L. V. Pirsson — Classification of Igneous Rochs. 277 



of rocks that we must consider, with the result of making 

 the system confused, and second, the failure to recognize 

 quantitative relations among the minerals, with the 

 result of making it inexact. "When we observe that which 

 is to entitle one kind of rock to recognition as an indi- 

 vidual entity is based on geologic occurrence and mineral 

 composition, whereas another kind is based on petro- 

 genetic relations and texture combined with mineral com- 

 position, and a third is based on geologic occurrence, 

 geologic age, and the minerals, it seems clear that there 

 is an attempt to accomplish too many things at one time. 

 Leaving aside the matter of geologic age, three systems 

 of classification, each of which endeavors to express 

 something different from the others, have been telescoped 

 together, and the result is to the advantage of none of 

 them. 



It now seems clear that we need three systems of classi- 

 fication of igneous rocks in order to express our knowl- 

 edge of their properties and relations and that we may 

 put like things together ; these are : 



A. A petrograpliic classification based on those inher- 

 ent characters of rocks, expressed by their minerals and 

 textures, which shall define in a material way the hind of 

 rock we are dealing with. This covers the idea, previ- 

 ously stated, that a granite porphyry is a granite por- 

 phyry, no matter where it occurs. 



B. A petrologic classification based on petrogenesis 

 which attempts to group the rocks according to their 

 family relationships and co-magmatic origin, as expressed 

 by their relative geologic positions, and the evidences of 

 consanguinity that they may present. Here, for example, 

 the dike rocks of Eosenbusch would find logical expres- 

 sion. This will be more fully explained later. 



C. A geologic classification based on the method of 

 occurrence that determines the form of the mass of an 

 igneous rock and its outward space relations to other 

 rocks. This states whether it is plutonic or volcanic, 

 whether a stock, laccolith, sheet, dike, etc. This kind of 

 classification is generally explained in more or less detail 

 in the standard text books of geology, and, with reference 

 to intrusive bodies, has been quite fully elaborated bv 

 Daly.^^ 



" Classification of Igneous Intrusive .Bodies, Jour, of Geol., vol. 13, p. 485- 

 1905. Also, Igneous Eocks and their Origin, p. 61, 1914. ' 



