PROBLEMS IX PETROLOGY. 



Bv JOSEPH P. IDDINGS. 

 {Read April 21, 1911.) 



The development of the science of petrology from that condition 

 of the study of rocks, properly termed petrography, is characterized 

 by the shifting of the emphasis from the purely observational and 

 descriptive phases of the work to those that relate to the origin and 

 formation of rocks, both with respect to their occurrence as integral 

 parts of the earth, and to their composition and structi^re. 



Not that there is less need than formerly for accurate observation 

 and study of rocks, and for thorough description of their composi- 

 tion, texture and occurrence, but the introduction of greater definite- 

 ness into conceptions of their modes of formation, and the widening 

 of the horizon of this field of research through the experimental 

 and synthetic investigations of the geophysicist, have advanced the 

 study of rocks from the accumulation of data and statistics, to the 

 formation of laws and relationships, both as regards minutest details 

 of composition and texture, and with respect to petrographical prov- 

 inces, and their connection with the dynamical history of the regions 

 of the earth in which they occur. 



As a consequence of this advance new problems present them- 

 selves, and invite the cooperation of workers in several branches 

 of inorganic science. Leaving out of consideration for the present 

 the great problems of metamorphism, some of which are being suc- 

 cessfully treated by Adams, or have been under investigation by 

 Van Hise and Leith, I wish to call your attention to certain phases 

 of the study of igneous rocks that may be grouped under three 

 heads for present purposes, as follows : ( 1 ) Tlie actual mineral 

 composition of igneous rocks, (2) the mathematics of the petrology 

 of igneous rocks, and (3) petrographical provinces. 



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