266 H. H. Robinson — Chemical Analyses of Igneous Rocks. 



Proportion of analyses 









Summation groups U. S. 



G. S. chemists 



All others 



99-00- 99-24 



•8 



4-2 



99-25- 99-49 



1-6 



6-6 



99-50- 99'74 



7-5 



11-0 



99-75- 99-99 



24-4 



15-9 



100-00-100-24 



35-7 



17-7 



100-25-100-49 



22-9 



'13-9 



100-50-100-Y4 



5-4 



12-1 



100-75-100-99 



1-7 



6-8 





100-0 



88-2 



Beyond above limits 



o-o 



11-8 



100-0 



100-0 



practice their distribution of summations would more nearly 

 approach that of the Survey chemists. In fact, the lower 

 grade for the work of other chemists is due to the inexperience 

 of many of the analysts. It is absolutely essential for a begin- 

 ner to make analyses in duplicate, no matter what may be the 

 nature of the rock, until he can secure accordant results. The 

 writer has been told by a teacher of quantitative analysis that 

 out of several hundred students the number who had obtained 

 uniformly good results could be counted on the fingers of one 

 hand. As Hillebrand has stated, " too great stress can not be 

 laid upon the importance, especially for the analyst of limited 

 experience, of consistently testing most of his final precipitates 

 and filtrates as carefully as possible in order to be sure, on the 

 one hand, that the substances reported by a given name are 

 wholly such and, on the other, that all is reported. In no 

 other way can an analyst so soon acquire the needed confidence 

 in himself and his methods."* 



A comparison of the distribution of summations for all other 

 chemists with that for the five chemists of the U. S. Geological 

 Survey (fig. 2) shows beyond argument how great is the room 

 for improvement in the general quality of silicate rock analysis. 

 The efficiency of the Survey chemists, which is fairly expressed 

 by their distribution curve for summations, may well be taken 

 as the standard. 



An idea of the general quality of silicate rock analyses may 

 also be obtained from the proportion of analyses that fall 

 within certain proposed limiting summations, as shown in the 

 following table : 



*Op. cit,, p. 28. 



