200 J. W. Jucld — On Volcanos. 



III. — Contributions to the Study of Yolcanos. — Second Series. 



By John W. Judd, F.G.S. 



On the Volcanic Outbursts which preceded the Formation op 



THE Alpine System. 



OF all the physical features of our continent, that which most 

 powerfully arrests the attention is the noble chain of the Alps, 

 with its wide-spreading system of subordinate mountain-ranges ; and 

 among the series of grand events, which by the researches of the 

 geologist are demonstrated to have taken place on this our own por- 

 tion of the globe, there are none more striking and conspicuous 

 than those manifestations of subterranean forces to which the vast 

 mountain-masses in question owe their origin. 



As we now know, the great centres of crystalline rock, which for 

 the most part constitute the axial portions of the Alpine ranges, do 

 not represent any primitive formation — portions of the surface of a 

 melted globe, as it originally cooled, or deposits of an ocean still in 

 ebullition, — as old writers on geology used to imagine ; but, od the 

 contrary, they are found to consist in great part of sediments formed 

 during different geological periods, which, long subsequently to their 

 deposition, were, by the action of tangential mechanical strains, 

 folded and crumpled, and by the agency of chemical forces developed 

 in their midst, were transformed into their present highly crystalline 

 condition. The sedimentary rocks out of which these crystalline 

 masses have been formed were, as wenowrecoguize, deposited during 

 different Paleeozoic, Mesozoic, and Tertiary epochs ; and the forces 

 by which they have been metamorphosed and have acquired their 

 present elevated, contorted, and even inverted positions, have for the 

 most part operated during what the geologist regards as compara- 

 tively recent periods ; indeed, the most powerful of all the series 

 of movements to which the Alpine chains owe their origin must 

 have taken place during the latter half of the Tertiary epoch. The 

 highly crystalline rocks of the central portions of the Alpine chain 

 are not the oldest formed portions of their masses, but the youngest 

 — and are the result of the extreme metamorphism of the most 

 violently disturbed portions of sedimentary deposits, some of which 

 are of as recent date as the Tertiary, and of the intrusion into their 

 midst of aqueo-igneous masses. The growth of the Alpine chain 

 was not, as formerly supposed, an exogenous one — resulting from 

 the successive deposition on the flanks of a granitic axis of layers 

 of gradually less crystalline character — but an endogenous one ; 

 masses of pre-formed sediments being upheaved along certain lines, 

 and at the same time folded, crumpled and crushed, while they 

 had a crystalline structure induced in them and liquefied masses 

 from below thrust into their midst. So recent, geologically 

 speaking, have been those manifestations of subterranean force to 

 which the Alpine system owes its origin, that the greater part of 

 the present elevation of these seemingly '' everlasting hills and 

 perpetual mountains" is unquestionably due to causes which have 

 operated since the period of the deposition of the clays and sands 

 on which London is built ! 



