ORIGIN OF LIMESTONE. 89 
the general history of the origin of limestone, 
from their affording strong evidence of the 
sources whence carbonate of lime has been 
derived.* 
* We see that thermal springs, in volcanic districts, issue 
from the earth, so highly charged with carbonate of lime, as to 
overspread large tracts of country with beds of calcareous tufa, 
or travertino. The waters that flow from the Lago di Tartaro, 
near Rome, and the hot springs of San Filippo, on the borders 
of Tuscany, are well known examples of this phenomenon. 
These existing operations afford a nearly certain explanation of 
the origin of extensive beds of limestone in fresh-water lakes of 
the tertiary period, where we know them to have been formed 
during seasons of intense volcanic activity. They seem also to 
indicate the probable agency of thermal waters in the formation 
of still larger calcareous deposits at the bottom of the sea, during 
preceding periods of the secondary and transition series. 
It is a difficult problem to account for the source of the 
enormous masses of carbonate of lime that compose nearly one- 
eighth part of the superficial crust of the globe. Some have 
referred it entirely to the secretions of marine animals ; an 
origin to which we must obviously assign those portions of 
calcareous strata which are composed of comminuted shells and 
corallines : but, until it can be shown that these animals have 
the power of forming lime from other elements, we must suppose 
that they derived it from the sea, either directly, or through the 
medium of its plants. In either case, it remains to find the 
source whence the sea obtained, not only these supplies of car- 
bonate of lime for its animal inhabitants, but also the still larger 
quantities of the same substance, that have been precipitated in 
the form of calcareous strata. 
We cannot suppose it to have resulted, like sands and clays, 
from the mechanical detritus of rocks of the granitic series, 
because the quantity of lime these rocks contain, bears no pro- 
portion to its large amount among the derivative rocks. The 
only remaining hypothesis seems to be, that lime was continually 
introduced to lakes and seas, by water that had percolated rocks 
through which calcareous earth was disseminated. 
