32 FLORIDA GEOLOGICAL SURVEY-EIGHTH ANNUAL REPORT. 
CALCAREOUS MATERIAL. 
Calcareous road materials occur in the form of shells, shell and 
coral limestones, oolitic limestone, and marls. All of these rocks 
consist essentially of calcium carbonate or of the double carbonate 
of calcium and magnesium, and have certain features in common. 
They are much less resistant to wear than is quartz. When pure 
and crystallized the mineral calcite (CaCCT) has a hardness of only 
three in the scale in which quartz is seven. It is thus much softer 
than the steel tires of wagons. In fact a chief item in the repair 
of calcareous roads arises from the fact that steel tires cut holes 
in the soft material. On the other hand, an advantageous property 
of calcareous material is the readiness with which it re-cements it¬ 
self. Calcium carbonate dissolves to an appreciable extent in water 
containing C 0 2 gas or weak organic acids. Chemical re-adjust¬ 
ment is therefore rapid in a mass of crushed or broken calcareous 
rock, the dissolved calcium carbonate acting as a cementing mate¬ 
rial. 
In practical application, the physical condition in which these 
materials occur must be taken into consideration. In the case of 
recent shells, the calcium carbonate is in a compact amorphous con¬ 
dition. The shells of a shell limestone are usually brittle and often 
crumble easily. The oolitic limestone is made up of innumerable 
round concretions barely large enough to be readily visible to the 
eye. These are held together by a calcareous cement. After crush¬ 
ing, the particles re-cement more or less perfectly. The marls are 
calcareous deposits which are sufficiently soft to be applied to roads 
without previous crushing. More or less perfectly crystallized 
limestone occurs locally in the State. Its formation is probably due 
to a replacement process similar to that described for flint and 
chert. The chemical changes in this case involve a rearrangement 
of the constituent molecules; as a result of which the non-crystal- 
lizecl material of the rock assumes a definite form. When partly 
crystallized the limestone becomes compact and close grained. 
Distribution and Amount of Calcaredus Rocks .—The calcareous 
rocks are widely distributed in the State. The Ocala limestone, as 
alreadv stated, is found near the surface over much of central Flor¬ 
ida. Oolitic limestones make up an extensive formation running 
north and south from Miami and forming the east border of the 
Everglades. Coral and oolitic limestones form the foundation of 
the keys from Miami to Key West. Shell limestone occurs exten- 
