56 BUNSKN OX THE FORMATION OF 



Galapagos, Dyke Mass. 



69. Oxygen found. Oxygen calculated. 



Silica .... 37-83 1964 19-24 



Alumina . . . 12-95 \ ^.^3 g.^g 



Protoxide of iron 9-93 J 

 Lime .... 7-49 -| 



Magnesia . . . 6-54 I ^,^ ^.^^ 



Potash .... 0-94 f 

 Soda .... 070 J 

 Water .... 23-00 

 Foreign residue . 0-96 



100-.34 

 Galapagos, Rock forming a Crater. 



100-00 



The analysis of the cementing matter of tuffs, occurring upon 

 the Cape Verde Islands, the Azores, Canaries, and in the basaltic 

 formations of Germany, yielded results corresponding perfectly 

 with the above. 



The frequent occurrence of these rocks, and their alternation 

 with the unaltered volcanic rocks by which they have been 

 penetrated, indicates that the palagonitic tuff rocks have essen- 

 tially contributed to the formation of the trachyto-pyroxenic 

 mixed members of this group, and a closer examination of the 

 phaenomena of plutonic contact proves that such is really the 

 case. The influence of the tuffs upon the formation of these 

 rocks consequently demands an especial investigation. The fol- 

 lowing analyses give the composition of the varieties of tuff 



