ITALIAN PETROLOGICAL SKETCHES 47 



It is also a Icucititc, and ol the same composition as the pre- 

 ceding". It is, however, of much finer structure and closely 

 resembles the leucitite of Sassi Lanciati near Bolsena. It carries 

 segregations of coarse-grained, granitic masses, made up of large 

 grains of green augite and a few well formed crystals of 

 olixine, with much leucite which shows excellent twinning 

 lamelLx, and which takes the place of the feldspar in typical 

 gabbros. Some cavities are filled with calcite. These segrega- 

 tions closely resemble the leucitic blocks briefly described by 

 Lacroix from Lake Bracciano, and may be compared with the 

 Missourite recently described by Pirsson.' 



THE TOLFA REGION. 



This region consists of a triangular group of hills, lying a 

 little more than lo*"" northeast of Civita Vecchia and some 20''" 

 west of Lake Bracciano. Its highest point is Monte Sassicari, 

 525"" above sea level. Since my visit of a few hours was con- 

 fined to the southern angle I will not go into the topography, 

 which moreover presents little of interest here. It may only 

 be noted that the hills are composed essentially of toscanite, 

 which in general is so extremely decomposed to alunite that a 

 fresh specimen of rock is indeed a vara avis. De Stefani points 

 out that the eruptions in all probability took place in post-Plio- 

 cene time, differing in this from Ponzi and others, who would 

 place them earlier. 



Petrography . — According to vom Rath there are two distinct 

 kinds of "trachyte" represented in the region. The one is a 

 "sanidine-oligoclase-trachyte" with light gray compact ground- 

 mass, in which lie phenocrysts of sanidine, oligoclase and bio- 

 tite. This is more abundant in the northeastern part of the 

 region, often shows a bed- like parting, and is comparatively little 

 subject to decomposition. The other is more like a pitchstone, 

 and is almost universally decomposed. Its groundmass is dark 

 brown and resinous, with phenocrvsts of sanidine, biotite and 



■ i'lRsso.N, Am. Jour. Sci., II, 315, iSgb. 



