J, W. Judd — On Volcanos. 209 



also, as presenting us with rock-masses of the most perfectly granitic 

 structure, of which the volcanic origin is certainly not open to 

 doubt ; this will at once appear when we come to notice their rela- 

 tions to the sedimentary rocks ; I am not indeed aware that the fact 

 has been ever disputed by any competent observer. We will now 

 proceed to notice some of the more salient features of the structure 

 and relations of these igneous rocks, which serve to throw light on 

 the questions of their age and mode of formation. 



As already mentioned, the whole of the smaller and scattered 

 volcanic outbursts in the Southern Tyrol took place during the 

 period of the Upper Trias. This is proved in the most conclusive 

 manner, by the way in which the igneous rocks are only found 

 intrusive among the lower members of the formation, while their 

 tuffs are interstratified with the beds of its higher members. Ac- 

 cording to Dr. Doelter's most recent researches, as we have before 

 pointed out, all the eruptions must have taken place during the 

 period of the formation of the "Wengener Schichten." In the 

 district which denudation has exposed to our study, we find, as 

 already remarked, a number of different centres of eruption, most 

 of which however have only produced rocks of basaltic character, 

 Melaphyres with Augite- and Uralite-porphyries. But two of these 

 centres of eruption are of larger size and more complex structure, 

 that of Predazzo having emitted the whole of the interesting varieties 

 of rocks we have noticed in the preceding pages, and that of Mon- 

 zoni all except the tourmaline -granite. 



The Monzonite rises in great intrusive masses through the midst 

 of the limestones and other rocks of the district, which assume, for a 

 considerable distance around, a highly crystalline character. Some- 

 times, as at Monzoni, vast masses of the metamorphosed limestone 

 are seen to be actually entangled and inclosed in the midst of the 

 igneous rock. Concerning the relations of the two varieties of 

 Monzonite to one another, very various opinions have been expressed 

 by different observers. Eichthofen regarded the more basic portions 

 of the rock, to which he gave the name of *'hypersthene-rock," as 

 traversing the Monzoni-syenite, which constitutes the larger part of 

 the mass, in veins. But although such veins of the basic variety of 

 the rock do occur traversing the more acid kind, as is well seen in 

 the Margola near Predazzo, yet the observations of Dr. Doelter 

 upon Monzoni would appear to lead to the conclusion that the great 

 masses of rock with plagioclase felspar are not sharply divided 

 from those in which orthoclase is the predominant constituent. 

 Indeed, as I have before observed, it is possible to find at Monzoni 

 every possible gradation, from the most acid to the most basic variety 

 of the rock. 



The Tourmaline -granite, which is seen only near Predazzo, also 

 rises in great intrusive masses, which traverse the Monzonite, and 

 send off veins into it ; so that there can be no doubt as to the more 

 recent date of its eruption. 



The Augite -porphyry, Uralite-porphyry, and Melaphyre consti- 

 tute intrusive masses rising through the Monzonite and the Tourma- 



DECADE II. — VOL. III. — NO. V. 14 



