50 
FLORIDA GEOLOGICAL SURVEY. 
era! by another is one of the important phases of chemical 
change. This process is again referred to in treating of 
the limestones. ^ 
Calcareous Material. 
Calcareous road materials occur in 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 crys¬ 
tallized the mineral cal cite (CaC0 3 ) has a hardness of 
only three in the scale in which quartz is seven and dia¬ 
mond is ten. It is thus much softer than the steel tires 
of wagons, and 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 itself. Calcium carbonate dissolves 
to an appreciable extent in water containing C0 2 gas or 
weak organic acids. Chemical readjustment is therefore 
rapid in a mass of ground up or broken calcareous rock, 
the dissolved calcium carbonate acting as a cementing 
material. 
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 condition. The shells of a shell lime¬ 
stone are usually more brittle and often crumble easily. 
The oolitic limestone is made up of innumerable minute 
round concretions barely large enough to be readily visi¬ 
ble to the eye. These are held together by a calcareous 
cement. After crushing the particles re-cement more or 
less perfectly. The marls are calcareous deposits con¬ 
taining more or less clayey impurities; they usually fall 
apart readily. More or less perfectly crystallized lime¬ 
stone occurs locally in the State. Its formation is prob- 
