THE VOLCANIC ROCKS OF ICELAND. M9 



16. 17. 18. 19. 20. 21. 



Silica .... 5005 47*48 49-17 47*69 49-37 47*07 

 Alumina and prot- 

 oxide of iron .30-47 31-22 3009 30*93 28-66 29-61 

 Lime .... 11-66 11-34 11-67 12-25 13-01 11-27 

 Magnesia . . . 5-20 6-47 6-82 5-83 7*52 9-50 

 Potash .... 0-38 0-60 1-67 0-48 0-20 0-58 

 Soda .... 2-24 2-89 0-58 2*82 1-24 1*97 



100-00 10000 100-00 10000 100-00 100-00 



22. 23. 24. 25. 26. 27. 28. 

 Silica . . . 75-91 77-92 75-29 78-95 76-42 76-38 75-77 

 Alum. & prot- 

 oxide of iron 13-62 13-33 15-54 13-13 14-67 15-12 14-14 

 Lime . . . 1-56 0-76 1-01 1-84 1-53 1*76 1-82 

 Magnesia . . 076 0-13 0-03 0-14 0-20 040 0-25 

 Potash . . . 5-64 3-27 5-42 1-76 194 1-88 2-46 

 Soda . . . 2-51 4-59 2-71 4-18 5-24 4-46 5-56 



100-00 100-00 10000 100-00 100-00 10000 100-00 



By taking the mean of these analyses, which form only a 

 small part of those which have been made, compositions are ob- 

 tained from which the results of the individual analyses differ 

 but little, and which may be regarded as the mean values of the 

 composition of the two liquid masses, the normal trachytic and 

 the normal pyroxenic, the two great sources from which have 

 originated, on the one hand, those Icelandic rocks containing the 

 largest proportion of acid, and on the other hand, those contain- 

 ing the largest proportion of bases : — 



29. 30. 



Normal trachytic Nonnal pyroxenic 



composition. composition. 



Silica 76-67 48-47 



Alumina and protoxide of iron . 14-23 30*16 



Lime 1-44 1187 



Magnesia 0-28 6*89 



Potash 3-20 0-65 



Soda 4-18 1-96 



100-00 100-00 



From these numbers we can at once calculate the mean re- 



