844 THE JOURNAL OF GEOLOGY. 



from which it was drawn, and the conditions of differentiation 

 existing there, and, further, upon whether it has undergone sub- 

 sequent differentiation within itself. 



The textural variations which were discussed in the first 

 part of this paper, and which may exist in diverse portions of 

 one rock body, or in different bodies of similar magmas, add still 

 further to the complexities in solidified magmas. Rock magmas 

 are thus known to vary frequently in chemical composition, min- 

 eral composition and texture. Names of rocks which are defined 

 in terms of these three characters, can only apply to that 

 portion of a rock body exhibiting the characters specified. 

 Other parts of the mass will have different names, and to this 

 extent be different rocks. The student should therefore recog- 

 nize the difference in the idea conveyed by the term rocks 

 as ordinarily used, and that which is involved in the expression 

 rock-body, as a geological unit. 



Joseph P. Iddings. 



