262 E. Greenly — The River Cefni, Anglesey. 



the rocks become more and more foliated in appearance, then 

 igneous material begins to show between the folia, and we come to 

 half-igneous, half-metamorphic ' banded gneisses ' at what we may 

 take as the junction. Inside the igneous mass huge fragments of 

 what was originally schist may often be seen, which by fusion and 

 impregnation, in the absence of stirring movements, fully bear out 

 Professor Cole's contention that " masses of quartz-diorite arise 

 as products of admixture where granite intrudes into more basic 

 masses." 



Of course, normal contact rocks are seen where the old 

 sedimentaries are invaded, but from what has been said on the 

 question of relative age, it will be inferred that this is comparatively 

 rare. I have nevertheless been able to record from time to time 

 the presence of nearly all the characteristic contact minerals, like 

 andalusite, fibrolite, kyanite, cordierite, etc., chiefly from ' Banded 

 Ironstone ' junctions. But the granite masses have kept as a rule 

 well within bounds, and rarely break through their covering of the 

 old Basement schists or those of igneous origin. But in the absence 

 of actual invasion by a granite mass, it is fairly obvious whether or 

 no the latter must be regarded as intrusive. The usual intense 

 folding of the schists leaves little room for doubt as a rule, for rocks 

 involved in movements which have left the granite undisturbed must 

 necessarily be of greater age. 



I am afraid some of the points discussed above may be regarded 

 as of a rather obvious nature. My excuse for putting them into 

 writing must be that they are seldom referred to in geological works 

 and are almost persistently overlooked in dealing with the structui'e 

 of metamorphic areas. 



IV. — The Kiver Cefni in Anglesey. 

 By Edward Greenly, F.G.S. 



MANY years ago, when discussing the origin of the Menai Straits, 

 attention was drawn by Eamsay to the existence of several 

 other valleys running in the same direction, north-east and south-west. 

 Not only is this the case, however, but the valleys of the dominant 

 system that traverses the plateau of Anglesey have the same trend, 

 ridge and trough alternating with wonderful regularity, as far, at 

 any rate, as Llanerchymedd, a distance of some 12 miles from 

 the Straits. 



Much less conspicuous, for the most part, though more numerous 

 than at first sight appears, are certain valleys running at right angles 

 to those of the dominant system, and therefore north-west and 

 south-east. They may drain either to north-west or south-east. 

 They are unlike those of the dominant system in almost every 

 particular. Those are long and straight : these are short and 

 sharply winding. Those are broad and relatively shallow, these 

 narrow and relatively deep. Those are generally bounded by gently 

 sloping sides, these by steep and even precipitous sides. Much the 



