352 



ME. P. LAKE ATs^D PEOF. S. H. EEYNOLDS ON [Sept. 1912^. 



beneath them. These are the actual relations observed in the 

 eastern part of Mynydd-y-Gader. 



If, however, erosion has not gone so far and the line CD is the^ 

 surface of the ground, in the north the dolerite will rest un- 

 conformably upon the Ehyolitic ISeries ; while in the south the 

 rhyolitic beds will lie upon the dolerite, but not conformably. The 

 junction will be a nearly horizontal plane, and the rhyolitic beds, 

 dipping steeply southwards, will abut against that plane. These 

 are the relations in the western part of Mynydd-y-Gader. 



It appears, then, that in section the dolerite is roughly L-shaped, 

 with one limb of the L nearly horizontal and the other dipping 

 steeply southwards. But the angle of the L is not at the same height 

 above the sea throughout the length of Mynydd-y-Gader. It slants 

 downwards from east to west, and the effect of the slant is increased 



Fig. 3. — Diagram illustrating the relations of the Mynydd-y-Gader 



dolerite to the lihyolitic and Ashy Series. 



[Symbols as in fig. 1.] 



to some extent by the transverse faults. In consequence of this the L 

 stands higher in the eastern part than in the western part of 

 Mynydd-y-Gader. It is accordingly more deeply eroded in the east, 

 and it is on this account that the relations of the dolerite to the 

 volcanic beds are so different at the two ends of the hill. 



The form of the intrusion is shown in fig. 4 (pp. 354-55), in which 

 Mynydd-y-Gader is represented as cut up into a series of blocks, 

 so as to exhibit the internal structure. The diagram is not drawn in 

 perspective, nor is it a true projection. It is a purely artificial but 

 convenient representation in which vertical heights and distances 

 along the base-lines (north 30° west to south 30° east, and east to 

 west) are drawn in their true proportions. 



In the present position of the mass, it appears as if the intrusion 

 had come up from below along the plane of bedding, and had then 

 cut through the beds and spread out almost horizontally. But the 

 intrusion probably took place before the beds received their present 

 dip; and, if we place the section so that the bedding is horizontal,. 



