302 



PRINCIPLES OF STRATIGRAPHY 



masses are younger than the subjacent, but older than the super- 

 jacent strata. 



Intrusive Igneous Bodies. 



The classification of intrusive igneous bodies as structural fea- 

 tures must be based primarily on their origin, form, and relation 

 to enclosing rock masses, while size and attitude with reference to 

 horizontal plane are criteria of minor significance. (Daly-7 :505.) 



Fig. 40. Diagrammatic map and section of Ascutney Mountain, Vermont, 

 illustrating a composite stock of successive intrusions, in stock or 

 boss form, of diorite, syenite, and granite. A small boss of 

 syenite (shown in black) cuts the diorite. These bodies cut crys- 

 talline schists, the attitude of which is shown by the dip-strike 

 symbols. (After Daly.) 



From the point of view of origin, or method of intrusion, two 

 great divisions of igneous intrusive masses may be recognized, (a) 

 the abyssal, deep-seated, or plutonic (Tiefengesteine), the method 

 of intrusion of which may be either a process of eating into the 

 enclosing rocks (cinfrcssen), or a pushing aside of the surround- 

 ing rocks, and (b) the injected masses, the Ganggesteine of Rosen- 

 busch, the Hypabyssal division of Brogger. The abyssal division 



