350 



ME. P. LAKE AND PROF. S. H. REYNOLDS ON [Sept. I912, 



this side it sends out several intrusive tongues into the surrounding 

 rocks ; but despite these, it will be seen from the map (PI. XXXV) 

 that at the eastern end there is a rough parallelism between the 

 edge of the dolerite and the boundary between the Rhyolitic and 

 Ashy Series. Moreover, in this part of the area the Rhyolitic 

 Series may be seen resting upon the dolerite like the beds upon the 

 top of a laccolite. 



Throughout the greater part of the eastern half of Mynydd-y- 

 Gader the surface of contact between the dolerite and the Rhyolitic 



Series on the south is approximately 

 parallel to the bedding of the latter. 

 But, west of the fault that crosses 

 the middle of the mass, there is a 

 sudden change in the nature of the 

 junction. Here the Rhyolitic Series 

 begins to spread northwards over 

 the dolerite, although it is no longer 

 conformable with its surface. The 

 dip of the rhyolite remains south- 

 erly, while the plane of contact with 

 the dolerite begins to slope towards 

 the north. The relations are as 

 shown in fig, 1. 



Farther still towards the west all 

 parallelism between the two is lost. 

 The Rhyolitic and Ashy Series still 

 dip steeply southwards, while the 

 upper surface of the dolerite slopes 

 at a low angle either northwards 

 or southwards. This is shown 

 clearly enough at the western end of 

 Mynydd-y-Gader, where some of the 

 little valleys are floored by dolerite 

 while the intervening spurs ara 

 capped by volcanic rock. Even 

 south of the boundary shown upon 

 the map it is probable that some 

 of the valleys are cut down to the 

 dolerite : for, although the rock is 

 not actually exposed, the valleys 

 all contain an accumulation of 

 doleritic boulders for which no 

 source is visible, the ridges between 

 them consisting of volcanic rock. 



Moreover, the nature of the 



junction is revealed in an actual 



section near the southern boundary 



of the dolerite, where a favourably placed crag shows the volcanic 



series resting upon a nearly horizontal surface of dolerite, the beds 



of the volcanic series dipping steeply southwards (fig. 2, p. 351). 



