ITALIAN PETROLOGICAL SKETCHES 377 



the extreme differentiation products — the toscanites and basic 

 leucitic rocks — or by the mean of the ciminites, vulsinites, and 

 acid leucitic rocks, which closely resemble each other. On 

 such a basis we could suppose it to have approximately a com- 

 position as follows: 8103 = 57-58, Al303 = i7-i8, total iron 

 oxides as FeO = 6-7, MgO = 2-3, CaO= 5-6.5, Na30=2-2.5, 

 K20 = 7-8, HoO=i-i.5 per cent. 



Whether the original magma differentiated horizontally from 

 north to south we are not yet in a position to discuss, but at 

 each center we may suppose the body of magma to have been 

 quite completely differentiated. We would thus have the tos- 

 canites and the basic leucitic rocks (leucitites and leucite- 

 basanites) representing the extreme products — the oxyphyric' 

 and lamprophyric types, respectively. Apart from questions 

 of conditions of solidification, the absence of leucite in the 

 vulsinites may be explained by their higher silica,'' while in the 

 ciminites and biotite-vulsinites the high magnesia conditioned 

 the formation of olivine, leaving the remaining part of the 

 magma sufificiently acid for the formation of orthoclase rather 

 than leucite. That these suppositions are possible may be seen 

 on comparing analyses i, 4, and 9 of Table I and i of Table II, 

 with II, 12, 14, and 15 of Table V, which are practically iden- 

 tical except for the silica in the first two, and the magnesia in 

 the second two of the trachydolerites. 



All this is, however, admittedly speculative to a large 

 extent, and these views are advanced in a provisional way to be 

 tested, and perhaps greatly modified, by future investigations. 

 Finally, it has perhaps have been noticed that little or no refer- 

 ence is made to the succession of the rocks. The omission is 

 intentional, because it does not seem to the writer, even o-rant- 

 ing that the order of eruptions is of the importance often 

 attributed to it, that our knowledge of the subject along the 

 Bolsena-Vesuvius line is sufficient to be of much value. 



Henry S. Washington. 



' PiRSSON, Am. Jour. Sci., L, 11 8, 1895. 



'The anomalous acid leucite-tephrites observed by vom Rath and Ricciardi are 

 against this view, and some of them need confirmation. 



