( 401 ) 



From this 1 lie following norm may be calculated: 



Sal =89.03 



Fern = (5. Mi» 



so thai according to the American system the rock belongs to Class 1 

 (Persalane) ; Subclass 1, (Persalone) ; order 4 (Brit.tanare); Rank 3 

 (Coloradase) ; Subrank 4 (Yellowstonose). The only positive deviation, 

 which the mode shows with respect to the norm, is the presence of 

 amphibole, so that the described rock ought to have the name of 

 Amphibolephvrivitrovellowstonose J ). 



The chemical analysis throws some light on the nature of the 

 rhombic pyroxene. The microscopic examination stopped at the 

 observation that the pyroxene was distinguished from the hypersthene 

 by its light colour. The above combination of the oxydes to mineral- 

 molecules now proves that in the pyroxene-molecule MgO predo- 

 minates strongly with respect to FeO. That the mode of the rock 

 by the appearance of amphibole deviates somewhat from the norm 

 calculated here, does not alter this fact considerably; even if we set 

 aside the circumstance, that the amphibole is quantitatively far inferior 

 to the crystalized rhombic pyroxene. For the fact that in the resorption 

 of the amphibole-phenocrysts only magnetite, no pyroxene as anew 



') In spite of the lack of quartz in this rock, it is, on the ground of the high SiO a 

 quantity, ranked not under the andesite but under the dacite; the same tiling was 

 done with the rock II 599 from the Soengei Embahoe. With the rock-analyses, 

 gathered by Osann, Beitrage zur chemischen Petrographic, li Teil, Stuttgart L905, 

 the above types are grouped as follows after the Si0 3 quantity (not free from H 8 0) : 



SiO a in ",i 75 74 73 72 71 70 G9 OS 67 66 65 04 f.3 CI 61 60 59 58 5 7 56 55 54 53 52 51 50 



Nr. of i Dacite I — i — 3 5 4 7 3 4 3 3 2 1 1 



analyses I Andesite 1 2 1 10 6 10 9 11 20 24 16 13 10 16 12 G 2 3 3 — 



From this may be calculated that the SiOa quantity with the dacite is fluctuating 

 round a mean of 67. 5° , with the andesite round 59.5%. Of course these figures 

 have only a very relative value: the rocks II 710 and II 599 however take up 

 places also in the projection- triangle of Osann which fall entirely within the sphere 

 of the dacite- and quartzporphyriteprojections, but only near the border of the 

 sphere of the andesites (glimmer-, hornblende-, hypersthene resp. augiteandesites and 

 porphyrites, cf. Osann, Versuch einer chem. Classification der Eruptivgesteine, Tsch. 

 Min. ivir. Mitt. XX, Heft 5. 6 190U, Taf. IX, X). 



