Formation in Brecknockshire. 
255 
wards in a line north of the village of Llandegly to Penny 
Bont. On the west, the, circuit is completed by the range run- 
ning through Unnear, New Bridge, Llanavon Vach to Builth. 
Not only is the trap deposit inclosed by this circular range of 
clay-slate, but the clay-slate stratification occupies the whole of 
the low ground between the hills of the two formations. In 
skirting round the trap, and in examining the quarries and sec- 
tions at the base of the formation, we every where found it in 
junction with the clay-slate. In attaining the, Llandegly rocks 
from the road, the schist appears at the foot of the hill lying on 
the trap, but is soon lost in ascending the hill. The same is 
the case at the outskirts of the trap at Bettys, and, in the road 
to Builth, the clay-slate has in many places been cut through, 
while immediately to the right is part of the trap range. Near 
Builth, the Wye winds round the trap, while the bed of the 
river is formed of clay-slate. On the road to Llandrindod, 
there are numerous displays of clay-slate, but by deviating a 
little to the right of the road, the line of the trap is soon reach- 
ed, and at Llandrindod, the high ground above the Wells be- 
longs to the trap, and the plain below is composed of clay-slate, 
as may be seen at the small stream near the Bock-house. 
Having thus described the range of the trap, and the mode 
of its association with the clay- slate formation, I shall now speak 
of the appearances which present themselves at the junction of 
the trap with the clay-slate. 
So much having been said with regard to the derangement 
of position and alteration of structure produced by trap upon 
the rocks, it was of course an object of primary interest, in so 
extensive a formation as the present, to see as much as possible 
of the contact of the two formations : but in two places only did 
I meet with any exhibition of the actual junction of the clay- 
slate with the trap. As far as I can understand, the schist, in 
its proximity to the trap, is either hardened into a complete 
flinty-slate, or else into a soft bituminous rock, a variety indeed 
of the black drawing-chalk. This soft bituminous slate we fre- 
quently found in our walks round the trap, but I never found 
it but once in actual junction with the trap. In gaining Llan- 
degly rocks from the road, we arrive at an opening near a tavern- 
house at the beginning of the ascent. This opening is in the 
black shale, and it must then lie nearly contiguous to the trap. 
