A GRAPHIC METHOD OF REPRESENTING THE CHEMI- 

 CAL RELATIONS OF A PETROGRAPHIC PROVINCE 



FRANK D. ADAMS 

 McGill University, Montreal 



During recent years questions concerning the chemical com- 

 position of rock magmas, and the relation of these to one another, 

 have been made the subject of extended investigation on the 

 part of petrographers. A great number of rock types, and of many 

 variations presented by these types, have been analyzed with 

 great accuracy, so that now the material available for such studies 

 has enormously increased. 



As it is a matter of difficulty in comparing a long series of 

 analyses to grasp clearly their various points of resemblance or 

 difference, and to recognize the relationships, often more or less 

 obscure, presented by the different types, investigators have 

 from time to time sought to express the results of these analyses 

 graphically so that by a comparative study of forms rather than 

 figures the relationships in question might be brought out more 

 clearly and presented in a more striking manner. 



It is unnecessary here to consider in detail the various methods 

 of graphic representation suggested by different workers. A 

 review of the various methods proposed will be found in a paper 

 by Iddings which appeared some years since.^ Some of these 

 methods, as for instance that of Reyer, show the relative propor- 

 tions of the several chemical constituents present in the rock by 

 means of a figure in which areas representing the various con- 

 stituents are variously shaded or distinguished by different con- 

 ventions. Others, such as that employed by Harker, indicate 

 the relative proportions of the constituents by means of a curve 

 on a plain surface. In a third class, such as those used by Brogger, 

 the composition of the rock is represented by a geometrical outline 

 whose shape would vary as the composition of the rock changed. 



' J. P. Iddings, Prof. Paper i8, U.S. Geol. Survey, 1903. 



