60 Professor Horsford on the 



and water is due to the more or less advanced stages of 

 change which the rock has undergone. In the ultimate form 

 of limestone all the water existing as hydrate in the earlier 

 stages will have become carbonate. 



These ingredients permit no action like that occurring in 

 hydraulic cements, in which silica plays an important part ; 

 or like that presented in the hardening of gypsum, in which 

 sulphuric acid is necessary. To one of the two remaining 

 processes, if to either, must it be ascribed ; and as hydrate 

 of lime is present, it cannot be exclusively assigned to a place 

 with calcareous spring deposits. Now, how could hydrate of 

 lime be provided from carbonate of lime % 



The completeness of the suite of collections provided for 

 me by Prof. Agassiz, has enabled me to answer this question 

 in such a manner as leaves, I think, little room for doubt. 

 On the main land against the Keys, there are depressions 

 which are filled with water only at long and irregular inter- 

 vals. This water, like that within and about the Keys, 

 abounds with animal life. As the water evaporates, these 

 animals die, and fall upon and mingle with the coral mud at 

 the bottonic As the beds become more and more completely 

 dry, the layer of mud and animal matter hardens till it forms 

 a mass resembling the surface or crust rock. 



Of this soft, growing rock, specimens were collected. 

 Agitated with water, it yielded a turbid, foetid solution. 

 Tested with acetate of lead, it betrayed the presence of hy~ 

 drosulphuric acid. After standing some hours, a delicate 

 white film was deposited upon the containing vessel, at the 

 surface of the water, which proved to be carbonate of lime. 

 Test-paper shewed the liquid to be alkaline. The addition 

 of soda solution set ammonia free, and the addition of chloride 

 of barium and hydrochloric acid shewed the presence of sul- 

 phuric acid. 



Conceiving this soft rock to be in the condition in which 

 the solidified crust was at first, the process of hardening 

 seemed of easy explanation. 



The animal matter mixed with the carbonate of lime, con- 

 taining sulphur and nitrogen, besides carbon, hydrogen, and 

 oxygen, in the progress of decay, which warmth and a small 



