Natural History of Yolcanos and Earthquakes. 259 



latter districts, the number of thermal springs whose tempera- 

 ture exceeds that of the soil at the most by a few degrees, are 

 enormous, and considerable exhalations of carbonic gas give 

 evidence of former galvanic action. It may, however, be con- 

 jectured, with some probability, that in the vicinity of the 

 Laacher See, and in the Eifel, springs may have existed, whose 

 duration depended on the cooling of the masses of lava. Sim- 

 ilar circumstances seem to have occurred in Auvergne and Vi- 

 varais, although the hot springs, which are not uncommon in 

 those countries, show that many of the former volcanic channels 

 are still unobstructed. 



The examination of deposits obviously formed from springs 

 which existed at a former time, may often present an indication 

 of their temperature. Thus, on the volcanic tongue of land, 

 called the Sneefield-Syssel, in Iceland, we find none of the hot 

 mineral springs which are so numerous in other parts of the 

 island, and which are distinguished by their holding silica in so- 

 lution, and exhahng sulphureted hydrogen gas. But, in former 

 times they existed here, for in many places we find siliceous in- 

 crustations in the form of tuffas and sinters. One cold spring, 

 which is now flowing, has certainly taken the place of a hot 

 siliceous spring, for its present deposits are only calcareous, and 

 quite distinct from the older incrustations.* The circumstance 

 that arragonite is deposited from hot springs, calcareous spar, 

 on the other hand, from cold ones, gives us also an indication 

 of this kind. Since G, Rosef pointed out that the former is 

 only deposited from a hot solution of carbonate of lime, the 

 occurrence of arragonite in any deposit, leads us to infer with 

 certainty, that these deposits owe their origin to a hot spring. 

 If, on the contrary, we find calcareous spar in any deposit, we 

 may infer with equal certainty, that it was produced by a 

 cold spring.| 



If the melted nucleus of the earth be the common seat of the 

 volcanic activity of the whole earth, subterranean communica- 



* K. V. Nidda, loco cit. p. 282. t PoggendorfF's Annal. t. xl. p. 353. 



t The following remark may not be entirely superfluous, viz. according to G. 

 Rose, arragonite is formed in a higher temperature only in the moist way, but 

 calcareous spar is formed in the dry way. Thus carbonate of lime crystallizes 

 from a state of fusion under strong pressure only in the form of calcareous spar. 

 Arragonite, exposed to a slight red heat, is easily converted into calcareous spar. 



