} 



2*13 1-32 2-60 2*91 5-10 3-59 3-85 



36 BUNSEX ON THE FORMATION OF 



granular structure, and a somewhat earthy compact appearance. 

 5 is a chnkstone from Falkaklettur, near Kalmanstunga, in 

 separate slabs, consisting of a compact, grayish-brown, dull mass, 

 speckled or striped with peroxide of iron, and in which there 

 are here and there traces of felspathic concretions. 6 is a black- 

 ish-gray, homogeneous, stony lava, with a great number of small 

 irregular cavities. It is apparently of very recent formation, 

 and occurs at Hrafntinnuhrygyr, near Krafla, at the far north- 

 east of Iceland. 7 is a black homogeneous obsidian, possessing 

 the characters peculiar to this rock, and alternating in beds with 

 the above lava : — 



1. 2. 3. 4» 5. 6. 7. 



Silica . . 75-91 77*92 75-29 78*95 76-42 76*38 75*77 



Alumina . 11*49 12*01 12*94 10*22 9*57 11*53 10*29 

 Protoxide 



of iron 



Lime . . 1*56 0'76 1*01 1*84 1*53 1*76 1*82 



Magnesia . 0*76 0-13 0*03 0-14 0*20 0*40 0*25 



Potash. . 5*64 3*27 5*42 1*76 1*94 1*88 2*46 



Soda . . 2*51 4*59 2*71 4*18 5*24 4*46 5*56 



100*00 100*00 100*00 100*00 100*00 100-00 100*00 



Universally- occurring non-metamorphic rocks, containing 

 more silicic acid than these, are not met with in Iceland. 



The normal-pyroxenic rocks, which as basic silicates of alu- 

 mina and protoxide of iron, in combination with lime, magnesia, 

 potash and soda, form the most extreme members towards the 

 opposite end of the series, present a similar correspondence in 

 their average composition. Here likewise again we find that 

 rocks belonging to the most different periods possess the same 

 mean composition, as is shown by the following analyses. 8 gives 

 the composition of the older coarse, granular, stratified trap rock 

 of Esjagebirge, which, when it alternates with tuff and trap con- 

 glomerate, is generally found in immense columns situated per- 

 pendicularly to the stratification. 9 is a similar, but somewhat 

 more compact rock, constituting the north-eastern coast cliffs of 

 the island Vidhey. 10 is a compact, lighter- coloured, fine, gra- 

 nular, massive basaltic rock, which traverses the palogonite tuff 

 of the Hagafjall on the right bank of the Thjorsa, near Storinupr. 



