46 BUNSEN ON THE FORMATION OF 



The streams of obsidian occurring on the north-eastern de- 

 clivity of Hecla approach very closely in composition to the 

 above. They represent a mixture of 1*0 trachyte and 0*2325 

 pyroxenic substance. 



42. 



Found. Calculated. 



Silica 71-35 71*35 



Alumina and protoxide of iron . 17'33 17*32 



Lime 1-24 3-41 



Magnesia 0*19 1*53 



Potash 4-23 2*73 



Soda 5-66 3'75 



100-00 100-00 



The extreme basic member is represented in the substance 

 of the great lava streams which has flowed from this volcano in a 

 W.N.W. direction to the banks of the Thjorsa. 



43. 

 Found. Calculated. 



Silica 49-60 48-47 



Alumina and protoxide of iron . 28-81 30-16 



Lime 13*07 11*87 



Magnesia 7-56 6-89 



Potash 0-20 0-65 



Soda 1*24 1-96 



100-00 100-00 



The lavas from the western foot of Hecla, examined in my 

 laboratory by Dr. Genth, are represented as resulting from the 

 mixture of the two extreme members by the following analyses: 



Lava at Hals. Efrahvols lava. Hecla ash of 1845. 



44. 45. 46. 

 Found. Calc. Found. Calc. Found. Calc. 

 Silica .... 55-95 55-95 59*45 59-45 56-76 56*76 

 Alumina and prot- 

 oxide of iron .29-83 25-93 27-68 23*96 27*47 25*48 

 Lime .... 6-54 9*10 5-50 7*80 6*75 8*79 

 Magnesia . . . 4*21 5*14 2*38 4*32 4*04 4*95 

 Potash .... 0-96 1*33 1*48 1*65 2*63 1*40 

 Soda .... 2-51 2-55 3-56 2*82 2*35 2*62 



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



1 trachyte+2-77 1 trachyte+ 1*568 1 trachyte-|-2-402 



pyroxenic mass. pyroxenic mass. pyroxenic mass. 



