composition, and AnaJlogies of Mocks, 35 



cnces in tlie chemical appearances of the rocks in these three 

 distinct productions. They have, however, been sufficiently 

 pointed out, pn otjher occasions j ^nd 1% haH been shewn, that they 

 probably consisted, in a great measure, in differences of the time 

 through which the fused materials had cooled, circumstances 

 confirmed by a great number of collateral appearances, although, 

 i|i many cases, there can be no doubt that great differences have 

 resulted from the different proportions of the several earths io 

 the fused compounds. ** 



Thus, from chemical analogies, there is assigned to all the un- 

 stratified rocks, that origin which was already deduced from 

 various other considerations ; and thus there is proved to exist 

 a division of rocks formed exclusively by the agency of heat. It 

 will now be convenient to begin the remainder of this exami- 

 nation at the other extreme. 



Where water holding carbonate of lime in solution is gradual- 

 ly evaporated, there are formed calcareous concretions, which 

 often attain a great size through age, and which, under peculiar 

 circumstances of crystallization, are sometimes not very diflferent 

 in aspect from certain limestone rocks. Under different circum- 

 stances, similar waters deposit their contents, so as to form rocks 

 of great depth and extent, producing real calcareous strata. 

 The Travertino of Italy appears to be one of the most perfect 

 examples of this nature. These simple and recent calcareous rocks 

 become compounds, in cases where the calcareous solution has 

 entangled fragments of shells, as it does in the West Indian 

 islands at this day, or where it has united fragments of discord* 

 ant natures, as it does on the shores of Messina, and on many 

 of our sea coasts. Thus calcareous rocks, both simple and com- 

 pound, are formed by water. Lastly, rocks of this nature are 

 now daily produced, in many parts of the great ocean, by the 

 efforts of marine animals ; the deserted coralline structure being 

 cemented partly by the actions of the animals themselves, and 

 partly by that of the sea on the calcareous earth. In the same 

 manner, ancient submarine piers, as at Carthage, become ce- 

 mented through lapse of time, by the intervention of shell-fish 



