186 Norman L. Silvester — Igneous Complex of 



By reflected light there seems to be a faint indication of the develop- 

 ment of two minerals producing a vague spotted appearance. 



At the base of the Aber Falls the slate shows distinct spotting in 

 the hand specimens. This rock when sliced will probably reveal 

 a greater development of contact minerals. 



The granophyre, when followed up either the Afon Bach or the 

 Cwm yr Afon Goch, becomes first much coarser in texture, then at 

 a distance of 200 feet from the falls junction develops patches of 

 darker hue, in which the dark minerals are segregated. Further 





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Fig. 1. — The Igneous Complex of Y Foel Fras. 

 upstream the rock becomes more fine-grained and darker and more 

 homogeneous in the hand specimen, but still with occasional j)atches 

 of coarser grain. Near the south-eastern margin of the intrusion 

 the rock becomes sheared. This is of interest, as it appears to be 

 evidence in favour of the theory put forward by Dr. A. Harker that 

 the direction of the thrust which produced the great earth-movements 

 in Bala times was from the south-east. There are no signs of shearing 

 on the north-west margin. This phenomenon is best seen on the 

 summit of Llwvdmor Bach. 



