Washington — Statement of Rock Analyses. 63 



After these it seems well to put the acid radicals which have 

 been determined, following the main principle of the other 

 division ; Ti0 2 and Zr0 2 should come first, then P 2 5 and 

 Y„0 6 . S0 3 is put next instead of at the head of the lesser acid 

 radicals, as it, like CI which follows, is a constituent of the 

 sodalite group. F comes after CI, and then S or FeS 2 may be 

 put. 



Then come the subordinate metallic oxides, in the order R 2 3 , 

 EO, K 2 0. These would include Cr 2 3 , (Ce 2 3 ), NiO, (CoO), 

 MnO, (CuO), JBaO, SrO, Li 2 0. Finally in rare cases C or N 

 may be put at the last. 



While the above order of arrangement is undoubtedly open 

 to criticism in certain respects, yet it seems to the writer, as 

 well as to other petrographers to whom it has been submitted, 

 to be on the whole the most logical and convenient. It may 

 be added that practical experience of it in the collection of a 

 large number of analyses sustains this view. 



I would also urge the advisability of printing the important 

 molecular ratios with each new analysis, wherever this is pos- 

 sible. The reader will be saved the trouble of calculating them 

 for himself, and will thus immediately have a fuller compre- 

 hension of the character of the rock. 



In this connection I would suggest, as a practical and con- 

 venient method for using analyses, the use of a card catalogue, 

 the cards being printed with the oxides in their proper order. 

 Experience has shown the utility of such a plan, and the time 

 given to its preparation will be more than compensated for by 

 that saved in reference and collation. 



