256 HENRY S. WASHINGTON 



mum amount of this alkali. If the feldspar is all plagioclase, 

 as Bucca's description would indicate, there would be no min- 

 eral capable of taking up any notable amount of potash, as its 

 percentage in biotite is never over 10., and the amount of this 

 mineral in these rocks is small. 



Furthermore we find at Radicofani, which is on the same 

 main line, " basalts" containing very considerable orthoclase 

 along with the plagioclase, and showing in one instance : SiO^ 

 = 55.13 ; K30 = 2. 43, and Na30 = 2.07. Again Abich (^Z. cit. 

 page 114) gives the silica percentage of the "basalt" from the 

 foot of Santa Croce as 54.62, and classes it as a trachy-dolerite, 

 though a rather basic one resembling the dolerites, while the 

 rock of Monte Santa Croce is more acid and like the trachytes. 

 It seems then very probable that these "basalts " of Rocca Mon- 

 fina are in reality not normal plagioclase basalts, but rather 

 approach the ciminites in composition both mineralogically and 

 chemically. Henry S. Washington. 



