34 J 



DR. E. GREENLY ON THE SUCCESSION AND [vol. lxxix. 



intrusions : they must be segregations, 1 and segregations from a 

 solid rock. Two generations of them can be distinguished, as in 

 fig. 6, where one is seen to cut the other, and both have ultrabasic 

 encasements. 



In connexion, however, with the origin of the gneissic structures, 

 our principal concern is with the encasements, and the most 

 interesting feature of these is that they are foliated. Where their 

 parent pegmatites conform to the gneissic banding (as in the 



Fig. 6. — Pegmatites with foliated ultrabasic encasements in basic 

 gneiss, showing deflected foliation : about 170 yards north- 

 north-west of Graig-allor (natural size). 



right-hand part of rig. 4, p. 344 ), their foliation is parallel to their 

 trend and to their own bounding-planes, as in that of the other 

 bands of the adjacent gneiss. Even where gneiss, pegmatite, and 

 encasement are all folded together, the foliation of the encasement 

 is folded with the whole triple system, and remains conformable to 



1 The pegmatites of the third generation, although much larger, and 

 quite as acid, have no dark encasements, but cut gneiss which has that 

 feature. Moreover, these veins are not confined to the basic, they appear 

 also in the acid gneisses which are probably of sedimentary origin. At the 

 crag west of the road-fork west- south- west of Clegir-mawr, one of them 

 cuts across the junction of the two. There are, consequently, grounds for 

 suspecting them to be intrusions, at any rate in the positions where we now 

 see them, although they may not have come from far away, or have emanated 

 from any large batholite, for their felspars also are those of the hornblende- 

 gneiss itself. 



