Page:The Journal of geology (IA journalofgeology11893univers).pdf/428

from which it was drawn, and the conditions of differentiation existing there, and, further, upon whether it has undergone subsequent 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, mineral 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 recognize 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.