THE 



GEOLOGICAL MAGAZINE 



NEW SERIES. DECADE II. VOL. III. 



No. VIII.— ATJGUST, 1876. 



oiRia-izT-A-Xj J^I^TICXJ:ES. 



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



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

 On the Interval which separated the two great Periods of 



Volcanic Activity in Connexion with the Formation of 



the Alpine System. 



THE study of the great mountain ranges of America by Eogers, 

 Hall, Dana, Le Conte, Hunt, and other geologists, has now 

 thrown much new light on the earth-movements which precede and 

 accompany the formation of mountain chains. As the result of these 

 researches, it appears certain that the preliminary stage in the forma- 

 tion of every mountain system has consisted in a long-continued 

 depression of the area which is afterwards to become its site ; and, 

 in consequence of this prolonged subsidence, the accumulation of an 

 enormous thickness of stratified rocks, within the great trough so 

 formed, has taken place. Of this character, as is now well known, 

 have been the earlier manifestations of the subterranean forces that 

 were concerned in the formation of the Appalachians, Green Moun- 

 tains, and other American ranges ; the districts in which they are 

 situated were subjected to long-continued depression, which per- 

 mitted of an abnormal development of all the members of the sedi- 

 mentary deposits formed during this initiatory period ; and it was 

 by the folding, metamorphism and crushing together of this abnor- 

 mally thickened portion of the earth's crust that the indurated and 

 elevated masses have been formed which denudation has sculptured 

 into the existing mountain chains. 



Now of the occurrence of just such long-continued periods of sub- 

 sidence as constituted the earlier stages of the formation of the 

 American mountain systems, we have very clear and unmistakable 

 evidence in the case of the Alps. After the series of volcanic out- 

 bursts, which we have described in detail in our last chapter,' as indi- 

 cating the existence of the Alpine line of weakness, a period of 

 almost uninterrupted, though irregular, subsidence of this portion of 

 the earth's crust commenced, and was continued throughout the epochs 

 of the Upper Trias, Ehaetic, Jurassic, Tithonian, Neocomian, Cre- 

 taceous and Nummulitic formations. 



In proof of this prolonged and continuous depression in the Alpine 

 region, we may appeal to a number of well-known geological facts. 

 In the first place, it must be remarked that almost every member of 

 the Mesozoic and older Eocene formations has attained in the Alpine 

 regions its maximum of thickness and development. Secondly, we 

 1 See the May Number, p. 200. 



DECADE II.— VOL. III. — NO. YIII. 22 



