8oo WHITMAN CROSS 



barium, strontium, etc., because the system, as published, does 

 not contain all the detail applicable to such cases. He cites, how- 

 ever, the remark by the authors of the system that the above- 

 named oxides are to be grouped with FeO or CaO "unless these 

 unusual components occur in sufficient amounts to make their 

 calculation as special mineral molecules desirable." It was 

 believed by the authors that the principles and methods of the 

 system were so clearly illustrated by the scheme published that 

 petrographers would understand the extension of the framework 

 of the system to rocks in which any rare elements might acquire 

 local prominence. Fermor's treatment of the manganese rocks 

 is in accord with that assumption. 



With regard to sections it was remarked: 



The application of the above principles [used in the construction of the 

 system] shows, however, that in certain points more numerous subdivisions 

 are needed. This necessity is met by the formation of sections of any of the 

 divisions above described. These sections will be based on more special 

 characters according to circumstances.^ 



It is of course possible that igneous rocks may sometime be 

 found containing in notable amounts rare substances which must 

 be introduced into the norm, and where the exact method of pro- 

 cedure in systematic treatment may not be plain. But even in 

 the case of barite, raised by Fermor, its position in the group A of 

 femic molecules is appreciated by him and the recognition of the 

 various molecules of this group in the system, while somewhat 

 complicated, will not prove very difficult. Until the occurrence 

 of barite as an original constituent of igneous rocks is con- 

 clusively demonstrated it seems unnecessary to work out the 

 details of its treatment in the Quantitative System. The igne- 

 ous nature of the quartz-barite rock of Salem, India, mentioned 

 by Fermor, can hardly be considered as proven, at the present 

 time. The new divisions of the Quantitative System which 

 may be made necessary for rocks of unusual chemical composi- 

 tion will not be appendices of the system, in the sense of unrelated 

 appendages. 



' Op. cit., p. 127. 



