346 ME. p. LAKE AND PROF. S. H. REYNOLDS ON [Sept. I 9 I 2, 



narrow gorges. The plateau extends southwards to a line running 

 from Gilfachwydd to Tan-y-Gader and Bryn-rhug, and thence, less 

 definitely and less regularly, to Pant-yr-onen and the Aran. From 

 this line the ground rises more steeply, and sometimes with con- 

 siderable undulations, to the foot of the crags which form the 

 northern face of Mynydd-y-Gader. 



Mynydd-y-Gader itself is a very rugged ridge about half a mile 

 wide, highest along its southern border and sloping unevenly down- 

 wards to its northern edge, where it presents a steep and rocky 

 face to the lower-lying ground. It terminates abruptly at both its 

 eastern and its western ends. 



Prom the crest of Mynydd-y-Gader the surface falls irregularly 

 to a broad depression at the foot of the Cader Idris range. 



The general structure of the area is simple. As we have shown 

 in our previous paper, the strike of the beds upon the slope at the 

 northern foot of Mynydd-y-Gader is approximately from north-west 

 to south-east, the dip being south-westerly. But this dip is not 

 constant throughout. Towards the east, beyond the limits of our 

 former map, a large mass of dolerite has been intruded, apparently 

 in laccolitic fashion, lifting the beds above it in the form of a 

 dome. 



On the southern side of Mynydd-y-Gader the dip is different. 

 In general it is almost due south at a high angle, but near the Aran 

 the strike bends round towards the north-east, following the curve 

 of the dolerite intrusion. It is, in fact, evident from the map 

 alone that there is a certain parallelism between the stratified rocks 

 and the dolerite mass of Mynydd-y-Gader. Bat this parallelism 

 is not complete, and the relations of the two will be considered 

 later. 



The structure is complicated by a number of faults, which appear 

 to form two distinct series : some running approximately from 

 north to south, and others from north-west to south-east. All 

 have their downthrow upon the west, so that the newer beds 

 appear in this direction. Only a few of these faults produce a,nj 

 effect upon the northern side of the Mynydd-5--Gader dolerite. 



There is, moreover, a fault, or rather a series of faults, along the 

 northern margin of Mynydd-y-Gader ; and this is approximately the 

 boundary between the prevalent south-westerly dip in the north 

 and the prevalent southerly dip in the south of the map. The 

 downthrow of this fault or series of faults is evidently upon its 

 southern side. 



South of Mynydd-y-Gader there are not uncommonly indications 

 of crushing, which lead us to suspect the existence of strike-faults. 

 Such faults are difficult to detect or prove. In the south-west of 

 our map, however, some of the other faults end abruptly against 

 an east-and-west line. They dislocate the rocks upon the one 

 side of the line, but do not affect the beds upon the other side. 

 We infer, therefore, the presence of a strike-fault, and our infer- 

 ence is confirmed by the abundance of quartz-veins and of signs of 

 crushing along this line. 



