556 CROSS, IDDINGS, PIRSSON, WASHINGTON 



PART III. METHODS OF CALCULATION. 



Chemical relations among salic minerals. 



Chemical relations among femic minerals. 

 Calculation of the norm. 



Percentage weights of minerals ; Examples of calculations. 

 Calculation of norm fro?n mode. 



Calculation of mode from chemical composition of a rock. 

 Calculation of alferric tninerals. 



Aluminous pyroxenes ; Aluminous amphiboles ; Ferromagnesian micas ; 



Garnet. 

 An example of calculation. 



Calculation of the norm ; Calculation of the mode ; Calculation of the norm 



from the mode. 



EPILOGUE. 



TABLES OF ALFERRIC MINERALS AND THE ROCKS IN WHICH THEY OCCUR. 



SUMMARY. 



We present in this work an entirely new system for the 

 classification and nomenclature of igneous rocks, which repre- 

 sents the results obtained after some years of consideration 

 and discussion of the subject in all its bearings. 



In uniting our efforts for this purpose we have been actuated 

 by a sense of the seriousness of the responsibility assumed in 

 such an undertaking, and by a conviction that the best results 

 were to be obtained by the co-operation of several workers who 

 agreed on fundamental principles and who were capable of har- 

 monious collaboration. 



Originally Professor George H. Williams was associated with 

 us in this work. By his death we have been deprived of a 

 counselor whose judgment we held in the highest esteem, and 

 whose loss we cannot cease to lament. 



Many attempts were made to modify in various ways the 

 existing systems of classification in the endeavor to make them 

 answer the demands of modern petrology, and each attempt 

 was successively cast aside on practical trial. It came thus 

 to be perceived by all of us that the main weakness of old sys- 

 tems lay in their fundamental principles and methods and that 

 any new, logical and comprehensive system based on the vast 

 amount of knowledge of igneous rocks acquired in these later 



