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SIR. J. V. ELSDEN 01ST THE IGNEOUS ROCKS [Aug. 1905, 



intrusions, that it seems reasonable to refer them to the same 

 period. It is, at least, certain that they are earlier than the period 

 during which the Pwll-Strodyr faulting took place. 



In the case of the quartz-norite intrusions the evidence is more 

 obscure. I have not been able to find any section in which they are 

 actually seen in contact with the lime-bostonite sills. At Ynys-y- 

 Castell they occur very near together, and here the lime-bostonite is 

 brecciated and tuff-like. It may, therefore, perhaps be inferred that 

 the brecciation is connected with the later intrusion of the Ynys-y- 

 Castell rock, although such an explanation does not necessarily hold 



Fig. 3. 



Diagram showing 



the Relation of 



the Abercastle Intrusions 



to those of the 



South-East of Ireland. 



£^V 



in the case of the similar brecciation at Priskilly Fawr. It is perhaps 

 more probable that the brecciation in each case is connected with the 

 period of faulting, the exposures lying comparatively near to the line 

 of the Pwll-Strodyr displacement. My evidence as to the relation of 

 the Pwll-Strodyr Fault to the quartz-norite intrusions is admittedly 

 imperfect ; but, so far as it goes, it seems to support the view that 

 the quartz-norites were intruded later than the main period of 

 faulting. This view is supported by the apparent absence of any 

 signs of brecciation or shearing in any of the rocks of this type which 

 have come under my notice. With some hesitation, therefore, I 

 ascribe the quartz-norites to a later period than the lime-bostonites. 



