208 J. W, Judd — On Volcanos. 



olivine (usually in a muck altered condition) and magnetite. Except 

 on the ground of their age, there appears to be no reason whatever 

 for removing them from the basalts. Sometimes the rock becomes 

 porphyritic in structure, from the diffusion through it of large 

 plagioclase crystals ; at other times the whole mass becomes coarsely 

 granular, and has been classed as a diabase, though it should prob- 

 ably be more properly called a dolerite. When the rock contains 

 large crystals of augite scattered through it, it becomes an augite- 

 porphyry, and where (through alteration, as Tschermak shows) these 

 augite crystals present the Uralite modification, the rock passes into 

 uralite-porphyry. All these basic rocks graduate into one another, 

 they frequently assume an amygdaloidal structure, and are associated 

 with enormous masses of agglomerates, tuffs and ashes of similar 

 composition. The chemical constitution of these rocks is illustrated 

 by the following mean analyses as calculated by Tschermak from the 

 closely approximating results arrived at by a number of different 

 chemists. 





Melaphyres. 



Augite-forphyries. 



Silica 



53-01 



49-78 



Alumina 



17-05 



18-83 



Protoxide of Iron 



11-95 



11-47 



Lime 



8-27 



10-44 



Magnesia 



3-77 



4-87 



Potash. 



3-42 



2-65 



Soda 



2-56 



2-0o 



100-00 100-00 



Of still younger date than the basaltic rocks just described are 

 others of restricted distribution, which Eichthofen called Syenite- 

 porphyry and Porphyrite, but which are by Dr. Doelter classed as 

 Orthoclase -porphyry. They are coarse-grained rocks of porphyritic, 

 occasionally approaching to granitic structure, consisting of a granu- 

 lar or felsitic base in which twinned crystals of red orthoclase, with 

 hornblende and magnetite, are scattered. This rock, like portions 

 of the Quartz-porphyry of Botzen, often contains considerable quan- 

 tities of the mineral of the pinite group known as Liebnerite. We 

 subjoin the following analj^sis by Kjerulf in illustration of the ulti- 

 mate composition of the Orthoclase porphyries : 



Snica 59-17 



Alumina 19-75 



Protoxide of Iron 1-71 



Lime 3-92 



Magnesia 0-40 



Potash 4-03 



Soda 3-54 



"Water, Carhonic Acid, etc 6-94 



98-43 



We have dwelt at considerable length on the petrological cha- 

 racters of these old volcanic masses of the Southern Tyrol, because of 

 the great amount of attention which they have attracted among 

 geologists ever since the days of Yon Buch, and the lively discus- 

 sions of which they have been the subject. They are interesting 



