20 HENRY S. WASHINGTON 



in which, so far as possible, analyses by both of us of rocks from the 

 same locality or flow are incorporated. 



Foerstner's ferric oxide, lime, and soda are uniformly, and often 

 much, higher than mine, except for the lime and soda in the basalts, 

 and the tendency of his magnesia is also to run higher. His alumina 

 is distinctly higher than mine in the trachytes, while in the pantel- 

 lerites it is generally lower, as it would also be in his basalts were 

 the 4 or 5 per cent of titanium dioxide and phosphoric oxide present 

 subtracted. His potash is markedly lower in the salic group. Only 

 his figures for silica and ferrous oxides run about the same as mine, 

 though even here some great discrepancies are to be noted. 



These differences, especially those in ferric oxide, lime, soda, 

 potash, and alumina, are very striking, and so uniformly in the 

 same respective directions as to suggest systematic errors in his ana- 

 lytical methods as the explanation. What these have been it is 

 difficult, if not impossible, to decide definitely, but, without enter- 

 ing into a detailed discussion, they may be ascribed to the inade- 

 quate methods prevailing at the time (prior to 1883), and probably 

 in part to impurities in his reagents. It will be noted that his 

 latest analysis, that of the lava of i8gi, most closely resembles the 

 corresponding one of mine. 



Whatever be the explanation, the discrepancies here pointed 

 out are, for the most part, so serious and so systematic, and the 

 incompleteness so marked, especially as regards titanium, phos- 

 phorus, and water, that Foerstner's analyses of the Pantellerian 

 rocks must be considered as of inferior quality and doubtful utility. 



Modal characters. — Leaving the basalts out of consideration the 

 lavas of Pantelleria show some very striking modal characteristics. 

 The small amount of quartz, considering the silicity of the rocks, 

 and its great rarity as phenocrysts, are very unusual. Its small 

 amount is, of course, due to the abundance of alkali feldspar and 

 of aegirite. As NaaO in aegirite takes up four times its amount 

 of siHca (molecularly) , much less silica is left uncombined than 

 would be the case in a rock of the same silicity but carrying only 

 ordinary pyroxenes in which the ratio of RO to SiOa is i to i. 



The exclusively alkalic character of the feldspars, and the 

 absolute lack of nephelite and soda-lime plagioclase are very strik- 



