CLASSIFICATION OF IGNEOUS ROCKS 565 



to be employed and their method of application, are as fol- 

 lows : 



The classification should be free from hypothesis, and be 

 based only on facts or relations determinable in the rock 

 itself. 



The classification should be quantitative as far as possible, 

 and each constituent, chemical or mineral, should be given 

 weight in proportion to the amount present in each case, irre- 

 spective of its rarity or unusual occurrence. 



The chemical composition of the rock is its most fundamental 

 character, being a quality inherent in the magma before its 

 solidification, and is therefore of greatest importance for its cor- 

 relation with other rocks. 



All rocks of like chemical composition should be classed 

 together, and degrees of similarity should be expressed by the 

 relative positions or values of the systematic divisions of the 

 classification. 



The mineral and textural characters, being dependent largely 

 on external conditions attending rock solidification, are to be 

 regarded as of subsidiary importance in classification, but should 

 receive due recognition in the system. 



Since it is the chemical composition of the magma that is the 

 fundamental character of igneous rocks by which they are to be 

 classified, only fresh, undecomposed, or unaltered rocks are to 

 be employed in establishing such a classification. 



A brief discussion of various principles that have entered 

 into systems of rock classification is introduced at this place in 

 order to make clear our reasons for selecting those employed 

 in creating the system proposed by us. 



Rocks have always been recognized as extremely difficult of 

 systematic classification, because of their infinite variation or 

 gradation from one kind to another in many ways. Reviewing 

 the many factors which may be employed in their classification, 

 they are found to fall into two groups, namely, geological rela- 

 tions, and inherent characters ; and the task of the systematic 

 petrographer is to select the characters and apply the criteria 



