618 F. F. GROUT GRAPHIC STUDY OF IGNEOUS ROCK SERIES 



ferent factors may dominate differentiation in different cases. If this 

 compilation of scattered data can be kept in mind in building a petrologic 

 theory for one particular case, its object will be amply fulfilled. 



Earlier Plots 



The diagrammatic presentation of rock series has been attempted by 

 several penologists.- Few of these plots are so constructed as to show 

 a series of rocks, and only one or two can be satisfactorily used to com- 

 pare one series with another. Iddings, in discussing rock classification, 

 used a diagram that served for plotting series 3 on the basis of silica and 

 the ratio of alkalies to silica. He used the method also for actually 

 plotting as many as five rock series at a time, almost exactly as here 

 planned, but without selecting geologically continuous bodies. Osann 

 has presented two forms of plot, 4 with triangular coordinates, which 

 might be used to plot actual series, but which he used for a more general 

 discussion of the chemical range of all igneous rocks and their differences 

 from sediments. Several other plots show relationships in a rock series 

 by lines connecting the names on a printed page, but do not make use 

 of any coordinate system. 5 



Method of Plotting 



constituents to use as coordinates 



It is, of course, evident to any student of igneous rocks that variations 

 occur in more than two directions. The rocks are mixtures of many 

 components. Any plane diagram must necessarily ignore some of the 

 features of the rocks, and the value of the plot depends on a wise selection 

 of coordinates. Of the several plots on which it is possible to compare 

 one series with another, most are found to use alkalies and silica in some 

 way. The arguments for the use of alkalies and silica are well stated 



2 Diagrams are reviewed by Iddings in U. S. Geol. Survey Prof. Paper 18 (1903). 

 Other series have been plotted by Adams, Journal of Geology, vol. 22, p. 689 ; by Grout, 

 Journal of Geology, vol. 26, p. 622, and by Reid, Bull, of Dept. of Geology, Univ. of 

 Calif., vol. 3, pp. 173-191. 



3 J. P. Iddings : Journal of Geology, vol. 6, 1898, pp. 92-111 and 219-237. 



4 A. Osann : Centralblatt fur Min., 1913, pp. 481-490. Osann's method of plotting, 

 however, was not as satisfactory for actual rock series as the method selected below. 



5 L. V. Pirsson : Tripyramid Mountain, New Hampshire. Am. Jour. Sci., vol. 31, 1911, 

 p. 430. Pirsson also gave his classes other diagrammatic series to illustrate differentia- 

 tion. 



Weinschenk : Fundamental principles of petrology, chapter iv. 

 Kolderup : Bergens Museums Aarbog, No. V, 1896, p. 14. 



Rogers (modified from Williams) : The Cortlandt series. New York Acad. Sci.. vol. 21, 

 1912, p. 58. 



