206 J, TF. Judd — On Yolcanos, 



ever miicli some of its minor conclusions may be modified "by later 

 investigations) as one of tlie most remarkable and valuable of geolo- 

 gical monograpbs. A more recent and very able investigator of the 

 district, Dr. Doelter of Vienna, finds evidence which leads him to 

 refer all the eruptions of the area to that portion of the Upper Trias 

 known as the " Wengener Schichten." 



Among the several varieties of volcanic rocks produced at the 

 two interesting vents of Predazzo and Monzoni, a definite order of 

 appearance has been recognized by all geologists who have inves- 

 tigated the subject. This succession is as follows : — 



1. Monzonite, including under this name the Hypersthene and Dial- 



lage rocks, the Diabase or Pyroxene Monzonite, the Monzoni- 

 or Augite- syenite, and the Amphibole Monzonite. 



2. Tourmaline-granite. 



3. Melaphyre, with Augite-porphyry, Uralite porphyry, and the 



tufi's associated with these several rocks. 



4. Orthoclase porphyry (porphyrites and syenite -porphyry of Eicht- 



hofen) . 



It will be interesting to notice, in the first place, the nature and 

 composition, both chemical and mineralogical, of these several volcanic 

 products ; then to point out the relations to one another and to the 

 sedimentary rocks of the masses which they constitute ; and lastly 

 to seek to determine the exact conditions under which they were 

 formed. 



The Monzonite, which was known to the older writers on geology 

 as '' the granite of Predazzo," is a rock of unmistakably granitic 

 structure, although its volcanic origin is also equally clear and in- 

 disputable. Under the general term have been included the two 

 rocks distinguished by Eichthofen as Monzoni-syenite and Hyper- 

 sthene-rock, by Tschermak as Monzoni-syenite and Diabase, by vom 

 Eath as Augite -syenite and Diabase, and by Doelter as Pyroxene- 

 Monzonite and Amphibole -Monzonite. 



The Monzonite would appear never to contain free quartz, but it 

 seems to me, from the examination which I made of the celebrated 

 mountain of Monzoni, to present every gradation between a rock of 

 decidedly acid character on the one hand to one of the most 

 eminently basic description on the other. Sometimes it is found 

 to consist almost entirely of an aggregate of orthoclase crystals, 

 with a small quantity of a plagioclase felspar, of biotite and of a 

 pyroxenic constituent. It is a singular circumstance that the last- 

 mentioned ingredient presents some very remarkable peculiarities 

 of character, that render it very difficult to determine its exact 

 nature. Tschermak regards it as a hornhlende, while vom Eath 

 considers it to be an augite, and in consequence calls the rock 

 " augite-syenite." 



In other varieties of the rock, however, we find the orthoclase 

 occupying a gradually less conspicuous place, and finally disappear- 

 ing altogether ; while the plagioclase felspar (usually labradorite), 

 the biotite and the pyroxenic constituent (which in the more basic 

 varieties of the rock, though regarded at first by both De Lapparent 



