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Vol. 66.] CLASSIFICATION OF IGNEOUS ROCKS. 473 



of the would-be systematist which is regarded, 'and the resulting 

 scheme cannot be permanent. 



The first step in all current petrographic systems may be called 

 a natural one. Igneous, sedimentary, and metamorphic rocks are 

 grand divisions made in recognition of the intimate relation 

 between the most important characters of rocks and certain great 

 earth-building or modifying processes. The further use of geological 

 factors in the classification of each grand division must depend 

 upon their suitability for this purpose. 



Chemical composition, mineral constitution, and texture are, in 

 this order, the generally recognized natural properties of igneous 

 rocks available for their classification. The chemical characters 

 are certainly fundamental, and most closely connected with the 

 molten condition. Are the chemical differences of igneous rocks 

 clearly determined by a geological factor which may be used in the 

 next step in classification? It does not seem to me that such is 

 the case, but that is the crux of* this discussion, and I desire to 

 consider the actual, that is, the natural, relations of rock-characters 

 to mode of origin or geological occurrence, and compare these with 

 the assumptions current in petrographic system. 



II. Factors of Chemical Composition. 



The chemical composition of igneous rocks as a class is now 

 fairly well known. No one can safely question the fact that the 

 great majority of rocks belong to a category in which there is 

 gradation in composition from one type to auother. Even the 

 extreme chemical types are no doubt connected with the average 

 rock by a continuous series, although some units may be, as yet, 

 undescribed. Classification on a strictly chemical basis is, then, 

 necessarily arbitrary or artificial, and a natural classification that 

 is to express the fundamental chemical characters of igneous rocks 

 must rest on facts of genesis, or of occurrence vitally related to the 

 known features of composition. 



Origin of differences of composition. — The observed 

 differences in chemical composition of magmas may be due to 

 several causes : 



(a) Primeval differences in the composition of the eai'th's crust at different 

 places. These variations are necessarily difficult to define, and are 

 hypothetical as to origin. They must be expressed as facts of distri- 

 bution. 



(//) Magmatic differentiation,— by which a homogeneous magma has split 

 up into component parts of differing composition. 



(c) Assimilation, — by which a given magma has been changed in composi- 

 tion through the fusion and absorption of unlike material. 



The first of these possible causes is quite hypothetical, and not 

 certainly distinguishable from the others in its effects. Probably 



tit would be most prominently visible in the geographical distri- 

 bution of chemically different rocks. Jt is not intended to imply 

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