44 BUNSEN ON THE FORMATION OF 



the Esja mountains, opposite Mossfell, the conglomeratic pyro- 

 xenic rock is traversed by a trachyte dyke, which consists in its 

 interior of a pure white mass, becoming gradually darker and 

 more ferruginous towards the surrounding rock. This is but 

 one among many examples which might be brought forward. 

 The analysis of the mass from this dyke, without taking into ac- 

 count the water which it contained, in consequence of especial 

 circumstances, gave for the part in the centre of the dyke (36), 

 for that situated nearer to the saalband (37), and for that part 

 of the rock immediately surrounding the dyke (38) : — 



36. 37. 38. 



Silica 78-95 66-18 50*25 



Alumina .... 7*71 9*74 1255 



Protoxide of iron . 4*32 1205 16-13 



Lime 155 449 11-10 



Magnesia .... 0*42 3*04 7*59 



Potash 2-48 0*94 0*34 



Soda 4-57 3-56 2-04 



10000 100-00 100-00 



A comparison of these results with the above average compo- 

 tions, shows that the interior mass of the dyke has almost ex- 

 actly the composition of the pure normal trachytic rocks, while, 

 on the contrary, the surrounding mass has the composition of 

 the normal pyroxenic rocks ; and the part situated near to the 

 surrounding rock is an admixture of both in the proportion of 

 0*5923 pyroxenic mass to 1*0 trachyte, for which the following 

 calculated composition closely corresponds with that found by 



actual analysis : — 



^ 39. 



Found. Calculated. 



Silica 66-18 66*18 



Alumina and protoxide of iron . 21*79 20*15 



Lime -K^> 4*49 5-32 



Magnesia ; 3*04 2*74 



Potash 0-94 2*25 



Soda 3-56 3*36 



100-00 100-00 



Upon closer examination, indeed, the most distinct indications 

 are found of a lateral fusion of the neighbouring rock and the 



