48 HENRY S. WASHINGTON 



augite. To these Busatti adds a third variety, though his 

 description hardly seems to warrant the separation. 



I am inclined to agree with de Stefani in thinking that there 

 is only one kind of rock, and that the differences noted are of 

 very small importance. From vom Rath's analysis of the pitch- 

 stone-like trachyte and from mine of the rock from Tolfa it will 

 be seen that the two varieties closely resemble each other 

 chemically and that the rock belongs to the group of Toscanites 

 as already defined. 



The only fresh specimens which I obtained were collected at 

 the hill on which stands the old castle above the village of Tolfa. 

 In all other places I found it so decomposed as to be absolutely 

 worthless for petrographical study. The groundmass is very 

 compact and bluish gray, and speckled with minute black spots 

 of augite. Very many large glassy feldspar phenocrysts, which 

 often show twinning, with some biotites are scattered through it. 



Under the microscope this rock presents an appearance 

 almost identical with that of the Monte Cucco rock. The rather 

 abundant plagioclase is an acid labradorite having the composition 

 Kh^ Auj, as determined by the extinction angles (22° and 23°) 

 of the albite-twinned lamellae of the two individuals of a Carls- 

 bad twin.^ The feldspar phenocrysts are quite often corroded. 

 The hyalopilitic groundmass has a colorless glass base, which is 

 thickly crowded with diopside prisms and laths, rather than 

 keraunoids of orthoclase. 



De Stefani'' mentions a leucitic rock which he obtained at the 

 base of Monte Elceto between the toscanite and the underlying 

 Cretaceous rocks. Rosenbusch, to whom a specimen was sent 

 for examination, reports that it is a "leucitite perfectly identical 

 with the leucitic rocks of Albano and the vicinity of Rome." 



^Busatti states that the plaglolase is oligoclase, but his determination seems to 

 rest on insufficient data, as he only mentions its " polysynthetic structure " but gives 

 no angles, as neither does vom Rath. ^ 



= De Stefani, Boll. Com. Geol. Ital., 1888, 224. 



