Prof. Horsford on the Rocks of the Florida Reefs. 245 
Arr. XX1X.— Solidification of the Rocks of the Florida Reefs, 
and the Sources of Lime in the Growth of Corals ; by Profes- 
sor Horsrorp, of Harvard. | 
I. It is required to ascertain by what processes, chemical or 
mechanical, or both chemical and mechanical, the surface and the 
submerged coral rocks have become hardened. 
By the surface rock is intended that Ahin brown crust, composed 
of numerous layers, which is distinguished by great compactness, 
and a peculiar ring, when, in detached condition, it is struck by a 
hammer, 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 appear- 
ance which has solidified under water, and which is of inferior 
hardness to the surface rock. 
The surface rock, so called, has in many places no longer the 
outermost position, though it had at the time of its formation. It 
is indeed interstratified with friable light-colored limestone. The 
epithet indicates the circumstances of its formation, not its pre- 
Sent position. rer 
1. We are familiar with the fact that a mixture of quicklime, 
water and sand, spread out upon walls and ceilings exposed to an 
atmosphere containing more or less of carbonic acid, ina few days 
becomes hard. Analyses have shown that two chemical phenom- 
ena are concerned in the solidification, to wit. : the absorption of 
carbonic acid from the air, forming carbonate of lime (which salt, 
uniting in equivalent proportions with the hydrate, forms, accord- 
ing to Fuchs, a compound of great stability); and the union of 
the outer portions of the sand-grains with the lime, forming a sili- 
Cate. Investigation has shown that sand fulfills mechanically a 
more important 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 
1 innte At 
: that c J i > J P ut , ‘eal 
lime in crystalline forms. The salt had been held in solution by 
of lime takes place. The process is exclusively chemical. 
3. The value of hydraulic cements is now conceived to depend 
' chiefly upon the presence of silica and lime, the oxyd of iron hav- 
Ang little or nothing to do with the process of solidification. The 
alumina, in the form of a silicate, yields its silica to the lime, 
which, for its transportation, requires water. This explains the 
hecessity of its being retained under water periods of variable 
