356 HENRY S. WASHINGTON 



feldspar — perhaps triclinic, and also that the plagioclase must 

 be either a basic labradorite or anorthite. These rocks may 

 then be regarded as among the more basic members of the 

 ciminites. 



We may finally allude briefly to the fact that at Rocca Mon- 

 fina so-called basalts occur as products of the last phase of 

 activity, and that there is reason for thinking that these rocks 

 much resemble those of Radicofani and may be regarded as 

 basic ciminites.' 



Vulsinite. — The rocks belonging to this group are porphy- 

 ritic in structure and megascopically look much like typical 

 trachytes. Feldspar phenocrysts are especially prominent and 

 with them are seen smaller crystals of augite and rare tables of 

 biotite. The groundmass is light gray and in most cases is 

 holocrystalline, or nearly so, glass being present in only small 

 amount in the specimens examined. The feldspars comprise ortho- 

 clase a?id soda ortlioclase and a basic plagioclase — labradorite to 

 anorthite — in quite large amount. The ferromagnesian min- 

 erals are typically represented by augite or diopside, though biotite 

 is accessory and is very abundant in some of the basic varieties. 

 Hornblende is lacking at all the Italian localities examined, 

 except for the occasional presence of sporadic crystals of bark- 

 evikite. Magnetite is generally present, but not in large amount. 

 Both quartz and olivine are wanting, or only present sporadically. 



The chemical composition of these rocks is shown in Table II. 



They are rocks of medium acidity, the silica varying from 55 

 to 60 per cent., or perhaps somewhat over. Alumina and iron 

 oxides are in about the same amounts as in the trachytes, mag- 

 nesia somewhat higher, lime quite high (3 to 6 per cent.), and 

 alkalies in the most representative rocks very high, with potash 

 largely preponderating over soda. No. i may be regarded as 

 the type analysis of a specimen from Bolsena. With this the 

 analyses of vom Rath (No. 2) and Ricciardi (No. 3) agree 

 fairly well, the greatest difference being in the alkalies. In No. 

 I these are high, with potash greatly in excess of soda, while in 



'Jour. Geol., V, 1897. 



