ITALIAN PETROLOGICAL SKETCHES 



837 



of orthoclase phenocrysts, the abundance of plagioclase and 

 [)rcscncc of olivine, with the andesitic structure of the ground- 

 mass, would incline one to call them andesites, while the abun- 

 dance of orthoclase in the groundmass inclines one towards the 

 trachytes. At the same time the olivine is much more abundant 

 than is usually found in an andesite, and the striking feature of 

 the comparative rarity of the feldspars among the phenocrysts 

 gives them a decidedly lamprophyric aspect. 



Similar difficulties are met with when we come to examine 

 the analvscs of these rocks striven below : 



TABLE I. 



1. Fontana Fiesole, Viterbo. H. S. Washington anal. 



2. West slope of Monte Cimino. Vom Rath, op. cit. 304. 



3. Mont' Alfina. Bolsena Region. Ricciardi anal. Klein, op. cit. 7. 



4. Sassara. Bolsena Region. Ricciardi anal. Klein, op. cit. 7. 



Here we see that while as regards the silica, alumina, iron 

 and lime thev approach the andesites rather than the trachytes, 

 yet that the potash is largely in excess of the soda and that the 

 rock is far richer in total alkalies than is the case with the true 

 andesites. The mao-nesia also is abnormallv high. 



We see, then, that this rock can be properly classed neither 

 with the andesites nor the trachytes, but that, like the vulsinite 

 prcviouslv described,' it occupies a position intermediate between 



'This JdURXAi.. IV, 547. 1S96. 



