THE ORIGIN OF THE INCLUSIONS IN DIKES 167 



their present position in Middle Ordovician shales. On the end of 

 Nash's Point a lo-foot sill of bostonite is filled with unaltered shale 

 inclusions from the immediate walls. 



Kemp and Marsters have the following suggestions to offer 

 concerning the manner in which the dikes acquired the inclusions: 



Two explanations may be offered for this rock. One, that it has been 

 intruded in a line of previous faulting and attrition, which has broken up the 

 walls and left loose material to be gathered up by the intrusive magma. This 

 explanation has the greater weight with the writers. The other is that it 

 represents only the upper portion of a dike and thus contains the float mate- 

 rial which the advance of an intrusive body, that forced its own passage, would 

 naturally gather from its walls. The lack of such inclusions in the neighbor- 

 ing dikes may be due to the fact that their tops have been eroded. 



The difificulty with the first explanation is that there is no evi- 

 dence of any faulting with brecciation at the particular places 

 where the dikes came up. Moreover, how did the sill which follows 

 a sharp fold in the shale obtain its inclusions by this hypothesis ? 

 It would appear extraordinary for faulting to occur in both a verti- 

 cal and nearly horizontal position (the shales having a low dip) 

 and to have brecciation occur only in these particular cases. The 

 second explanation is partly in accord with the theory about to be 

 proposed, but there is no reason why these should be the top of the 

 dikes, or why the dikes for some distance vertically should not be 

 equally filled with inclusions. 



The origin of the inclusions appears to the writer to have been 

 the shattering of the walls of the dike by the intrusive magma as 

 it ascended through the pre-Cambrian, Cambrian, and Ordovician. 

 Little corrosion could take place in fragments caught in such a 

 magma and shot upward to their present position where they would 

 be quickly chilled because of the inadequate supply of heat and 

 lack of time. The invaded rocks would be comparatively cold 

 when ripped off, and would therefore have a chilling effect upon the 

 surrounding dike-magma in a 20-foot dike. Kemp and Marsters 

 found the bostonite to be porphyritic, with a fine groundmass show- 

 ing flow-structure around the inclusions. The rounded character 

 of some of the inclusions is probably due to mechanical causes, 

 principally the friction of the blocks against the walls of the dike 



