CLA SSIFICA TION OF IGNE OUS RO CKS 557 



years must be built up from the very beginning on different 

 lines from those heretofore employed. After many endeavors 

 in various directions, we gradually evolved the system here 

 presented. Along with its evolution has gone hand in hand the 

 calculation of thousands of analyses by which it has been 

 tested and its formation in large part controlled. It is 3.chemico- 

 minera logical system based on its own principles, and is in nowise 

 an attempt to reduce any one of existing systems to a chemical 

 basis or to formulate one of them in a chemical way. Its con- 

 cepts of rocks are in a large measure new, and hence, except in 

 a very small degree, it demands a new nomenclature. 



In brief outline, what we propose is as follows : All igneous 

 rocks are classified on a basis of their chemical composition; all 

 rocks having like chemical composition are grouped together. 

 The definition of the chemical composition of a rock and of a 

 unit of classification is expressed in terms of certain minerals 

 capable of crystallizing from a magma of a given chemical com- 

 position, and the expression is quantitative. For this purpose 

 the rock-making minerals are divided into two groups, consist- 

 ing on the one hand mainly of the more highly siliceous 

 alkali- and calci-aluminous ones, and on the other of the ferro- 

 magnesian ones. The first group is called mnemonically the 

 salic group, the second is the femic group. From this category 

 the micas and aluminous augites and amphiboles are excluded 

 for reasons given in full. 



For the purpose of completely classifying a rock by this 

 system its chemical composition must be actually known by 

 chemical analysis, or approximately so by physical means or by 

 microscopic, optical methods indicated by the authors. The 

 thousands of rocks of various mineral compositions and tex- 

 tures whose places in this system are indicated by chemical 

 analysis become types for comparison, analogous to type speci- 

 mens of zoology and botany, by which similar rocks may be 

 approximately classified. 



Since it is known that many magmas may crystallize into 

 quite different mineral combinations, according to the influence 



