2 SIDNEY POWERS 



PART I 



INTRODUCTION 



Several dikes containing numerous inclusions have recently 

 been seen by the writer, and the origin of these inclusions appears 

 to be of sufhcient interest to warrant a brief notice of these and other 

 cases found in the literature/ The dikes have received various 

 descriptive names, but none of them is sufficiently comprehensive 

 to include all the examples. 



Inclusions are not infrequent in all types of both extrusive and 

 intrusive rocks. Those in extrusive rocks have been specially 

 treated by Lacroix. Intrusive rocks are so extensive and of such 

 varying forms that only dikes and a few related intrusions will be 

 considered here. The xenoliths in the large intrusive bodies have 

 formed the basis for the stoping hypothesis, but only part of the 

 inclusions in dikes are of this origin. 



Dikes acquire inclusions by shattering blocks off the walls of a 

 fissure during their ascent through it. These fragments may remain 

 near the place from which they came, or they may move up or 

 down. In most cases the fragments rise, whether of greater specific 

 gravity than the molten dike-rock or not, because they are forced 

 upward by the magma. In the cases where the fragments sink, 

 either they are heavier than the magma or they are carried down. 



Some dikes have invaded conglomerates, from which they have 

 dissolved the cement and included the bowlders. In other cases the 

 fissures through which the magma came may have been open to the 

 surface, so that stream gravels fell in and were caught in the ascend- 

 ing magma. In rare instances a dike may ascend through a fault 

 breccia and thus acquire its inclusions. These special cases are 

 not separated from the other examples. 



To classify dikes according to the direction of movement of the 

 inclusions is difficult, because in almost every case some inclusions 

 rise and some remain near the place of origin. A classification 

 is attempted which will at least serve to emphasize the number 

 of examples in which inclusions sink. 



' The writer is indebted to Professor R. A. Daly for suggestions concerning this 

 paper. 



