312 ORIGIN OF ERUPTIVE AND PRIMARY ROOKS. 



The mean of these analyses is : 



Silica 48.472 



Alumina.. / 14.781 



Ferrous oxide I 15.3S3 



Lime ) 11.866 51.528 



Magnesia j 6.890 



Soda / 1.957 



Potash I 0.651 



100.000 



This Bunsen assumes to be the composition of the normal 

 pyroxenic mass, which fills the second supposed reservoir of igne- 

 ous fluid material in the centre of the earth. He further argues 

 that all volcanic rocks, that is to say rocks belonging to the 

 trachytic, basaltic or lava eruptive formations, may be regarded 

 as mixtures of these two fluid materials, and shews that after 

 merely determining how much silica they contain, it can be 

 ascertained by calculation in what proportions these two materials 

 from the different reservoirs are present. With regard to this 

 theory Sartorius von Waitershausen remarks : " It is evident 

 "that this average (that of the normal pyroxenic mass) can 

 "just as little be regarded as the limit on the basic side, as the 

 " so called normal trachytic average on the other. Nor is it 

 " apparent why the above-mentioned six analyses only were used 

 " in computing the average, while others, such as lava from Thicrsa, 

 "and trap rock from Esia were neglected."* 



Mr. Sterry Hunt, who as we shall see, rejects altogether the 

 theory which derives the eruptive rocks from a portion of the 

 primitive fused mass of the globe, and supposes them to consist 

 of altered, fused, and displaced sediments, (Can. Naturalist, Dec. 

 1859], remarks, with regard to Bunsen's hypothesis, that the cal- 

 culated results as deduced from the volcanic rocks of Hungary 

 and Armenia, often differ considerably from those obtained by 

 analysis ; a result which will follow, when as is often the case, dif- 

 ferent triclinic feldspars replace each other in the pyroxenic rocks. 

 He also shows that the composition of certain eruptive rocks, 

 like phonolites, (which are highly basic, and yet contain but little 

 lime, magnesia, or iron-oxide) is such that they cannot be derived 

 from either of the magmas of Bunsen. 



* Ueber die vulcanische Gesteine in Sicilien und Island, und ihre 

 lubmarine Umbildung ; Gottingen, 1853. 



