50 FLORIDA GEOLOGICAL SURVEY—FOURTH ANNUAL REPORT. 
great resistance, remains as sand even after the disintegration and 
disappearance of most of the associated minerals. The feldspars 
break up through decay, furnishing the clayey element in soils, while 
from certain of the feldspars are derived also the potash so neces 
sary to soil fertility. The ferromagnesian silicates include pyrox 
enes, amphiboles and mica. From these minerals soils derive the 
iron which is so large an element in the coloring of soils, and 
various other bases, as calcium and magnesium. Phosphorus* is 
present in igneous rocks, although usually in small quantities in the 
form of apatite. 
SECONDARY OR SEDIMENTARY ROCKS. 
The secondary, derived, or sedimentary rocks are grouped into 
several classes determined by their manner of formation and chemi 
cal composition. These rocks are much less complex both in struc¬ 
ture and in chemical and mineralological composition than the igne 
ous rocks. They are derived from other rocks and in the process of 
reworking there is necessarily a selective separation of materials. 
The more soluble constituents of the original rocks are carried to 
the rivers, lakes and the ocean in solution, while the less soluble 
are mechanically transported and are separated according to specific- 
gravity and size of particles. 
Such common rocks as shales, sandstones, and conglomerates 
represent material mechanically transported and reaccumulated. 
The transporting agents are chiefly running water and the wind. 
The shales represent the finer sediments carried as mud, accumu 
lated in quiet water and subsequently consolidated. The sand 
stones are the result of the accumulation of sands either by water 
or by the wind. After being accumulated these sands may become 
cemented and thus form sandstone. Sands accumulated on land 
by the wind form sand dunes in which the sand may remain in a 
loose uncemented condition or under certain conditions may be 
more or less perfectly cemented. The conglomerates are made of 
the heavier materials such as pebble and small rock, which is the 
first to drop out of suspension in running water. The clays, likewise, 
are in some instances mechanically accumulated, although many of 
the clays are residual, having formed in place from the decay of such 
clay-bearing minerals as the feldspars. 
The clays and shales consist of a mixture of several minerals, 
among which hydrated aluminum silicates predominate. With 
