BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 



VOL. 33, PP. 617-638 SEPTEMBER 30, 1922 



GEAPHIC STUDY OF IGNEOUS EOCK SEEIES 1 



BY FRANK F. GROUT 



{Presented before the Society December 29, 1921) 



CONTENTS 



Page 



Scope of the paper 617 



Earlier plots 618 



Method of plotting 618 



Constituents to use as coordinates 618 



Form in which to use them 619 



General statement 619 



Silica 620 



Alkalies 620 



Selection of data 621 



The diagrams 623 



The "trend" of differentiation 625 



Normal sequences 628 



Syntexis 631 



"Primary magma" 631 



The problem 631 



Evidences from the diagrams 632 



Degree of uniformity of primary magma 633 



Summary 634 



Selected bibliography of rock series 635 



Scope of Paper 



The increasing number of well studied examples of igneous differentia- 

 tion and comagmatic regions indicates that a compilation of available 

 data may be suggestive to those who are building petrologic theories on 

 the basis of the idea of differentiation. A fairly satisfactory graph has 

 been found for the concise presentation of the data. On this graph are 

 plotted data for 40 rock-masses in which gradational series can be traced, 

 and about 20 series of less definite connection but strong signs of con- 

 sanguinity. The several series are widely different, showing that dif- 



1 Manuscript received by the Secretary of the Society January 18, 1922. 



(617) 



