2 GEORGE V. SESSIONAL PAPER No. 25a A. 1912 



CHAPTER XXV. 



CLASSIFICATION OF IGNEOUS INTRUSIVE BODIES. 



Introduction. 



In 1905 the writer published a paper on the classification of intrusive bodies> 

 a problem which had already been forced upon his attention during the Boun- 

 dary survey.* Since that date some discussion of the subject has taken place,, 

 especially by Harker, and certain new types may be added to the first classifica- 

 tion. In the present chapter the matter of the 1905 paper is summarized, with 

 the inclusion of many of its paragraphs, and some additional considerations will 

 be offered. 



Principles of Classification. 



A review of the definitions given by the leading authorities on this subject 

 shows that each definition has been based on one or more primary features of 

 igneous intrusions, namely: 



(a) The method of intrusion. 



(b) The relation of the body to pre-intrusion structures in the invaded 

 formation. 



(c) The form of the body. 



(d) The size of the body. 



(e) The attitude of the body with reference to the horizontal plane. 



For a given body the method of intrusion is the most important criterion 

 that could be used in classification. If it might be determined in every detail 

 just how the igneous mass reached its present position, the form of the body 

 and its relation to structural planes in the country-rock would therewith be 

 known. A genetic, and therefore natural, classification should thus be founded 

 on the method of intrusion. In the present state of geological science it is r 

 however, impossible to apply this fundamental principle throughout the estab- 

 lished list of intrusive bodies. 



The greater number of recognized types are those of bodies of magma 

 which is exotic except for a small, variable proportion of it due to contact fusion. 

 In each of these cases the magma has come into its chamber through channels 

 which have fed the growing body from larger, deeper-lying, generally invisible 

 reservoirs. The chamber is due to a parting of the country-rock into which the 

 magma is injected. An injected body is thus one which is entirely inclosed 



* Journal of Geology, Vol. 13, 1905, p. 485. 



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