THE VOLCANIC ROCKS OF ICELAND. 



41 



only two great piutonic foci which have supported the volcanic 

 action since its first commencement to the present time, and if 

 the contents of these foci have respectively the compositions be- 

 longing to the above-considered extreme members, then the 

 composition of all the non-metamorphic rocks of Iceland may 

 be determined by means of one constituent, and most advan- 

 tageously by the per-centage of silica. 



Pyroxenic rock '^ 

 to 1 part of > 

 trachyte ... J 



Silica 



Alumina and prot- 

 oxide of iron. . . 



Lime 



Magnesia 



Potash 



Soda 



0*000 



76-67 



14-23 

 1-44 

 0-28 

 3-20 

 4 



76-00 



14'6l 

 1 



0-43 

 3-14 

 4-14 



0-106 



74-00 



15-73 

 2-43 

 0-91 

 2-96 

 3-97 



100-0 



0*199 



0-310 



72-00 



16-86 

 3*17 

 1-38 

 2-78 

 3-81 



70-00 



18-00 

 3-90 

 1 84 



100-0 100-0 



I 



0-444 0-609 



68-00 



1913 

 4-65 

 2-31 

 2-42 

 3-49 



66-00 



20-26 

 5-38 

 2-78 

 2-23 

 3-36 



64-00 



21-39 

 6-13 

 3-25 

 2-05 

 3*18 



100-0 



1-084 1-446,1*959 



-00,60-00.58-00 



22-52 23-65 24-78 



6-87 



3-72 

 1-87 

 3-02 



100-0 



7-61 

 4-19 

 1-69 



4-65 

 1-51 



2-86, 2-71 



,100-0 lOO'O 

 I I 



2-745 4-099 



-00 54-00 52-00 



50-00 



25*92 27-04 '28- 17 29*29 

 9*08 9'83 10-5711-31 



17-43 



5-12' 5-69 

 1*33| 1*15 

 2-55 2-39 



6-06 6-53 

 0-96 0-79 

 2*24 2-08 



100-0 1000 100-0 100-0 



I I I 



48-47 



30-16 

 11-87 



6- 



0- 



196 



100-0 



If these numbers should be found to correspond with the 

 composition of all the original varieties of Icelandic rocks which 

 have not yet undergone a chemical metamorphism, it may be 

 considered as proved that they are all either merely mixtures of 

 the above-mentioned acid and basic extreme members, or those 

 members themselves ; and that the great mineralogical and pe- 

 trographic ditferences which these rocks present, are only con- 

 sequences of the varying proportions in which the mixture took 

 place, and of the prevailing physical conditions under which the 

 rocks reached their present situation and assumed their present 

 form. Among the large number of analyses of Icelandic rocks 

 which have been carried out in my laboratory, there is not one 

 in which the results differ from the calculated composition ob- 

 tained in the above-mentioned manner, to a greater extent than 

 might reasonably be expected in such a calculation, based merely 

 upon the mean results of analyses. The following experimental 

 and calculated results were obtained for a granular whitish gray 

 trachyte, containing hornblende and orthoclase crystals from 

 Oexnadalr in the Norden Islands ; — 



