

Ch. XVII.] 



CALCAEEOUS SPEINGS. 



399 



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the siliceous layers which, encrust the circular basin of an 

 Icelandic geyser, we should soon see a considerable cone 

 built up, with a crater in the middle ; and if the action of the 

 spring were intermittent, so that ten or twenty years should 

 elapse between the periods when solid matter was emitted, or 

 (say) an interval of three centuries, as in the case of Vesuvius 

 between 1306 and 1631, the discharge would be on so grand 

 a scale as to afford no mean object of comparison with the 

 intermittent outpourings of a volcano.* 



Calcareous springs. — Our first attention is naturally directed 

 to springs which are highly charged with calcareous matter, 

 for these produce a variety of phenomena of much interest 

 in geology. It is known that rain-water collecting carbonic 

 acid from the atmosphere has the property of dissolving the 

 calcareous rocks over which it flows, and thus, in the smallest 

 ponds and rivulets, matter is often supplied for the earthy 

 secretions of testacea, and for the growth, of certain plants 

 on which they feed. But many springs hold so much car- 

 bonic acid in solution, that they are enabled to dissolve a 

 much larger quantity of calcareous matter than rain-water : 

 and when the acid is dissipated in the atmosphere, the 

 mineral ( ingredients are thrown down, in the form of porous 

 tufa or of more compact travertin. 



Auvergne. — Calcareous springs, although most abundant in 

 limestone districts, are by no means confined to them, but 

 flow out indiscriminately from all rock formations. In 

 Central France, a district where the primary rocks are un- 

 usually destitute of limestone, springs copiously charged 

 with carbonate of lime rise up through the granite and 

 gneiss. Some of these are thermal, and probably derive 

 their origin from the deep source of volcanic heat, once so 

 active in that region. One of these springs, at the northern 

 base of the hill upon which Clermont is built, issues from 

 volcanic peperino, which rests on granite. It has formed, 

 by its incrustations, an elevated mound of travertin, or white 

 concretionary limestone, 240 feet in length, and, at its ter- 



mination, sixteen feet 



high and 



twelve wide. Another 



* 



Lyell. Anniversary Address, British Association, 1864. 



