THE VOLCANIC ROCKS OF ICELAND. 51 



casian rocks, on the other hand, afford a possibility of ascertain- 

 ing, by means of calculation from the extreme acid members 

 which concur in conjunction with the accompanying basic sub- 

 stances, the composition of the pyroxenic substance which 

 filled the non-trachy tic focus of volcanic rock-formations in Ice- 

 land. The various constituents of such a substance may be 

 easily ascertained by means of the following equation derived 

 from (2), 



a 



-Pny 



in which A„ represents, in per cents, the separate constituents of 

 a mixed rock. This formula gives, for reasons which may readily 

 be understood, results which are more accurate, in proportion 

 as An differs more from the numerical value of the normal tra- 

 chytic composition. This is the case only with the analysis (60). 

 If therefore from this the value of Pn is calculated, we obtain — 



Caucasus. Iceland. 



Calculated. Found. 



Silica 48-47 48-47 



Alumina and protoxide of iron . 31*97 30*16 



Lime 11*56 11*87 



Magnesia 4*72 6*89 



AlkaUes 3-28 2*61 



100-00 10000 



It cannot therefore be doubted that both the extensive vol- 

 canic elevations constituting the high table-land of Armenia and 

 the island Iceland have flowed from sources which were chemi- 

 cally identical. The idea that perhaps all the volcanic forma- 

 tions on the earth's surface have originated from the same source, 

 or even, indeed, that it is from this very source that all the ferrugi- 

 nous and non-ferruginous plutonic rocks have originated by fu- 

 sion together, is rendered the less improbable by the fact that the 

 mineralogical differences existing between those Caucasian and 

 Icelandic rocks which present the same mean composition, are 

 not less marked than those observed among other ferruginous 

 rocks of plutonic origin. It would therefore be very interesting 

 to trace the genetic relations existing among the more ancient 



E2 



