THE VOLCANIC ROCKS OF ICELAND. 4? 



The order of the analyses corresponds with the succession of 

 the streams examined, from the more ancient to the younger. 

 They show that the flow from the two volcanic foci which main- 

 tain the activity of the volcano, is as irregular as this activity 

 itself. On the formation of the lava streams of Thjorsa the 

 pyroxenic focus alone acted, and in the formation of that at Hals 

 the action of the pyroxenic focus preponderated over that of the 

 trachytic. The contrary relation is observed in the Efrahvols 

 lava; and during the most recent eruption in the year 1845, the 

 action of the pyroxenic focus again preponderated over that of 

 the trachytic. 



The great resemblance which is everywhere found to exist 

 between the rocky masses poured out from volcanoes, jus- 

 tifies the conjecture that these processes of rock formation are 

 not limited to Iceland alone. A number of analyses of analo- 

 gous kinds of rocks from other localities, executed in my labo- 

 ratory, prove that this is really the case. The volcanic system 

 of the high table-land of Armenia is in this respect especially 

 remarkable. The mean results of analyses of rocks from that 

 locality, for which we are indebted to the excellent researches of 

 Abich, likewise give for the extreme acid member occurring there 

 precisely the same composition as the Icelandic. 



Normal trachytic The same from the high table- 



substance of lands of Armenia *. 



-^ Iceland. 47. 48. 49. 50. 



Silica 76-67 77*27 77*60 77*42 76-66 



Alumina and prot- 

 oxide of iron 

 Lime . 

 Magnesia 

 Potash . . 

 Soda . . . 

 Loss by ignition 



This resemblance is recognizable, however, not only in the 

 extreme acid member of the series, but is still more distinct 



14-23 14-14 13-72 14-83 15*17 



1-44 1-31 1-40 2-73 1-25 



0-28 ? ? ? ? 



3-20 2-44 2-301 . .^^ 2*94 



^\ 4-32 



4-18 4-15 4-21J 3-52 



000 0-51 0-61 0-57 1-12 



* Ueber die geolog. Natur des Armen. Hochlandes, von Dr. H. Abich, 

 Dorpat, 184.3. 



47. Brown obsidian from the Lesser Ararat. 48. Obsidian porphj'ry from 

 the Greater Ararat. 49. Obsidian from Kiotangdag. 50. Dioritic porphyry 

 from Besobdal. 



