ITALIAN PETROLOGICAL SKETCHES 357 



vom Rath's their total amount is about the same but the relative 

 quantities quite different, and in Ricciardi's they are both much 

 lower than in No. i, but with potash in excess. It may be 

 remarked that these mutual differences are fairly constant else- 

 where and point to differences in analytical methods rather than 

 to differences in composition. 



Another typical vulsinite was found near Vetralla in the 

 Viterbo region, and its analysis (No. 4) closely agrees with that 

 of the Bolsena rock. The San Magno vulsinite (No. 5) is quite 

 similar, though the silica is a little higher and the alkalies low. 

 In regard to the rocks from Torre Alfina and San Lorenzo (Nos. 

 6 and 7) I must confess to some hesitation. Klein mentions 

 olivine as occurring in notable quantity, though both analyses 

 show small amounts of magnesia and high silica. The analyses 

 resemble, indeed, much more'those of the acid toscanites, and it 

 seems probable that they were not made on the specimens exam- 

 ined by Klein, No. 8 is by Ricciardi of a specimen from the 

 Piano delle Macinaje at Monte Amiata sent him by Verri.' The 

 analysis is essentially that of a vulsinite. This view is con- 

 firmed by the description of Artini,^ who states that it is gray, 

 porphyritic, and composed of large phenocrysts of sanidine, 

 smaller but more numerous phenocrysts of plagioclase, showing 

 a core of bytownite, a border of andesine, with augite and biotite, 

 lying in a groundmass of feldspar, augite, and glass. 



Analyses i and 4 yield the following approximate composi- 

 tions, though the results are uncertain, owing to our ignorance 

 of the composition of the biotite and augite : 



I 4 



Orthoclase, . . . . 



Albite, - . - - 



Anorthite, . - . . 



Diopside, . - - - 



Biotite, - - - - - 



Accessories, 



' Ricciardi speaks of it as labeled "the main mass of Monte Amiata." Verri 

 corrects this error later (Boll. Soc. Geol. Ital., VIII, 1889). 

 »Artini, Giorn. Miner., IV, 7, 1893. 



