THE VOLCANOES AND ROCKS OF PANTELLERIA 



23 



cussed this point, and points out that the great majority of the 

 normative mineral molecules are those that Marker himself would 

 necessarily choose. 



In many of the rocks now under discussion accurate chemical 

 analysis shows that more than enough soda is present to combine 

 with silica and alumina, or with silica and ferric oxide, as potential 

 or actual albite or aegirite respectively. This excess of soda is an 

 important chemical feature of the rocks and, as among rock-making 

 minerals we find soda always as silicates (except in the sodalite 

 group) , it is natural and justifiable to state this excess in the norm 

 as NaaSiOj, of course without the implication that a mineral of this 

 composition actually exists. When we find, as we do here, that the 

 rocks showing this sodium metasilicate in the norm are likewise 

 rich in cossyrite, and that this mineral itself contains a large excess 

 of soda, presumably as metasilicate, the procedure adopted seems 

 amply justified. A similar reasoning applies to the other " unnatu- 

 ral minerals" objected to by Marker, kaliophilite, akermanite, wol- 

 lastonite, and potassium metasilicate. The last of these, by the 

 way, is present in the norms of only three rock analyses. 



TABLE IV 

 Averages of Rock Types of Pantelleria 



Si02. . 

 AI3O3 

 FeaOj 

 FeO.. 

 MgO 

 CaO. 

 Na^O, 

 K3O.. 

 TiO. 



PA- 



MnO 



la 



66.85 

 11.82 



5-29 

 2 .gi 



0.73 

 1 .04 



5-32 

 4.71 

 0.96 

 o. 16 



O. 21 



I 



69.30 

 II .02 



0.78 



5-25 

 4.69 



o. 14 



lb 



71-75 

 10.23 



4-94 

 1-33 

 0.25 

 0.52 

 5. 18 

 4.67 

 0.80 

 o. 12 

 o. 21 



Ila 



63.46 



16.36 



2 . 29 



26 

 0.81 



1.66 

 6. 55 

 4-37 

 0-95 

 0.25 

 0.04 



II 



66.27 



1347 

 2. 20 

 4.38 

 o. s6 



■32 

 .60 

 .90 

 .18 



116 



69.08 



10.58 



2 . II 



5 50 



0.31 



0-37 

 6. 10 

 4.82 

 0.85 



O. II 



o. 17 



III 



46.09 



12.77 



2-95 

 9.99 

 6. 17 

 9.82 



3-43 

 1. 16 



591 

 151 

 o. 20 



Succession of magmas. — The change in composition of the 

 magma during the successive eruptive phases offers some features 

 of interest. The averages of the analyses of the several types — each 

 representing a distinct volcanic episode — are shown in Table IV. 



