564 CROSS, IDDINGS, P IRS SON, WASHINGTON 



The gradations in the chemical and mineral composition of 

 rocks, within limits. 



The limited number of the important rock-making minerals. 



The existence of rocks having but one mineral component. 



The variable crystallization of a chemically homogeneous 

 magma, or of several chemically similar magmas, whereby differ- 

 ent combinations of minerals may be produced from chemically 

 similar magmas. In other words, the fact that rocks having 

 diverse mineral compositions may be chemically alike. 



The fact that diverse textures develop from the same magma 

 and from chemically similar magmas, and conversely that similar 

 textures develop from chemically different magmas. 



The incomplete crystallization of magmas or their solidifica- 

 tion into glass in many instances. 



The identity of rocks of different geological ages. 



The identity of many rocks with different modes of geologi- 

 cal occurrence. 



The existence of petro graphical provinces in which the igneous 

 rocks genetically related are distinguishable from those of other 

 regions, when considered in connection with the occurrence 

 elsewhere of similar petrographical provinces and with the 

 gradual transitions between provinces, should be treated as a 

 larger phase of differentiation. Moreover it has been shown by 

 Brogger 1 that similar rocks may be differentiated from different 

 parent magmas in several petrographical provinces, and may 

 occur in two or more unlike series. That is, rocks belonging to 

 distinct provinces may resemble one another so closely in their 

 dominant characters that they would naturally be defined by the 

 same terms. Hence, we conclude that all igneous rocks should 

 be correlated and classified in one comprehensive system based 

 upon principles common to igneous rocks in general. 



The principles which we consider applicable to the classifica- 

 tion of igneous rocks, as criteria by which to judge of the facts 



1 Quart. Jour. Geo!. Soc, Vol. L (1894); P- 36, and Die Eruplivgesteine des Kris- 

 tianiagebietes, Vol. Ill, p. 57, Christiania, 1898. 



