J. W. Judd — On Volcanos. 203^ 



more or less rounded grains, and these, with crystals of orthoclase 

 felspar, are imbedded in a paste, which is sometimes micro -crystalline, 

 at others more or less glassy, and occasionally exhibits a sphge- 

 rulitic structure. 



Closely associated with the quartz-porphyry of Botzen is a rock 

 called by Tschermak " quartz-porphyrite," and which differs from 

 the former principally in being composed of a plagioclase felspar 

 instead of orthoclase. This rock is best seen in the San Pellegrino 

 Valley lying to the south of the mountain of Monzoni, but it has also 

 been detected at some other points in the district. It is usually of a 

 greyish-green colour, and exhibits numerous large grains of quartz 

 with smaller crystals of plagioclase felspar and biotite, imbedded in a 

 compact dark-coloured base. The rock in question is of much interest 

 as presenting us with an exact representative among the volcanic 

 products of older date of the remarkable modern quartz-Andesites or 

 Dacites, which are so largely developed in Eastern Europe. The 

 view once held by petrologists, that the prevailing felspar in all rocks 

 which contain free quartz must necessarily be an orthoclase, is now 

 shown l^to be altogether untenable. Among granitic rocks we have 

 the Tonalite of vom Eath (the so-called granite of the Adamello 

 group), composed of a plagioclase felspar with much free quartz. 

 Among the porphyritic rocks, or those with a texture intermediate 

 between granites and lavas, and also among the latter class of rocks 

 themselves, we find innumerable examples of rocks of similar mine- 

 ralogical constitution, — both in the modern Hungarian Dacites and 

 the ancient quartz-porphyrites of the Southern Tyrol. 



With regard to the epoch at which these quartziferous lavas of the 

 Southern Tyrol were erupted, all geologists are now agreed that it 

 must have been that of the Dyas or Permian. 



In order to illustrate the ultimate composition of these quartziferous 

 volcanic rocks, which during the Permian period were so abundantly 

 erupted in the Alpine region, we may cite the following analyses : — • 



(1) (2) (3) (4) (5) 



Silica, with Titanic acid 76-14 73-10 74-62 67-98 66-75 



Alumina 12-69 12-84 11-94 14-01 16-53 



Peroxide of Iron — — — — 2-76 



Protoxide of Iron 1-78 3-02 2-59 5-00 1-66 



Protoxide of manganese 0-17 trace 0*23 — — 



Lime 0-51 1-22 0-73 2-47 4-71 



Magnesia 0-32 0-44 0-31 2-41 2-64 



Potash 5-81 4-32 5-29 3-55 1*82 



Soda 1-82 3-33 2-93 2-25 2-86 



Water 1-03 1-20 0-90 1-67 2-12 



Totals 100-27 99-47 99-54 99-34 101-85 



(1) is a typical example of the quartz-porphyry of Botzen from St. UMcli in the 



Grodener Thai. The analysis, which is by Dr. Ruhe, was published by Scheerer. 



(2) and (3) are analyses of somewhat similar rocks made by Scheerer and Ruhe 



respectively ; (2) is the rock of Moena, which contains much puiitoid. 



(4) is a rock, which is regarded by Tschermak as to a certain extent intermediate 



in character between the quartz -porphyries and the quartz-porphjTites. It 

 is from the Travignolo Valley, and the analysis given is by Scheerer. 



(5) the interesting quartz-porphyrite from the" San Pellegrino Valley. The 



analysis, which is by S. Kenya, was pubhshed by Tschermak. 



