CLA SSIFICA TION OF IGNEO US RO CKS 563 



groups, a few names are made to do duty for many obviously 

 distinct rocks. 



Upon the grounds stated, a new system of petrography seems 

 to us necessary. This conclusion has been reached by us only 

 after many efforts to revise or remodel existing schemes. Our 

 first conferences upon this undertaking were participated in by 

 George Huntington Williams, and our first exchange of sugges- 

 tions bears the date of May 1, 1893. These efforts on our part, 

 extending over ten years, have brought us from somewhat 

 diverse views to full accord in the system here presented. 



BASIS OF CLASSIFICATION. 



The progress in the knowledge of the composition, texture, 

 and occurrence of igneous rocks, as well as in the conceptions 

 of the probable causes of variation among different bodies and 

 within one body of rock, has been such as to justify us in con- 

 sidering the problem of the classification of igneous rocks in the 

 light of certain established facts of a general nature. The 

 recognition of these leads to the establishment of general prin- 

 ciples upon which a new system of classification may be based. 



The facts to be considered may be stated categorically as fol- 

 lows : 



The once igneous or molten condition of the magmas from 

 which rocks of this kind have solidified. 



The characters of such magmas as solutions. 



The physical and chemical properties of such solutions. 



The ability of complex solutions to differentiate by osmotic 

 diffusion, liquation, fractional crystallization, or by other pro- 

 cesses, under varying conditions of temperature and pressure. 



The variations in chemical composition resulting from such 

 differentiation. 



The existence of marked chemical variations within igneous 

 masses of small volume in some instances, and of slight varia- 

 tions within bodies of large volume in others. 



The absence of fixed proportions of constituents in rocks 

 with two or more mineral components. 



