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so radical a change in chemical composition. These inclusions 

 must, by this theory, be considered to be fragments of older 

 rocks, formed in this way. Still basic inclusions may be sup- 

 posed to have been formed by mechanical agglomeration, and no 

 doubt this has often been the case. But, in opposition to both 

 these theories, it is in many cases evident that the inclusions 

 were soft, and then the simplest view is that they were drops, or 

 portions, of a partial magma, which at the temperature, existing 

 immediately before crystallization, could no longer be held in 

 solution by the principal magma, but separated out, 



The great petrographical province of Iceland is characterized 

 principally by enormous eruptions of plagioclase-basalts and 

 exceedingly subordinate eruptions of rhyolites, which, however, 

 are very numerous. No other eruptive rocks are known from 

 Iceland up to this time. 1 If we considered the differentiation of 

 the primary magma, which here was very basic, as a diffusion- 

 phenomenon, according to "Soret's principle," it would be incom- 

 prehensible why the differentiation never stopped with the pro- 

 duction of an intermediate magma, and, moreover, this theory 

 would demand that every little rhyolite-magma previous to the 

 eruptions would have been surrounded by a broad zone, showing 

 all transitions to the basaltic magma. In both cases these inter- 

 mediate magmas should have been erupted at some time, but, 

 as already mentioned, we know a hundred eruptions of rhyolite 

 but not a single one of andesitic rocks. It therefore seems more 

 probable that these intermediate magmas never existed in the 

 petrographical province of Iceland, but that the acid partial 

 magmas were separated out directly from the basic original 

 magma, which by lowering temperature lost its homogeneity. 

 The conditions of temperature and pressure being different in 

 different places these acid partial magmas also became somewhat 

 different, but may all be classified as soda-rhyolites. The 

 chemical compounds, which constitute the silicate magmas — 

 and which are not necessarily identical with the rock-forming 



1 Refer to H. Backstrom : " Beitrage zur Kenntniss der islandischen Liparite " in 

 Geol. Foren. Forh. 13, 667. (Stockholm, 1891). 



