THE VOLCANIC ROCKS OP ICELAND. 97 



a compact trap rock alternating -with zeolitic, amygdaloid and 

 palagonitic tuff. The trap, which belongs to the gray coarse- 

 grained variety occurring all over Iceland, passes very gradually 

 into a tough, bluish-gray, zeolitic amygdaloid, presenting an 

 almost earthy appearance, and whose substance consists, to the 

 amount of almost one-third, of crystal druses and rough masses 

 of chabasite. No perceptible line of separation between the two 

 rocks can be detected in the fissures and imbedded masses. 

 When the homogeneous mixture of this amygdaloid admits of 

 an average composition being ascertained by analysis, it is found 

 to have exactly the constitution of the trap into which it passes ; 

 and it follows from the accompanying analyses, calculated for 

 anhydrous substance, that both rocks consist of pure normal 

 pyroxenic or palagonitic substance. 



116. 117. 



Trap. Amygdaloid. 



Silica 49-87 49-60 



Alumina 14-66 13-98 



Protoxide of iron . . 13*57 14*60 



Lime 12-56 11*78 



Magnesia 6-55 6*90 



Potash ...... 0-42 0*22 



Soda 2*37 2-92 



100-00 100-00 



An equally gradual transition may be observed from the 

 amygdaloid into a red, friable, argillaceous bed, only a few feet 

 thick, but extending for miles round, the substance of which, 

 under the microscope, appears as an altered amygdaloid, in 

 which the unaltered chabasite is found with all its characteristic 

 peculiarities of distribution and concretion, surrounded by the 

 decomposed rock. The succeeding beds of palagonitic tuff are 

 again connected by gradual transitions throughout all the phases 

 of a progressive decomposition in the most intimate manner with 

 the above argillaceous bed, in such a manner, indeed, that separate 

 concretions may be traced from the sohd trap rock down into 

 the unaltered beds of tuff. This metamorphosis, which may be 

 imitated on a small scale with any piece of palagonite, has here 

 been effected on a large scale in Nature, at the time when the 

 erupted melted trap caused the fusion of the underlying pala- 



SCIEN. MEM.— Aa^ PhJL Vol. I. Part I. H 



