/ TA LI A N PE TR OL O GICA L SKE TCHES 4 1 



altered biotite, as may also be the case with part of the diopside 

 microlites. An analysis of this rock by Dr. Rohrig is given in 

 Table I, No. i. It is seen to be as high in silica as a trachyte, 

 low in alumina, with rather andesitic amounts of lime and mag- 

 nesia, and in alkalies standing between the two groups, though the 

 relative amounts of potash and soda are not what we might expect. 

 The rock just described, as well as similar ones from other 

 localities in the region, are called by Tittoni "trachytic retin- 

 ites." Bucca describes them as augite-andesites, with which 

 name his descriptions agree very well, though it seems to me, as 

 it does to de Stefani, that he unreasonably neglects the abundant 

 sanidines and quartz. He speaks of hypersthene as abundant, 

 and gives extinction angles for the plagioclase' from which he 

 concludes that they must be very basic — "from labradorite to 

 anorthite." According to him orthoclase only occurs as the 

 laro-e tabular phenocrysts, which is certainly not the case in my 

 specimens. 



Leiicitite .—l:\\& leucitic rocks collected by me belong to three 

 groups, leucitite, leucite-tephrite, and leucite-phonolite. Speci- 

 mens of the first were obtained south of Lake Bracciano, from 

 a flow at Crocicchie, from a similar, or the same, flow about one 

 kilometer west of this and from the quarry at L'Uomo Morto 

 southwest of the lake. A similar rock was seen also at Oriolo, 

 northwest of the lake, but I have no specimen of it. They are 

 all compact, dark gray, basaltic looking rocks, with some fresh 

 well-shaped leucite phenocrysts (0.2 to i '="^) and a few smaller 

 ones of augite. 



Under the microscope the phenocrysts present no remarkable 

 features. The leucites are clear, somewhat cracked, show quite 

 strong double refraction, and contain few inclusions. The 

 augites are well shaped, pale green and not pleochroic, generally 

 darker toward the center, and with an extinction angle of about 

 42°. Some are seen with a fringe of later dark green augite at 

 the ends, which extinguishes at an angle of 50^ and includes 

 some magnetite grains. 



'He says that they vary from 35" to 59°. There seems to be some error here. 



