ITALIAN PETROLOGICAL SKETCHES 553 



From the above description and analyses we may then define 

 the vulsi/iites as effusive rocks occupying an inter?nediate position 

 between the trachytes and the andesitcs. They are characterized 

 mineralogically by the presence of alkali feldspar with a large 

 amount of basic plagioclase (labradorite to anorthite) together 

 with augite and diopsidc. Hornblende and biotite are not abun- 

 dant in the type specimens, though they may be present in large 

 amounts in other varieties, as will be seen later. Olivine is want- 

 ing, or if present is so in only accessory amounts. Chemically 

 they are rocks of medium acidity, SiOg, from about 55 to about 

 60 per cent., though it may run slightly above or below these 

 figures. Alumina and iron are present in medium amounts, mag- 

 nesia is low, lime rather high (3 to 6 per cent.), and alkalies 

 (especially potash) high. Of the analyses above No. 3 may be 

 regarded as typical. From an examination of this it is seen that 

 after the formation of magnetite and pyroxene considerable lime 

 is left over. Since part, if not most, of the alkali feldspar is a 

 soda orthoclase and the amount of soda present is small this 

 lime must go to form anorthite. 



Klein describes very similar rocks which belong to the vuj- 

 sinites. Those from Torre Alfina and San Lorenzo, north of 

 Lake Bolsena, are very high in silica and lower in potash (accord- 

 ing to Ricciardi's analyses) than we would expect (c/ anal. 5, 

 p. 565). Their groundmass is brown, phenocrysts of glassy feld- 

 spar, biotite and augite are visible, and they seem not to be quite 

 fresh. Among the phenocr3'sts sanidine largely predominates over 

 the plagioclase, which is basic. Some olivine is present, but since 

 the magnesia is low its quantity cannot be very great. The ground- 

 mass is very glassy, the base being brown through the presence 

 of globulites. Orthoclase, plagioclase and magnetite are present 

 in the groundmass, but augite is not mentioned. These rocks 

 seem to resemble those of Monte Amiata described by Williams, 

 and their relations will be discussed later. 



The "trachyte" of San Magno, to the west of the lake, 

 approaches nearer to the Bolsena rock, the silica being 60.03 

 but the potash is still rather low, though higher than the soda 



