210 J. W. Judd—On Volcanos. 



line-granite and giving off veins wliicli traverse both of these rocks. 

 These basaltic rocks are, as already stated, by far the most wddely 

 distributed products of the volcanic ejections of the Trias period in 

 the Southern Tyrol, and are found at all the centres of eruption. 



The last-formed of these Triassic volcanic rocks are the porphyrite 

 and syenite- porphyry of Eichthofen (the orthoclase-porphyry of Dr. 

 Doelter), which, however, only occurs in the form of veins travers- 

 ing all the other rocks, but does not constitute any considerable- 

 proportion of the intrusive masses. 



- From the peculiar arrangement affected by the stratified rocks 

 around the different centres of eruption, Eichthofen, as we have 

 already seen, applied to them the name of " craters." It must, how- 

 ever, be clear to every one who carefully examines the question 

 that the actual volcanic cones, and therefore the craters themselves^ 

 must have been destroyed by denudation before the deposition of the 

 highest beds of the Trias. It seems to be certain, from an examina- 

 tion of the tuffs produced by them, that the eruptions of this period 

 must have taken place under the waters of a comparatively shallow 

 sea, and that the cones, as they were built up, gradually rose to the 

 surface, when (as in the well known case of Graham's Isle) they 

 would be gradually destroyed, and their materials distributed by the 

 waves. Near the centres of eruption we find the coarsest agglomerates 

 containing bombs and ejected masses of the surrounding sedimentary 

 rocks ; but as we recede from these centres the ejected materials become 

 of gradually finer character, and more mingled with aqueous sediments. 

 In the latter condition they are often seen to be crowded with the 

 beautiful fossils of the St. Cassian series, and alternate with other 

 beds of the '' Wengener Schichten." 



Although it cannot be proved that the Monzonite-rock and the 

 Tourmaline granite actually reached the surface, yet it is in the 

 highest degree probable that they did so. They certainly could 

 not have formed extensive lava-streams, nor have given rise to any 

 great quantity of tuffs hy explosive action ; indeed we are unable to 

 determine exactly the form which these particular granitic rocks 

 would assume on reaching the surface. Their composition is, how- 

 ever, identical with that of certain sanidine-trachyte and andesitic 

 lavas. Probably, after the manner of many lavas of this class (such 

 as the Domites of the Auvergne), they constituted very imperfectly 

 liquid masses, which quietly welled forth, forming dome-shaped hills, 

 the extrusion of which was accompanied by but little explosive 

 action. 



Yery different, however, must have been the phenomena attending 

 the eruption of the more basic rocks,— the so-called melaphyres, 

 augite-porphyries, and uralite-porphyries. Not only do these appear 

 to have flowed from the igneous centres in lava-streams of consider- 

 able length and thickness, but the explosions which accompanied 

 them certainly gave rise to the formation of the most prodigious 

 quantities of scoria? and ashes. From the study of these vast deposits 

 we obtain the clearest insight into the conditions under which the 

 volcanic outbursts to which they owed their origin must have taken 

 place. 



