THE SOILS OF FLORIDA. 
49 
SOIL FORMATION. 
ROCKS OF THE EARTH’S CRUST. 
The rocks of the earth’s crust from which soils are formed may 
be grouped under two main divisions, igneous (or primary) and 
sedimentary (or secondary.) The igneous rocks are those which 
appear to have cooled from a molten condition. The earliest rocks 
of the earth’s crust are of this type, as well as the more recent ma¬ 
terials brought up from deep within the earth by volcanic action. 
Secondary, or sedimentary rocks on the contrary, are those which 
have been derived either directly or indirectly from igneous rocks 
Chemical changes, however, are going on incessantly within the 
earth and affect all rock formations. Chemical and physical forces 
have in many instances so profoundly altered formations that it is 
no longer possible to determine whether they were originally igneous 
or sedimentary. For these the term metamorphic rocks has often 
been used. 
IGNEOUS ROCKS. 
The igneous rocks are very complex chemically, and include most 
of the chemical elements. In structure and mineralogical composi¬ 
tion they are likewise variable. The structure is determined largely 
by the rate of cooling and other conditions under which the rocks 
were formed. When cooled quickly the time necessary for crystal 
lization is not available and the rocks are of a glassy texture. When 
cooled more slowly various minerals are formed and the rock assumes 
a more or less distinctly crystallized structure. Volcanic ash and 
obsidian are examples of rapidly cooled rock ; while the granites and 
similar coarsely crystallized rocks may result from a molten mass 
cooling slowly deep within the earth and under great pressure, 
which latter conditions favor a more perfect crystallization. 
Mineralogically the igneous rocks are likewise complex. The 
presence of the chemical elements under varying conditions of cool¬ 
ing give conditions favorable for the formation of many minerals. 
The leading minerals formed under these conditions are the sili¬ 
cates, of which there are a great number. In addition to the sili 
cates, sulphates, sulphides, phosphates, phosphides, chlorides, oxides 
and other minerals abound. Among the most abundant minerals 
in these rocks may be mentioned quartz, feldspars and the ferro- 
magnesian silicates. The first of these, quartz, is to be noted in 
this connection as the mineral which, owing to its abundance and 
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