J. Tf. Judd — On Volcanos. 343 



any part of the earth's surface, is by studying the whole history of 

 a volcanic outburst from its commencement to its close, by examin- 

 ing the record of the rocks with the aid of the palseontological 

 scheme of chronology. But here the investigator finds himself beset 

 with another difficulty. The violent and destructive effects of the 

 later and grander periods of eruption having sufficed to obliterate, in 

 too many instances, almost every trace of the products of the earlier 

 and feebler manifestations of volcanic action. 



One conclusion on the subject we may now, however, regard as 

 being sufficiently well established ; namely, that the first appearance 

 of volcanic action in a district is usually, if not always, of a gradual 

 character, and that it is preceded by a series of phenomena, similar in 

 character, but taking place in a reverse order to those which accom- 

 pany the decline of volcanic forces in any area. In the Auvergne, 

 and the Euganean Hills, in Hungary, and Bohemia, and indeed in all 

 the old volcanic districts surrounding the Alpine system, we find 

 sufficiently clear proofs that the close of the long period of the cessa- 

 tion of volcanic violence in the area was heralded by the apjDearance 

 of springs containing large quantities of carbonic acid, or holding 

 silica and other substances in solution ; then of similar springs with 

 an elevated temperature, and of sources of heated vapour and acid 

 gases; eventually actual fissures were formed, giving rise to the 

 formation of cinder-cones and lava-streams of insignificant dimen- 

 sions (like "the puys") ; till, gathering fresh force, the igneous 

 activity was in the end equal to the task of building up the largest 

 volcanic cones, and these became the seats of paroxysmal or long- 

 continued outbursts on the grandest scale. Such a history appears 

 to belong to all the centres of volcanic forces around the Alpine 

 system, from the first premonitory symptoms in the Oligocene to 

 their climax in the Miocene. 



In the present chapter we have had such frequent occasion to 

 speak of the tremendous efforts of elevatory or igneous forces, that 

 we may possibly be misunderstood as referring the actual forms 

 of mountain masses to such causes. Nothing could, however, bo 

 possibly further from our intention. The existing surfaces of the 

 Alps we regard as being entirely the result of the sculpturing action 

 of denuding forces. In three different ways, however, have the sub- 

 terranean forces contributed towards the formation of this and other 

 great mountain systems. First, by leading to the abnormal thickening 

 of the earth's crust along a certain line. Secondly, by folding and 

 crumpling together this thickened mass, and producing from it a 

 crystalline axis, capable of withstanding the action of denuding 

 forces to a much greater extent than the surrounding unaltered 

 masses. Thirdly, by elevating the masses into regions of the atmo- 

 sphere in which the destructive effects of moisture, frost and heat, 

 and the transporting powers of water and ice act with their maximum 

 intensity. That during the great movements, lines of weakness have 

 been produced, and fresh lines of drainage originated or old ones 

 modified, and thus the action of denudation greatly controlled, we 

 are far from denying ; but we regard all these actions as exercising 



