496 Rev. A. Irving — Triassic Bejjosifs of the Alps. 



volcanic activity. An example of tbis is seen in the extensive 

 development of volcanic rocks (commonly known as ' quartz-por- 

 phyries '), which form the principal feature of the Bozen district. 

 Here we find probably the most extensive display of such rocks 

 which the present surface of the earth furnishes. The eruptive 

 character of these ' porphyries ' is clearly illustrated in the case of 

 the Rittner Horn, the culminating peak of a district between the 

 valleys of the Sarn and the Eisack known as the Eitten. Walking 

 from Bozen to the summit of the mountain, and more especially in 

 the tipper part above Klobenstein, one passes in succession over 

 alternating 'quartz-porphyries' (such as are extensively use for build- 

 ing purposes in Bozen) and ash-beds.' The stratification of these 

 'ash' deposits is brought out in the clearest possible manner by 

 weathering, and frequently angular fragments (blown by steam 

 along with finer debris from the ancient volcanic canal) are seen in- 

 cluded in them : on the most highly weathered surfaces these frag- 

 ments may be detached easily with a common alpenstock. In many 

 cases a hand specimen (except on the point of hardness) could 

 scarcely be distinguished from a specimen of tufa or peperino taken 

 from the Roman hills, the colours of both these being here repro- 

 duced. The juxtaposition of the tnff-deposits and the ancient lava- 

 flows may be frequently observed in situ. The summit of the 

 mountain preserves the outline of a portion of the ancient crater, 

 the remaining walls of which can be traced as distinctly as one can 

 trace the remaining crater- walls of slaggy basalt of Tertiary age at 

 Daun, or in the Moselberg in the Eifel. It would almost appear 

 even that some of the beds were originally vesicular acidic lava- 

 flows, and that the steam-cavities have been subsequently filled with 

 quartz, perhaps (as M. Daubree has taught us) by the action of 

 super-heated water upon the siliceous materials of the rock. This 

 hypothesis is, I think, warrantable, when we take into account the 

 results obtained by M. Daubree (especially the deposition of crystalline 

 quartz) by heat from such a siliceous composition as we commonly 

 call "glass," under sufficient pressure in mere super-heated water.^ 

 These Bozener volcanic rocks being in all probability of Dyassic 

 ao-e, the history of the Alpine mountain-system supplies ample data 

 for the application of the physical principle so ably demonstrated 

 by the great French savant. 



The phenomena presented to us in this mountain seem to me to 

 go altogether to confirm the view maintained by Prof. Credner, of 

 Leipzig,^ that the true place of these rocks known as ' quartz-por- 

 phyries,' in any system of classification of the crystalline rocks, is 

 among the ' Older Eruptive Rocks,' and that they ought not to be 



1 The earth-pillars (' Erd-pyramiden') of the neighbourhood are, or have been, 

 severally capped Avith blocks of quartz-porphyry, but I think the finer materials 

 which constitute them are more largely derived from the tuffs (clastic rocks) than 

 from the crystalline rocks ; both those of the Finsterbach near Lengmoos, and the 

 group lower down near Bozen. 



2 Vide Eludes Synthetiques de Geologie Experiment ale, par A. Daubree, pp. 

 165-6. 



3 Elemente der Geologie, 3rd edition, pp. 274, 275. 



