34 BUNSEN ON THE FORMATION OF 



constituents of which have not yet passed into the state of defi- 

 nite compounds, or are grouped together in crystalline aggre- 

 gates without any matrix. Such purely crystalline aggregates 

 are not abundant among the products belonging to the most 

 recent eruptive periods. Among these are found far more fre- 

 quently such rocks as consist of an amorphous and a crystalline 

 mass, the former corresponding with the solvent, and the latter 

 with the dissolved parts of the solidifying eruptive mass. The 

 petrographic character which manifests itself in this condition of 

 the existing rocks, depends immediately and essentially upon the 

 chemical constitution of the original homogeneous mass of liquid 

 silicates. The mere investigation of the minerals which have 

 been formed in the rock cannot lead to any knowledge of the 

 constitution of the mass itself, which has wholly or partially 

 separated into crystalline minerals. If, therefore, we are desirous 

 not to restrict ourselves to a mere natural history description of 

 the different varieties of rocks, but to go back to the actual con- 

 ditions of their origin, it is evident that the average composition 

 of a rock, without reference to the more recently formed minerals 

 which it may contain, can alone constitute the starting-point of 

 such an investigation. The extensive volcanic system situated 

 near the northern polar circle, and constituting the island of 

 Iceland, affords the best opportunity of obtaining these data, 

 because it is not associated with any calcareous or siliceous 

 sedimentary beds which could have exercised any disturbing 

 influence upon the normal composition of the eruptive volcanic 

 rocks, and because these rocks, which in turn present themselves 

 as the representatives of universally-occurring formations, are for 

 the most part amorphous, or at least almost always bear the 

 stamp of such a uniform and intimate mixture as to render pos- 

 sible the determination of their average composition. 



Although the great diversity displayed in the mineralogical 

 and chemical composition of the non-metamorphic rocks of Ice- 

 land would appear at first sight to exclude any idea of a recog- 

 nizable uniformity in their origin, still there is one character 

 which is found, upon more close observation, to connect together 

 in the most intimate manner all these products, from the most 

 recent lavas to the most ancient eruptive masses, however widely 

 different their mineralogical constitution may be. 



