CLASSIFICATION OF IGNEOUS ROCKS 603 



1. Let us consider the case of somewhat similar rocks 

 having like textures but somewhat different mineral composi- 

 tions which have been classed together under one name on a 

 basis of texture, and subsequently separated on a basis of min- 

 eral differences, the gabbros. Among several of these rocks 

 with phanerocrystalline granular texture there are those with 

 hornblende, or mica, or olivine, and they are known as horn- 

 blende-gabbro, mica-gabbro, and olivine-gabbro. 



2. There is a case of a number of rocks having nearly 

 the same mineral composition (labradorite, augite, olivine, and 

 magnetite) and called basalt, which possess quite different tex- 

 tures, such as evenly granular, ophitic, intersertal, porphyritic, 

 or microlitic glassy. 



We can see strong grounds for both of these methods of 

 classification, and each may serve an important purpose in petro- 

 graphical work. There are instances in which one method of 

 correlation is more valuable than another. Freedom of choice 

 should be allowed if consistent with definiteness and stability of 

 the system of classification. 



For these reasons the variable characters of each petro- 

 graphical unit established by the system of classification we 

 propose, which characters are results of the conditions attending 

 solidification, or crystallization, are treated in a different manner 

 from the chemical characters. They are treated as variable 

 factors in the system, although inherent and persistent qualities 

 of a particular rock. They are assigned the role of qualifiers 

 of the magmatic units wherever there is need of further subdi- 

 vision. They consequently appear in the production of a system- 

 atic nomenclature, which is an integral part of comprehensive 

 classification. 



Actual Mineral Composition. 



Let us first consider what minerals may actually be developed 

 in rocks and what relations they bear to the standard minerals 

 which are used to express the chemical composition of a magma 

 unit. And let us assume that the rock is holocrystalline. Those 



