178 
GALCAREOUS SPRINGS. 
with strata filled with cold water, they would give 
rise to thermal springs of every degree of tempera- 
ture. 
Calcareous Springs. — Our chief object in noticing, 
springs in this place is to point out how they are 
agents in the formation of rocks : and we first call 
the attention of the reader to those charged with 
lime or calcareous matter, as the phenomena con- 
nected with them are of high interest in geology. 
It is a fact generally known, that water has the 
property of dissolving the calcareous rocks over 
which it flows. Springs that contain carbonic acid 
in solution are capable of dissolving a much larger 
quantity of calcareous matter than rain water ; and, 
owing to the dissipation of the acid in the atmo- 
sphere, such springs throw down large quantities, 
of lime, in the form of a loose and porous rock, call- 
ed tufa, or in a more compact form, called travertin.^ 
Sucn springs are more commonly found in lime- 
stone districts, though not confined to them ; for iix 
Clermont, near Central France, one of these springs 
issues from a volcanic rock resting on granite, which 
has formed, by its deposites, a mound of white con- 
pretionary limestone 240 feet in length, 16 feet high, 
and 12 wide. 
Among the Apennine hills in Italy, and espe- 
cially in Tuscany, so much calcareous matter has 
been deposited by the innumerable springs scatter- 
ed over the country, that the whole ground is cov- 
ered with limestone, and sounds hollow beneath 
the foot. In other places in the same country, Mr. 
Lyell states that he saw compact rocks formed in 
this manner, descending the slanting sides of hills 
much in the manner of lava currents, except that 
they were of a white colour, and terminate abrupt- 
ly when they reach the course of a river. 
* This stone was called by the ancients lapis Tiburtinus, being 
formed in great quantity by the river Anio, at Tibur, near Romew 
Travertin is an abbreviation from Transiihurtinus. 
