GEOLOGY. 275 



ON THE SOLIDIFICATION OF THE ROCKS OF THE FLORIDA REEFS. 



THE following is an abstract of a paper read at the American Asso- 

 ciation, Albany, by Prof. E. N. Horsford : 



It is required to ascertain by what processes, chemical or mechani- 

 cal, or both, the surface and the submerged rocks have become hard- 

 ened. By the surface rocks is intended that thin brown crust, 

 composed of numerous layers, which is distinguished by great compact- 

 ness, and which occurs on the abrupt ocean side, and more abundantly 

 on the long slopes on the land side of the Keys. By the submerged 

 rock is intended the rock of oolitic appearance, which has solidified 

 under water, and which is of superior hardness to the surface rock. 

 The surface rock, so called, has, in many places, no longer the outer- 

 most position, though it had at the time of its formation. It is indeed 

 inter-stratified with friable light-colored limestone. The epithet indi- 

 cates the circumstances of its formation, not its present position. WQ 

 are familiar with the fact that a mixture of quicklime, water and sand, 

 spread out upon walls and ceilings exposed to an atmosphere contain- 

 ing carbonic acid, in a few days becomes hard. Analyses have shown 

 that two chemical phenomena are concerned in the solidification, viz., 

 the absorption of carbonic acid from the air, forming carbonate of lime, 

 (which salt, uniting in equivalent proportions with the hydrate, forms 

 a compound of great stability,) and the union of the outer portions of 

 the sand-grains with the lime forming a silicate. Investigation has 

 shown that sand fulfils, mechanically, a more important office by in- 

 creasing the extent of surface to which the compound of the hydrate 

 and carlDonate may attach itself. The latter office may also be per- 

 formed, and equally well, by pulverized limestone. It is well known 

 that calcareous springs deposit carbonate of lime in crystalline forms. 

 The salt had been held in solution by carbonic acid contained in the 

 water. Upon reaching the surface, under less pressure and the influ- 

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

 precipitate of carbonate of lime takes place. 



The value of hydraulic cements is now conceived to depend chiefly 

 upon the presence of silica and lime, the oxide of iron having little or 

 nothing to do with solidification. The alumina, in the form of a sili- 

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

 water. This explains at once the necessity of its being retained under 

 water periods of variable length, according to the proportions of the 

 ingredients. Gypsum, from which water of crystallization has 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 furnish a suggestion as to the 

 methods by which the rocks of the Florida reefs have been hardened ? 



The facts presented in furnished, specimens are as follows : The 

 rock formed under water exclusively is composed of grains of size less 

 than that of a mustard seed, which, to the naked eye, appear quite 

 globular and of uniform diameter. More carefully examined with a 

 microscope, they are found to be far from regular in form or uniform in 

 size, but present numerous depressions and prominences. Distributed 



