Rocks of the Florida Beefs. 57 



1. We are familiar with the fact that a mixture of quick- 

 lime, water and sand, spread out upon walls and ceilings ex- 

 posed to an atmosphere containing more or less of carbonic 

 acid, in a few days becomes hard. Analyses have shewn that 

 two chemical phenomena are concerned in the solidification, 

 to wit — the absorption of carbonic acid from the air, forming 

 carbonate of lime (which salt, uniting in equivalent propor- 

 tions with the hydrate, forms, according to Fuchs, a com- 

 pound of great stability) ; and the union of the outer portions 

 of the sand-grains with the lime, forming a silicate. Investi- 

 gation has shewn that sand fulfils mechanically a more im- 

 portant office, by increasing the extent of surface to which 

 the compound of the hydrate and carbonate may attach itself. 

 The latter office may also be performed, and equally well, by 

 pulverized limestone. 



2. It is well known that calcareous springs deposit carbo- 

 nate of lime in crystalline forms. The salt had been held in 

 solution by carbonic acid contained in the water. Upon reach- 

 ing the surface, under less pressure and the influence of a 

 high temperature, its carbonic acid is given up, and with it 

 a precipitate of carbonate of lime takes place. The process 

 is exclusively chemical. 



3. The value of hydraulic cements is now conceived to de- 

 pend chiefly upon the presence of silica and lime, the oxide of 

 iron having little or nothing to do with the process of solidi- 

 fication. The alumina, in the form of a silicate, yields its 

 silica to the lime, which, for its transportation, requires 

 water. This explains the necessity of its being retained 

 under water periods of variable length, according to the pro- 

 portions of the ingredients. The processes are both chemical 

 and mechanical. 



4. Gypsum, from which the two atoms of water of crystal- 

 lisation have been expelled by heat, rapidly hardens upon 

 being mixed with water. This is ascribed to the reunion of 

 the sulphate of lime with the water. 



Do either of the above processes suggest the method by 

 which the rocks of the Florida reefs have been hardened ? 



The facts presented in the furnished specimens are as 

 follow : — 



