ITALIAN PETROLOGICAL SKETCHES 37 



Toscanitc. — The non-leucitic eruptive rocks of this region 

 resemble the vulsinites and ciminites in containing basic plagio- 

 clase, as well as orthoclase, and are consequently quite rich in 

 lime. They differ, however, in being much more acid, with 

 SiO., from 63-72, and sometimes contain quartz. They, there- 

 fore, occupy a place intermediate between the rhyolites and the 

 dacites. They correspond, in fact, very closely, both mineral- 

 ogically and chemically, with the rocks of Monte Amiata 

 described by J. F. Williams.' They also resemble the quartz- 

 trachytes of Campiglia^ and Roccastrada.3 As these earlier known 

 localities are in Tuscany (Ital. Toscana) this group of acid effu- 

 sive rocks, characterized mineralogically by the presence of 

 basic plagioclase, as well as orthoclase, with occasional quartz, 

 and chemically by high silica and alkalies and (for the acidity) 

 high lime, and low alumina, may be called tosca?iite. It may be 

 mentioned that they also resemble certain rhyolites from Ponza 

 and from the Euganean hills near Padua. They thus occupy the 

 place in Brogger's^ table filled by the group of quartz-trachyte- 

 andesites, and in some cases are so acid as to fall in with his del- 

 lensite (dacite-liparite). They approach this especially in their 

 low alumina. Analyses of typical toscanites will be found in 

 Table I. 



The rock of Monte Calvario is very much decomposed, so 

 much so that good fresh specimens are dif^cult to find. I finally 

 obtained some which are quite, though not entirely, fresh at a 

 quarry on the southeast side where work was going on. The 

 rock is rather coarse-grained and resembles many of the por- 

 phyries of our western states. The groundmass is light gray, 

 and glassv feldspar phenocrysts abound, which are colored light 

 yellow by the infiltration of ferruginous water. They are chiefly 

 of sanidine with a smaller quantity of acid labradorite. Some 



'Williams, Neu. Jahr. B. Bd. V, 381, 1885. 



^VoM Rath, Zeit. d. d. geol. Ges. XVIII, 639, 1866. Also Dalmer, Neu. Jahr. 

 1887, II, 206. 



3MATTEUCCI, Boll. Com. Geol. Ital. 1890, 284 ff., and Boll. Soc. Geol. Ital. X, 

 670 ff., 1891. 



^Brogger, Eruptionsfolge bei Predazzo, Kristiania, 1895, 60. 



