Vol. 68.] 



THE GEOLOGY OF MTNTDD-Y-GADEK. 



351 



Prom this account it will be seen that along its northern boun- 

 dary the dolerite always cuts across the bedding of the volcanic 



series. 



Eig. 



2. — Shetch of a crag at the 

 western end of Mynydd-y- 

 Gader^ shoiving the relation 

 of the Rhyolitic Series to the 

 dolerite. 



£The overhanging ledge at the top 

 consists of rbyolite dipping steeply 

 southwards (to the right in the 

 figure). The undercut portion is 

 dolerite. The plane of junction 

 between the two is nearly hori- 

 zontal.] 



Along the southern 

 boundary its upper surface is 

 nearly parallel to the bedding 

 in the eastern part of Mynydd-y- 

 Gader, but transgressive in the 

 western part. 



These relations appear at 

 first sight rather complex, but 

 the key to the explanation will 

 be found in fig. 1 (p. 350). 



In this section the thickness 

 of the dolerite is necessarily 

 arbitrary, for the under sur- 

 face is exposed only at the 

 northern end, where it is seen 

 resting on the Rhyolitic Series. 

 But the general form must be 

 that shown in the figure. It 

 is clear that it consists of a 

 nearly horizontal limb resting 

 in the north trausgressively 

 upon the Rhyolitic Series, 

 and a descending limb which 

 plunges conformably beneath the rhyolitic beds on the south. It 

 is only the thickness and the precise shape of these limbs that 

 remain doubtful. As the rhyolitic beds in the north of the section 

 generally dip at a rather lower angle than those in the south, it is 

 probable that the descending limb thins downwards and approaches 

 in shape a typical laccolite rather than a sill, but no attempt has 

 been made to indicate this in the figure. 



In the actual line of section the dip of the rhyolitic masses 

 which rest upon the horizontal limb is difficult to determine, for 

 they are so altered that their stratification is almost lost. We do 

 not know, therefore, exactly where the rhyolitic beds cease to run 

 parallel to the upper surface of the dolerite. 



It is, moreover, possible that these rhyolitic masses may not be 

 so entirely superficial as is shown in the diagram. They may be 

 slightly sunk into the dolerite, but apparently not to any great 

 depth : for, wherever the ground-level falls much below the line 

 which in the figure is taken as their base, the dolerite is exposed. 



Using this section as a foundation, and restoring the beds above 

 the horizontal limb, we obtain the diagram shown in fig. 3 (p. 352). 

 Prom this diagram it is clear that the visible relations of the dolerite 

 to the beds in contact with it will depend upon the depth to which 

 the whole mass has been eroded. 



If, for example, the line AB is the surface of the ground, the 

 dolerite will be seen at its northern end to lie unconformably 

 upon the rhyolitic beds, at its southern end to plunge conformably 



