Vol. 68.] THE GEOLOGY OF MYNTDD-T-GADER. 349 



The Pare dole rite — In the angle between the Aran and the 

 Ceunant there is a large area of dolerite of yery irregular outline. 

 It is the western end of a mass which extends beyond the limits 

 of our map. The little farm of Pare stands near its northern 

 border. 



This mass of dolerite is intruded into the Middle Lingula Plags, 

 and in its midst is a small patch of the same beds. At its northern 

 margin the Plags dip beneath it, but along its southern and western 

 borders the beds dip away from the dolerite, and appear to rest 

 upon its surface. Hence we conclude that the intrusion is laccolitic 

 in character, and has lifted up the beds above it. Surrounding the 

 laccolite on its southern and western sides there are several doleritic 

 sills which follow very closely the bedding of the Lingula Plags iu 

 which they lie. 



The Mynydd-y-Gader dolerite. — By far the most important 

 and most interesting of the intrusive masses is the dolerite of 

 Mynydd-y-Gader. It forms an irregularly elliptical patch, with its 

 long axis running from west to east. Its length is about 2 miles 

 and its greatest breadth about half a mile. At each end it thins 

 abruptly, and almost or entirely disappears. It is generally a fine- 

 grained rock, and often shows marked columnar jointing. 



The relations of this dolerite to the surrounding rocks are 

 somewhat complex. A glance at the map (PI. XXXY) shows that 

 it is closely associated with the rhyolitic beds, but it does not lie 

 entirely within that series. 



The northern edge is comparatively simple. The crags which form 

 the northern face of Mynydd-y-Gader consist chiefly of dolerite ; 

 but at the base there is generally a narrow strip of volcanic rocks 

 belonging to the Ehyolitic Series. Wherever the relations of the 

 two are visible, it is clear that the dolerite lies upon the volcanic 

 beds ; and, where the line of crags has been cut back, the rhyolite is 

 almost invariably exposed in the floors of the gullies. Along this 

 edge, therefore, the dolerite lies upon the Khyolitic Series ; yet the 

 two are not conformable. The base of the dolerite is not a plane, 

 but it usually dips at a low angle towards the north. The volcanic 

 rocks, on the other hand, wherever the bedding can be made out, 

 <lip southwards at an angle of 30° to 50°. Here the dolerite is 

 certainly not laccolitic in its relations. 



Eut, although along this edge the dolerite is generally in contact 

 with the Ehyolitic Series, in some places, where its margin bends 

 northward, it touches the Lingula Plags. Wherever this happens 

 the junction is vertical, and the flags themselves show no signs of 

 metamorphism. We conclude, therefore, that these junctions are 

 faulted. The same fault or series of faults, separates the Lingula 

 Mags from the narrow strips of rhyolite at the base of the dolerite. 

 Towards the west the fault enters the Ehyolitic Series itself ; but, 

 on account of the similarity of the rocks upon both sides, it can no 

 longer be traced. 



The southern edge of the dolerite is much more complex. On 



