50 BUNSEN ON THE FORMATION OF 



A somewhat decomposed porous Rockfro^n Ararat. 



58. 

 Found. Calculated. 



Silica 65-39 65 39 



Alumina and protoxide of iron . 22*65 20*61 



Lime 7*40 5*61 



Magnesia 3*00 2-92 



Potash and Soda 1*56 5*47 



100*00 100-00 



V ^ > 



1 trachyte and 0*6666 pyroxenic mass. 



Unknown Crystalline Rock from between Keschet and Kobi, 



59. 



Found. Calculated. 



Silica 61*25 61*25 



Alumina and protoxide of iron . 25*72 22*95 



Lime 6*27 7*14 



Magnesia . 3*77 3*89 



Potash and Soda 2*99 4*77 



100*00 10000 



V ^ > 



1 trachyte and 1-207 pyroxenic mass. 



Abich mentions only one variety of rock which approximates 

 nearer in its composition to the normal pyroxenic substance. 

 This is an amygdaloid basalt from Ararat, which may be regarded 

 as consisting of 1 trachyte and 3*427 pyroxenic substance, thus: 



60. 



Found. Calculated. 



Silica 54*84 54*84 



Alumina and protoxide of iron . 27'96 26*56 



Lime 9*28 9*51 



Magnesia 3'72 5*39 



Potash and Soda * 4*20 3*70 



100*00 100*00 



While, therefore, on the one hand, the composition of the Cau- 

 casian rocks may be deduced (their per-centage of silica being 

 known) from the composition of the Icelandic rocks, the Cau- 



• The alkalies are here estimated from the deficiency. 



