40 BUNSEN ON THE FORMATION OF 



lation between the oxygen of the acid and that of the bases, 

 which for the trachytic rock is as 3 : 0*596, and for the pyro- 

 xenic as 3 : 1*998. All the other unaltered rocks of Iceland 

 which do not immediately belong to these two groups have such 

 a composition, that the oxygen in their bases varies between 

 0*579 and 1*948 for 3 oxygen in the acid. There is strictly only 

 one relation met with, and that is such as necessarily belongs to 

 rocks consisting of intimate mixtures of these two extreme 

 members. Consequently the question directly suggests itself as 

 to whether the non-metamorphic rocks of Iceland have really 

 resulted from such an admixture of fused masses, or in other 

 words, whether there were only two separate foci, which have 

 maintained the plutonic activity there observed from its first 

 commencement up to the present day. This problem admits of 

 a direct solution. 



If we characterize by S the per-centage of silica in a mixed 

 rock, by s the per-centage of silica in the normal trachytic, and 

 by cr the per-centage of silica in the normal pyroxenic rocks, then 



S^ = « (1) 



in which equation a represents the quantity of normal py- 

 roxenic rock mass which must be mixed with one part of 

 normal trachytic rock mass in order to give the composition 

 of the mixed rock in question. This value a may indeed be cal- 

 culated from any one of the constituents of the rock : silica is 

 however selected for this purpose, as best adapted, because it 

 can be most accurately determined and is the least variable 

 constituent in the rocks. All the other constituents of the mixed 

 rocks are then determined by means of a. For if the weight of 

 the separate constituents in one part of the normal pyroxenic 

 rock mass is taken as PoPi'^Pm and in the same manner the 

 weight of the separate constituents in the unity of weight of the 

 normal trachytic rock mass as Iq ^i...t„, the weight of all the 

 other constituents of the mixed rock may be ascertained by 

 means of the equation 



^~ (a+1) -^ (a+1) +-"(^Tiy • • ^^^ 

 If, therefore, it is really the case that there exist in Iceland 



