ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 351 



Bunsen and Strang are inclined to extend the theory of the 

 normal trachytic and basaltic magmas, mentioned in the first part 

 of this paper, so as to include the plutonio rocks, and to maintain 

 that granites, porphyries and the older eruptive rocks are capable 

 of being also regarded as mixtures of the two hypothetical melted 

 masses. Of course the same objections which apply to this 

 theory so far as basaltic and trachytic rocks are concerned, apply 

 also in the case of the older rocks. On the other hand von 

 Waltershausen's theory, also previously described, furnishes a 

 complete explanation of the cause of the more siliceous character 

 of the older rocks. The same increase of density and of basic 

 constituents, which he supposes now to take place from the surface 

 to the centre of the earth, existed in the oldest geological periods. 

 The fused material, which, on breaking through the earth's crust 

 solidified to granite, was the uppermost concentric layer then 

 existing. It was lightest in weight and richest in silica. 

 Beneath it lay successive layers graduating into each other, and 

 with the depth increasing in density and basic constituents. But 

 according to this theory, the magmas from which granites, porphy- 

 ries, and trachytes resulted ought to have had a position nearer 

 the surface than the fused matter which on its eruption yielded 

 syenites, greenstones, melaphyres, and basalts. Hence the former 

 rocks ought to have been the first to appear upon the earth's sur- 

 face. Porphyries ought to have preceded syenites, and trachytes 

 ought to have broken through the earth's crust and solidified 

 prior not only to basalt but to greenstone and melaphyre. This 

 is, however, not the case, and Sartorius von Waltershausen fully 

 appreciates the difficulty, mentioning that the trachyte of Esia 

 near Reikjavik, " which according to its mineralogical character 

 " belongs to a higher-lying zone nevertheless intersects in the 

 " form of a vein the strata of Iclandic trap, which in general 

 " originate from deeper regions.""' To explain the difficulty he 

 resorts to the theory " that the earth's crust possesses different 

 " thicknesses in different places, or that the surface of separation 

 " between the already solidified and the still fluid masses represents 

 " a relief turned inwards, of mountains and valleys. Now where 

 " such a mountain accidentally reaches down into greater depths, 

 " melted masses might be able, through fissures in it, prematurely 

 " to escape to the surface, which masses might be broken through 

 " later by rocks of higher zones which had remained longer fluid. 



* Die Vulk^nische Gesteine, etc. p. 337. 



