414 T. L. Walker — Causes of Variation in the 



It is improbable that this is ever the only cause tending to 

 produce variations in the composition of dikes and stocks. 



Frequently eruptive stocks possess basic borders and acid 

 centers, but on the other hand there are a few well known 

 ernptives where the central portion is much more basic than 

 the margin. Vogt* says that in many Norwegian occurrences, 

 the central portions are very basic, even to the extent of large 

 masses of titaniferous iron ore. Though there are only a few 

 such cases, yet they must be taken into account. Such occur- 

 rences are anomalies according to all the old theories of dif- 

 ferentiation outlined above, but they are quite regular when 

 considered in connection with the explanation offered in the 



Vertical. 



Horizontal. 



Upper. 



Middle. 



Lower. 



s 





CS 





TD 





OJ 





~ 









<D 









a 





i— i 



v.,:': ;'•;'.•'.'•'••'•'■;. ••?. y ' 





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o 



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I<^i^fe;: : 



Acid. 



Intermediate. 



Basic. 



Ultra-basic. 



previous paragraph. In fact we should expect to find such 

 cases. In order to obtain a section of a stock showing a basic 

 border and an acid center, it is necessary that the original 

 magma should have been basic, and to have a section from an 

 upper horizon of the reservoir. If, however, the original 

 magma were of an intermediate composition, say 60 per cent 



* Zur Classification der Erzvorkommen. Zeitschr. f. prakt. Geologie, 1894. 



