6b 
FLORIDA STATF GEOLOGICAL, SURVEY. 
conditions have changed, be favorable to the disintegration of 
these deposits. Moreover, any change in levels, either elevation 
or depression, affects the water level and hence modifies condi¬ 
tions. Such changes in elevation have undoubtedly occurred. For 
instance a rise‘in elevation of 15 to 25 feet along the east side of 
Florida and a similar depression along the west coast as late as 
Pleistocene times is fairly well established. This, together with 
any further changes that occurred in the elevation of the peninsu¬ 
lar, must be taken into account in its bearing on the change of 
water level and the corresponding change in deposition, and dis¬ 
integration. It is not held that the accumulation of the rock in 
no case occurs above water level. In fact the secondary stalactitic 
deposits seen in many boulders evidently form as in caves above 
water level. The earth is a complex chemical laboratory in which 
chemical reactions take place in accordance with constantly 
changing conditions. 
THE FORMATION OF BOULDERS. 
The phosphate boulders have evidently been formed chemically 
through the agency of ground water. The boulders of silica are 
formed by a similar process by which silica taken into solution 
near the surface is redeposited at a greater depth. 
SILICA BOULDERS. 
Most of the flint or silica boulders were originally masses of 
limestone and still retain, in recognizable form, the shells and 
other fossils of which the limestone was originally composed. In 
these boulders the calcium carbonate has been replaced by silica. 
This process is common in nature. Petrification, another term 
for a similar process, is the slow removal in solution of the sub¬ 
stance of which an object is composed and its replacement by 
some other substance. In the case of petrified wood the wood has 
been removed and replaced by silica, calcium carbonate, iron car¬ 
bonate or whatever the petrifying agent may be. Silicified wood, 
silicified shells, silicified bone all refer to petrification in which 
silica was the petrifying agent. 
