THE VOLCANOES AND ROCKS OF PANTELLERIA 27 



only rarely contain soda-lime feldspars, the felsic minerals being 

 almost without exception alkali feldspars, with or without quartz 

 or nephelite. Corresponding to this, the norms of such rocks seldom 

 show anorthite-, the apparent presence of this in some cases being 

 certainly due to a too high figure for alumina because of the non- 

 determination of titanium and phosphorus. The norms of such 

 rocks very frequently show acmite, and less often sodium metasili- 

 cate, or would show it were the alumina correctly low, while acmite 

 seldom occurs in the norms of any but such sodic pyroxene and 

 amphibole-bearing rocks. 



