166 Volcanoes. 



rica ; while oy other writers it is applied in the same page to 

 a fissure through which a cohimn of smoke is projected, and 

 to immense mountains that eject liquid rocks. Now, how 

 pertain soever it may be, in the theories of some speculatists, 

 that they all derive their origin from the same cause, some 

 difference should be made in language : and therefore, in- 

 stead of applying the term to every situation in which heated 

 substances are ejected, we shall confine it to those elevations 

 which are terminated by an interior cavity, and appear to have 

 gected liquid earths. 



There has been much doubt among geologists whether we 

 are to attribute hot springs to the same agent as volcanoes. 

 In some instances there can be no doubt that they derive 

 their caloric from the same source, but in others there seems 

 every probability that it arises from other causes. Few per- 

 sons, for instance, would doubt that the geysers of Iceland 

 have an intimate connection with the numerous volcanoes 

 with which that singular island abounds : but there may be a 

 reasonable scepticism in attributing the hot springs of our own 

 country, most of which arise in the lias beds, so remarkable 

 for sulphuret of iron, to the same cause. 



Volcanoes, according to our definition of them, may be 

 divided into two classes, extinct and active. There is some 

 difficulty in determining when a volcano may be considered 

 extinct; for, in our own times, craters which have not suf- 

 fered eruption in the age of history, have suddenly assumed 

 all their activity, and, with renewed energy, spread their de- 

 structive effects over the adjacent districts. We must, then, 

 regard all volcanoes extinct which have not been active in the 

 memory of history, till some proof of present activity be given. 



One of the most interesting series of extinct volcanoes with 

 which we are acquainted is that of Auvergne in France. The 

 most recent of these had certainly no relic of activity at the 

 time Julius Caesar invaded Gaul ; for although he encamped 

 upon them, he has not referred to their volcanic origin. 

 How long before this they became extinct we cannot say, as 

 no historical records refer to them. 



The most recent part of this district is that to the west of 

 Claremont (Scrope's Central France). This fact is not deter- 

 mined by any distinctive difference in the characters of the 

 lava, for the deductions formed on these have ever been found 

 erroneous, but from its position. For in some instances it has 

 flowed down the contiguous valleys, and hence was ejected 

 after the valleys were excavated ; while in other instances it 

 only occurs as an outlier, or, in other words, caps the hills 

 which surround these valleys ; and hence was formed before 



