202 J". W. Judd — On Volcanos. 



Yolcanic outbursts, we propose to give a sliglit sketch, in the present 

 chapter. 



In studying the history of these early outbursts of the volcanic 

 forces in the Alpine district, we have unfortunately some great diffi- 

 culties to contend with. Throughout the greater part of the Alpine 

 range, sedimentary rocks of younger date have been piled to the 

 height of many thousands of feet above the volcanic products of the 

 period we are considering. At three points, however, denuding 

 forces have exposed them for our study — namely, near the Lake of 

 Lugano on the borders of Switzerland and Italy, in the Southern 

 Tyrol, and in the country about Kaibl in Carinthia. At a number of 

 intermediate points and at some others in the same line, but to the 

 eastward of all of them, more partial exposures of volcanic rocks of 

 contemporaneous date are found ; these serve to indicate that, if we 

 could remove the mass of superincumbent rocks of later date, we 

 should probably discover beneath them a continuous range of volcanic 

 masses along a line nearly coincident with that of the future Alps. 



Of the several exposures of the pre- Alpine volcanic rocks, that 

 which affords us by far the best insight into the nature and succes- 

 sion of the operations which took place, occurs in the district of 

 the Southern Tyrol. The old volcanos of this country we shall now 

 proceed to describe, and then afterwards briefly notice the features 

 of some of the contemporaneously formed igneous masses at other 

 parts of the Alpine system. 



The greatest breach in the continuity of the Alpine chains is that 

 which has been effected by the denudation of the river-systems of 

 the Inn and the Adige, flowing northwards and southwards to the 

 Danube and Po respectively ; and it is at the watershed between 

 these two systems of valleys that the lowest of the great Alpine 

 passes — the Brenner — occurs. Now, it is just in this area of maximum 

 denudation that the finest exposure of the pre-Alpine volcanic rocks 

 is found. 



The oldest of these is the well-known quartz -porphyry of Botzen, 

 which by the earlier geologists was regarded as a granite. Although 

 only very partially exhibited by the denudation of the vast overlying 

 masses of the Triassic, Rhsetic, Jurassic, Tithonian, Neocomian and 

 Cretaceous rocks, it can be traced over an area of more than 1,000 

 square miles, and is found to constitute mountain-masses of more 

 than 9,000 feet in height. It is probable, moreover, from the 

 appearance of similar rocks at the bottoms of deep valleys, and at 

 other points where the younger formations have been removed by 

 denudation, that a belt of similar rocks extends below the southern 

 limestone zone of the Al23S, from the borders of Switzerland on the 

 west to Carinthia in the east. 



The rock of the quartz-porphyry of Botzen presents many varia- 

 tions in structure and appearance ; while it sometimes approximates 

 in its characters to the granites, with which class of rocks it was, 

 as we have already remarked, identified by the older writers, it at 

 other times closely resembles the recent Liparites of Hungary and 

 other countries. The quartz is almost always distinctly visible, as 



