THE VOLCANIC ROCKS OF ICELAND. 57 



examined^ calculated for anhydrous substance and protoxide of 

 iron : — 



Laugar- 

 Selia- Troll- Gala- Reykm- vatn- Krfsu- Naefr- Fosb- 

 dair. konugil. pagos. hildh. shellir. vik. holt. vogr. Laxa. 



Silica 48-29 M-20 5018 4969 6071 4763 46-29 47-78 51-36 



Alumina 14-41 10-58 1718 15-01 13-55 1708 10-30 15-55 13-53 



Protoxide of iron... 16-47 2034 11-85 1740 1544 15-53 21-30 1417 18-27 



Lime 11-31 1085 9-94 1001 1075 814 958 1009 6-91 



Magnesia 7-79 5-70 8-68 6-83 789 8-95 8-64 9-39 776 



Potash 0-88 0-55 0-93 0-35 0-81 0-52 111 1-61 217 



Soda 0-85 0-78 1-24 0-71 076 2-15 2-78 1-41 000 



100-00 10000 100-00 100-00 100-00 100-00 100-00 10000 100 00 



Oxygen in SiO^ and the bases 



3:2-032 3: 1-703 3 : r992 3: 1-906 3:1-835 3:2-178 3:2-039 3:2*099 3:1-746 



Silica 48-29 51-20 5018 49-69 50-71 47-63 4629 4778 51-36 



Alumina and prot- 

 oxide of iron 30-88 30-92 29-03 32-41 28-99 32-61 3160 29 72 31-80 



Lime 11-31 10-85 9-94 1001 10-75 814 958 1009 6-91 



Magnesia 779 5-70 8-68 6-83 7*98 8-95 8-64 939 7'76 



Potash 0-88 0-55 0-93 0-35 0-81 0-52 Ml 161 217 



Soda 0-85 0-78 1-24 0-71 0-76 215 2-78 1-41 000 



100-00 100-00 100-00 100-00 100-00 10000 100-00 100-00 100-00 



These analyses lead to the unexpected result, that the sub- 

 stance of the melted palagonitic silicates almost exactly corre- 

 sponds in its composition with the normal pyroxenic rocks, with 

 the single exception of a somewhat greater variation here and 

 there in the relative proportions of the several constituents. In 

 fact, the following mean composition, deduced from all these ana- 

 lyses, scarcely shows any perceptible difference from that of the 

 normal pyroxenic rocks, when we overlook the trifling excess 

 in the quantity of magnesia and the somewhat smaller quantity 

 of lime : — 



Normal pyroxenic 

 Palagonite. substance. 



Silica 49-24 48-47 



Alumina and protoxide of iron . 30-82 30-16 



Lime 9-73 11-87 



Magnesia 7*97 6-89 



Potash 0-99 065 



Soda 1-34 1-96 



100-00 10000 



The proportion of the oxygen in the acid to that in the bases 

 is for palagonite as 3 : 1*948, and for the pyroxenic rock as 



