THE CLASSIFICATION OF THE 

 PLUTONIC ROCKS 



By F. H. HATCH 



In the quantitative classification of igneous rocks devised by 

 Messrs. Whitman Cross, Iddings, Pirsson, and H. S. Washington, 1 

 the hitherto existing nomenclature of rock-types is entirely dis- 

 carded, and a new nomenclature introduced which is based on 

 purely chemical considerations without regard to mode of origin. 

 Consequently plutonic, volcanic, and differentiated dyke rocks, 

 if they have similar chemical compositions, find a place in the 

 same divisions, no matter how dissimilar they may be in physical 

 character. 



I desire in this paper to show that it is not necessary to 

 throw over the existing rock nomenclature, nor to disregard 

 mode of origin, in basing a natural system of classification on 

 chemical considerations. Apart from mode of occurrence, the 

 difference in internal structure or texture, between a eugranitic 

 plutonic rock, that has cooled slowly as a deep-seated mass 

 under pressure and in the presence of occluded water, and a 

 porphyritic or semi-vitreous volcanic rock, that has been erupted 

 at the surface and has cooled quickly, is so great that a system 

 can scarcely be termed natural that places two such different 

 rocks in the same category. In a natural classification, there- 

 fore, the plutonic and the volcanic rocks must be recognised 

 as distant classes. It is not necessary for my purpose to 

 discuss here whether a third division for the hypabyssal rocks, 

 to use Brogger's 2 convenient term for Rosenbusch's Ganggcsteine, 

 is equally justified or not. 



Accepting, then, the plutonic rocks, as a primary sub- 

 division of the igneous rocks, we can apply to them the 

 principles of a chemical classification. The best results are 



1 Quantitative Classification of Igneous Rocks, based on Chemical and Minera- 

 logical Characters, with a Systematic Nomenclature, 8vo, pp. 286, Chicago, 1903. 

 For a critical review of this, see A. Harker, Geol. Mag. December 4, vol. x. 1903, 

 pp. 173-8. 



2 Die Gesteine der Grorudit-Tinguait-Reihe, Vidensk. Skrifter Math-naturv. 

 Klasse. Christiania, 1894, p. 123. 



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