548 HENR Y S. IV A SHING TON 



These rocks are not very abundant in the region, and seem 

 to be more common in the northern part than elsewhere. In 

 certain cases they belong to the earlier eruptions, prior to those 

 of the leucitic rocks. Purely alkali feldspar-trachytes seem to 

 be unknown about this center, unless Moderni's trachyte of Ris- 

 pampani belongs here, a point which his brief description does 

 not permit us to decide. The plagioclase end of the series is 

 represented by the augite-andesite of Monte Rado, which is also 

 olivine-bearing. 



The only specimens of vulsinite in my possession were col- 

 lected from a small eminence on the eastern shore of the lake, 

 immediately to the north of the small town of Bolsena. The 

 locality is mentioned by Verri and de Stefani, and the rock is 

 the same as that described by Klein (p. 8), with whose descrip- 

 tions my specimens agree very closely. It is probably identical 

 also with the trachyte of vom Rath (p. 2gi), though Nassini's 

 quarry, whence he obtained his specimen, does not seem to be 

 known at present. The rock belongs, as we shall see, to one of 

 the earliest outflows. 



Megascopically it is fine-grained and compact, with marked 

 eutaxitic structure. The greater part is light ash gray, but 

 mottled with spots and streaks of dark gray and yellowish brown. 

 The structure here seems to be due to a mingling of magmas of 

 somewhat different composition. Many glassy phenocrysts of 

 feldspar, as well as smaller ones of augite and a few of biotite, 

 are to be seen. The specific gravity is 2.534 at 25° C. 



The feldspar phenocrysts are both alkali feldspar and plag- 

 ioclase, the former in the majority but the latter quite common. 

 Twinning, according to the Carlsbad law is not rare in the ortho- 

 clase, and one very perfect example of a Baveno twin was seen. 

 While twinning is the rule with the plagioclase, yet multiple lam- 

 ellae are not often seen, and some untwinned crystals occur. 

 Zonal structure is not frequent. The polarization color furnishes 

 the readiest method of distinguishing between the two feldspars, 

 that of the plagioclase in my sections, which are about o™™.2 

 thick, reaching the light straw yellow of the first order. The 



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