Prof. J. W. Jiidd — On Vohanos. 533 



The greater oscillations, attended with the more sweeping action 

 of marine denudation, which have affected the southern range of 

 Miocene volcanos, — those north of the Alps having for the most 

 part suffered only from subaerial waste, — has resulted in reducing the 

 former to a much more ruinous condition than the latter. In the 

 numerous intrusive masses of gabbro, serpentine, diorite, etc., which 

 abound in the Iberian and Italian peninsulas, and which are, in 

 many cases at least, of post-Eocene age, as well as in some skeleton 

 volcanos, like those of the Euganean Hills and Styria, with vents 

 like those of Sardinia, Central and Southern Italy, Sicily, and the 

 iEgean Sea, which have continued in action to more recent periods, 

 we find the different centres of volcanic action on the southern side 

 of the Alps, exhibiting every stage of dissection by denudation. This 

 southern chain is continued eastwards by the volcanos of Asia Minor, 

 Persia, and Afghanistan into Northern India. 



The extinction of the great volcanic cones, both on the north and 

 south sides of the central mountain axis, was followed, as we have 

 already remarked, by the appearance of scattered "puys" which can 

 be reckoned by thousands, and were evidently thrown, up along lines 

 of fissure opened in the plains surrounding the extinct cones. 



It is impossible to regard as accidental the close agreement which 

 we have thus shown to exist between the critical periods in the 

 history of the last great movements in the Alpine axis, and those of 

 the volcanic outbursts in the surrounding areas ; more especially 

 as we have seen that the igneous activity was developed along 

 lines having the most unmistakable parallelism to the principal 

 Alpine chains. 



Now it is of the very highest importance that we should fully 

 recognize the vastness of the changes which have taken place in the 

 physical geography of the continent during a comparatively recent 

 geological epoch : for in the action of the great subterranean move- 

 ments during the later Tertiary periods we can scarcely fail to 

 recognize — if the amount and effects of the disturbances be fully 

 realized — a competent cause for some of the more important of the 

 physical phenomena that mark the history of this epoch in Western 

 Europe ; and among these we may especially mention that extension 

 of the Alpine glaciers, which seems to have characterized several of 

 the most recent of the geological periods. 



In seeking for the causes of the extension of glaciers during 

 former geological periods, we cannot but regard it as a most unfor- 

 tunate circumstance that geologists have often appealed for explana- 

 tions of the phenomena they witness to the little-known and less- 

 understood action of the ice-fields that are supposed to cover 

 Greenland and the Antarctic continent. Hence has grown up the 

 hypotheses of vast movements of -'continental ice," of "ice-sheets," 

 and " polar ice-caps," the very existence of which rests on the 

 smallest and slenderest basis of evidence. For the supposition that 

 the icebergs of Arctic regions are not derived from large and 

 confluent glaciers, but from detached portions of ice-sheets which 

 have the power of over-riding hill and valley alike, I must confess 



