THE GEOLOGICAL VERSUS THE PETROGRAPHICAL 

 CLASSIFICATION OF IGNEOUS ROCKS. 



The rocks which make up the solid earth are of interest 

 from a great many standpoints. A large part of geology is 

 more or less directly concerned with them — as to their char- 

 acteristics, their origin, their relationships in the mass of the 

 earth, the changes in the rocks themselves, either metamorphism 

 or decay, the mechanical destruction of rock masses, and 

 structural changes in the earth dependent in many ways upon 

 the characters of the rocks affected. From all of the points of 

 view just indicated, and from still others, the geologist — often 

 a specialist — has need for a nomenclature by which he may 

 name the rocks as objects, and for various classifications express- 

 ing their observed relationships in different directions. 



But while an adequate and expressive classification is a 

 matter of great importance to many, it is commonly agreed that 

 the systems of classification and nomenclature now in use are in 

 a state of great confusion — of rapidly increasing confusion. To 

 my mind the principal cause for this deplorable state of things 

 is the lack of a clear conception of the natural relationship 

 between the systematic classification of rocks, upon which their 

 specific nomenclature must be based, and various other, necessary 

 classifications of the same bodies. It seems that, while the 

 multiform relations and affinities of rocks and their complex 

 inner nature are more or less clearly understood, the futility of 

 endeavoring to express all of these factors in one system of clas- 

 sification has not come sufficiently to recognition. It is my firm 

 belief that no great progress in systematic petrography is possible 

 until a more rational view of the relationship of that science to 

 geology prevails among its devotees. 



What is a rock ? This question has often been found diffi- 

 cult to answer in satisfactory form. It is commonly said that 



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