108 J. P. IDDINGS 



ing to different genetic series, or to different petrographical 

 provinces, may be chemically alike. 



The same is true with regard to the mineralogical char- 

 acteristics of rocks. Since these are primarily dependent on the 

 chemical composition of the magma of each rock, and also on 

 the physical conditions attending its solidification, it follows 

 that the minerals composing certain members of any genetic 

 series of rocks must be like the minerals in some members of 

 other genetic series. Definitions based on the character and 

 proportions of these minerals must apply equally well to mem- 

 bers of several genetic rock series. 



Moreover, the texture, granularity, and physical aspect of 

 rocks depend on both the composition of the magma and the 

 physical conditions under which solidification took place. The 

 latter are independent of geographical position ; that is, are not 

 localized, but are universal, and cannot be peculiar to any petro- 

 graphical province. They depend on the environment of the 

 magma in each instance, and magmas in different provinces 

 may have existed amid similar environment and have been 

 exposed to almost identical physical conditions. 



The most distinctive features of igneous rocks, then, their 

 chemical and mineral composition and texture, cannot serve as 

 means of discrimination of rocks of all genetic series, conse- 

 quently a group or class of rocks which may be of essentially 

 the same natures as regards these essential qualities may 

 embrace rocks belonging to different genetic series. It follows 

 from this that the classing of all known igneous rocks into groups 

 that shall have essentially the same chemical and mineral com- 

 position and texture, and which may be designated by definitions 

 expressed in terms of these qualities, must disregard the genetic 

 relations among the rocks of different classes. 



The nature of a rock-body or geological unit. — In the foregoing 

 discussion the term rock has been used as though applied to a 

 mass having in each case some definite composition and texture. 

 But it is well known that in large masses this is not always the 

 case. In some masses the texture varies in different parts of 



