THE VOLCANIC ROCKS OF ICELAND. 45 



substance of the dyke, gradually decreasing from the surface of 

 contact towards the central part of the dyke, and to which they 

 do not extend. These observations and experiments prove that 

 it is not necessary to regard the chemically intermediate mem- 

 bers between the extreme acid and basic members of the Ice- 

 landic group of rocks as resulting from simultaneous eruptions 

 from the two great plutonic foci, but that even the already formed 

 rocks may, during eruptions or injections, have furnished one 

 part of the material for the formation of mixed rocks. 



A confirmation of this law is likewise met with in applying 

 it to the rock formations upon the lava streams of the Icelandic 

 volcanoes. The final members, and the intermediate rocks re- 

 sulting from their mixture, are not absent even among these 

 most modern formations. The enormous lava streams which 

 have rent the palagonitic ranges of hills of Krafla and Leirh- 

 niikr, afford the most striking evidence of this, and Hecla not 

 less so. However, the law is in this instance not so distinctly 

 recognizable, because the volcanic mountains penetrated by 

 these lava streams consist of metamorphic products of the pala- 

 gonitic tuff, which, although it corresponds in composition with 

 the normal trap mass, is subject to greater variation in the pro- 

 portions of its constituents. The discrepancies accounted for 

 by this circumstance were rendered still greater by the impos- 

 sibility of obtaining for examination any other than hand spe- 

 cimens, which contained separate and not very uniformly mixed 

 felspathic masses. But notwithstanding these disadvantages, 

 the law is still found to obtain. 



The blackish gray stony lava of Hrafntinnuhryggr (40) on 

 the south-eastern foot of the Krafla, and the obsidian (41) al- 

 ternating within beds, have exactly the composition as the 

 normal trachytic mass. 



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