34 
FLORIDA state: geological survey. 
large. With a conservative estimate of the average mineral content 
of the spring water (219 parts per million) and the assumption that 
about one-half 1 the rainfall of Florida entered the earth and removed 
this amount of material, Dr. Sellards reached the conclusion that the 
rate of solution was sufficient to remove, in the limestone section 
of Florida, about 400 tons per square mile annually. If evenly dis¬ 
tributed, this would lower the surface of the limestone about one foot 
in five or six thousand years. The concentration of this solution along 
certain beds or channels of active circulation would permit the forma¬ 
tion of large underground passages in a comparatively short period 
of geological time. These channels, known as caverns, are sometimes 
hundreds of feet in diameter and several miles in length. A level sur¬ 
face and a porous soil favor the development of caverns because most 
of the rainfall sinks into the earth instead of flowing off over the 
surface. In the past the region under discussion appears to have fur¬ 
nished an excellent opportunity for the formation of caverns because 
its surface was comparatively level and covered by a mantle of porous 
material. 
Sink Holes: — As solution progressed the cavern roofs became 
weakened at various points and collapsed, forming the depressions 
known as sink-holes. In some areas these depressions are so numer¬ 
ous that they occupy a large part of the surface and give the region 
its characteristic topography. Splendid examples of ancient sinks 
such as the Devil’s Mill Hopper are to be found in various parts of 
the State, and instances where sinks have been formed by the collapse 
of cavern roofs within the memory of persons now living are known 
in different parts of the Lake Region. A good example of a recently 
formed sink is to be seen on the road between High Springs and the 
“Sink” of the Santa Fe River. In the phosphate region a large 
quantity of water, which has been used in mining operations, is al¬ 
lowed to enter the ground. That this water frequently has a notice¬ 
able effect in weakening the roofs of the underground drainage chan¬ 
nels is shown by the following quotation from the unpublished notes 
of Geo. H. Eldridge: 
Sinks have frequently been formed since the mining of phosphate has been 
undertaken in the vicinities of the settling ponds, or in the line of drainage from 
the mine washers. The writer has passed over an apparently solid road in the 
morning to return at night to find in his way a chasm fully 40 feet across, earth, 
shrubs and trees engulfed, and with the water pouring down to an underground 
passage in the weirdest way. Again, at one of the Southampton mines, the 
floor of the old pit, together with an adjoining area of the sand overburden, has 
sunk several feet .with a rift in the earth 4 or 5 feet across in which is shown 
on the one side the smooth walls of lime spires and connecting ridge; on the 
1 Ibid. p. 16. 
