204 J. W. Jiidd — On Volcanos. 



Witli respect to the mode of formation of these vast masses of 

 Tolcanic rocks in the Southern Tyrol, the only conclusion at which 

 we can anive is, that the highly siliceous materials which compose 

 them constituted a lava of only very imperfect liquidity, and that, 

 like many modern lavas of the same class, it quietly welled forth 

 from volcanic fissures, building up vast dome-shaped mountains, the 

 formation of which was attended with but comparatively little 

 explosive action. Masses of tuffs, however, clearly resulting from 

 the latter kind of action, do occur in association with these widely 

 spread quartziferous lavas, and the manner in which they are found 

 to alternate with beds of sandstone and conglomerate, as in the 

 Gro doner Thai, and at many other points, indicates that the outbursts 

 by which these volcanic rocks were formed must have taken place 

 either on the land or in shallow water. Eichthofen has endeavoured 

 to refer these Permian volcanic outbursts to a number of distinct 

 periods, and it is certain that they must have extended over enormous 

 intervals of time. Some idea of the scale on which these displays 

 of volcanic acti^dty took place may be gathered from the fact that, 

 although they were certainly greatly denuded before the deposition 

 of the superincumbent rocks upon them, and are now only very im- 

 perfectly exposed to our observation, yet the quartziferous volcanic 

 rocks of the Botzen district constitute rock -masses more than 9,000 

 feet in thickness ! 



The eruption of the quartz -porphyry of Botzen, which, as we have 

 shown, must be regarded as the first sj^mptom of the existence of that 

 line of weakness in the earth's crust, to which the formation, at a 

 later period, of the Alpine chain must be ascribed, was followed by 

 a prolonged period of subsidence. During this epoch the vast 

 thickness of sediments which constitute the Alpine Trias was de- 

 posited, consisting of sandstones, limestones, and shales piled upon 

 one another to the aggregate thickness of several thousands of feet. 

 This quiet accumulation of sediments during the Triassic period was, 

 however, interrupted by a number of volcanic outbursts on a far 

 less grand scale than those which had taken place during the 

 Permian period ; the remarkable variations in the thickness and mode 

 of formation of the several divisions of the Alpine Trias also indicate 

 the very local and interrupted nature of the movements which were 

 taking place in the area, and prove the continued activity of those 

 subterranean forces of which the manifestations at the surface by 

 volcanic outbursts had become so much more feeble. 



If, however, the volcanic outbursts of the Trias appear insignificant 

 in extent when compared with those of an earlier period, they arrest 

 the attention of the geologist much more powerfully than the latter, 

 on account of the variety and interesting characters of the rock- 

 masses produced by them ; and especially by the assemblage of 

 beautiful crystallized minerals, which have resulted from the action 

 of the igneous intrusions on the surrounding stratified masses. 



There are indeed few localities in Europe more justly famous 

 among men of science than the vallej'-s of the Southern Tyrol. To 

 those of the most varied tastes, the district offers equally powerful 



