 
able to take it from the water and build it into their 
skeletons. All the shells of ocean mollusks, and the 
