136 IP. Cross — Igneous Rocks in Wyoming. 



Classification and Nomenclature. 



In the foregoing pages new names have been proposed for 

 three rock types described. It is now desired to explain as 

 clearly as possible the grounds for adding three new names to 

 the rapidly growing list of rock varieties, and this involves 

 more or less discussion as to principles of classification. 



With regard to the present tendency to confer names upon 

 many more or less distinct, newly recognized or more narrowly 

 defined, rock types, it must be admitted that from several 

 sources the protests against this course are most natural. 

 Teachers, geologists with whom petrography is a side issue, 

 and those to whom all rocks are merely accidental mixtures of 

 various minerals, — to all these the new terms are abhorrent. 

 But while a period of confusion is to be regretted, it appears 

 to me that the recognition and naming of every truly distinct 

 rock type may be a necessary prelude to the much needed 

 reform of onr present illogical and inadequate petrographical 

 scheme. 



Igneous magmas must be classified on chemical grounds ; 

 their crystalline equivalents principally upon mineralogical 

 constitution, as the more or less evident expression of chemical 

 composition and as the cause of the principal characteristics of 

 rocks. It does not follow, as is sometimes asserted, that 

 because rock-making minerals may be developed in infinitely 

 varying proportions, that there are no natural rock types or 

 groups. There is, for each prominent rock constituent, a con- 

 siderable range in its development within which it places its 

 own stamp upon the rock containing it. It may play the lead- 

 ing role, or share the honors equally with others, or be subordi- 

 nate. With a given structure the habit of the rock depends 

 largely upon the minerals which are its leading constituents. 

 Most new rock names of the last few years have been con- 

 ferred, consciously or unconsciously, in recognition of this nat- 

 ural law. But this law has not yet been fully recognized in 

 the system of petrography, and until it is so recognized the 

 system will be unsatisfactory. The character-giving relative 

 abundance of minerals in rocks is not awarded proper weight 

 in classification. 



The weakness of the present scheme in the direction alluded 

 to lies in giving to the feldspars and feldspathoids far too much 

 weight, and to the dark silicates far too little, in constructing 

 the frame work. "Bocks with feldspar," — u Bocks without 

 feldspar," — these two divisions comprise all igneous rocks. 

 Gabbro is mineralogically a rock composed of basic plagioclase 

 and pyroxene, and within it are included everything from 

 anorthosite to the vanishing point of the feldspar, and we are 



