/ TA LI A N PE TR OL O GICA L SKE TCHES 253 



medium gray compact rock showing small biotitc, augite, and 

 feldspar phenocrysts. In thin sections the phenocrysts of alkali 

 feldspar and basic plagioclase are present in about equal amounts. 

 They contain many glass inclusions. The biotite phenocrysts 

 are all deeply altered, and a few large augites met with show no 

 noteworthy features. The groundmass is trachytic and com- 

 posed of flakes and laths of alkali feldspar, with laths of plagio- 

 clase, diopside needles, and magnetite grains. There is little or 

 no glass present. 



Rocks which represent transitional forms between the above 

 and true trachytes are found in specimens from the ravine at 

 Molino di Casa Fredda, from a flow northwest of Tuoro, near 

 Teano, and from a loose block in the valley of Casi. They 

 resemble the rocks already described, but plagioclase is much 

 less abundant, and in some almost entirely wanting. The 

 groundmass of the Casi rock is much darker and finer grained 

 than in the other cases ; feldspar phenocrysts are abundant, but 

 few of ferromagnesian minerals are seen. Some good examples 

 of microperthite seen in it indicate that the alkali feldspar con- 

 tains considerable soda. In the specimen from Tuoro a note- 

 worthy feature is the presence in one slide of a single, large, 

 well-shaped crystal of colorless diallage, showing its character- 

 istic dusty inclusions and parting parallel to the orthopinacoid 

 These, as well as other rocks similar to these, are all described 

 by Bucca under the head of trachytes, his descriptions agreeing 

 very closely with the characters of my specimens. He closes, 

 however, with the significant remark (p. 257) that they 

 "approach so closely to the andesites that I was tempted sev- 

 eral times to refer them to these." 



Trachyte. — A rock from a small quarry on the road to 

 Conca, on the northern outer flank of the volcano, belongs to 

 the trachytes proper rather than the vulsinites. It is the same 

 as a rock described by Bucca (p. 255), whose description agrees 

 very well with the characters of my specimens. The main mass 

 of the rock is compact and light gray, showing few phenocrysts 

 of augite and feldspar. Scattered through this are rounded 



