111? Rev. W.D.Conybeare on the Phenomena of Geology 



pear generally to characterize the youngest sera of volcanic 

 action. 



Brongniart is also of opinion that the period of activity of 

 the extinct volcanos of Auvergne belonged to the close of the 

 tertiary rather than the commencement of the actual epoch ; 

 and believes it to have preceded the action of the causes, 

 whatever they may have been, which produced the vast accu- 

 mulations of gravel, which he designates " Clysmien," a term 

 which, having the fear of Mr. Lyell before my eyes, I dare 

 not translate 'diluvial' (Tall. p. 364). He admits, however, 

 that any well authenticated example of the lava of those vol- 

 canos actually overlying the said gravel, would negative this 

 presumption ; but contends that the instances hitherto cited 

 have been only beds of volcanic fragments, which might readily 

 originate in the same causes which produced gravel of the 

 other rocks. 



Proceed we now to the volcanic vents which still continue 

 in a state of activity. If we compare the proportion of the 

 surface actually thus occupied with that of one-fifth, which 

 we have seen must have prevailed in the primitive and tran- 

 sition periods, we shall again have a ratio which I can only 

 express as tangent : cotangent of the infinitesimal part of a 

 second, which (as far as I am acquainted) the advocates of 

 existing causes alone consider as that of equality. It might 

 however be objected that this is an unfair view of the case ; 

 that in all the earlier formations we have the accumulated 

 products of the volcanic action of many remote successive 

 ages of immense duration; that perhaps but a very small 

 proportion of this total resulted from the volcanos of any 

 one single age ; and that the new vents which have in the 

 actual sera succeeded to those now extinct may, therefore, 

 be as numerous as those which previously existed at any 

 single date. But to this we answer, that if the actual sites be 

 examined, far from appearing to be new vents which have re- 

 placed extinct old ones, they will be found to be only small 

 residual portions of much more extensive volcanic districts, 

 which appear from the rocks characterizing them, trachyte, 

 basalt, &c., to have been contemporaneous with those of Au- 

 vergne, the Rhine, &c. Thus in Iceland, Hecla only is active ; 

 but the whole island is obviously the product of ancient vol- 

 canos. In Italy, Vesuvius occupies but a small part of the 

 Campi Phlegrsei; and there are many other like districts near 

 Rome, about the Alban lake, &c. &c. On this subject I can- 

 not do better than quote the very words of Brongniart, whose 

 authority deserves the more attention, as he has most care- 

 fully studied the whole subject, and drawn up one of the most 



complete 



