CLASSIFICATION OF IGNEOUS INTRUSIVE BODIES 505 



intruded in two or more periods of irruption, the materials being 

 originally derived from two or more kinds of magma. (Fig. 9.) 



A multiple or composite batholith may thus be in part made up 

 of stocks. 



Magmatic differentiation or other influences may render hetero- 

 geneous the material composing a simple batholith; or each member 

 of a multiple or a composite batholith. 



No author has attempted to fix a lower limit to the areal dimen- 

 sions of a batholith. Since there is no certain distinction either in 

 form or relations between stocks and batholiths, an arbitrary limit 

 may be set between the two on the score of areal extent. It may be 

 proposed that a body of the kind exposed in an area of less than 200 

 square kilometers is a stock; a similar body with a larger area is, 

 accordingly, a batholith. 



PRINCIPLES OF CLASSIFICATION 



A review of the foregoing definitions shows that each of them 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 deter- 

 mined 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 there- 

 fore natural, classification should thus be founded on the method of 

 intrusion. In the present state of geological science it is, 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 portion of it due 

 to contact fusion. In each of these cases the magma has come into 



