254 Dr Gllby jun. m the Trap and Clay-slate 
consists; but, in the present instance, a particular detail of this sort 
would be quite unnecessary. Whoever has seen much of trap-rocks 
must have observed, that, by continually passing into each other, 
they present the greatest possible diversity of structure. I never 
was more impressed with this fact, than in traversing the trap 
hills of this district. It was hardly possible to move twenty 
yards, without finding the rock exhibit quite a different charac- 
ter from that which I had just left. Any person will easily sa- 
tisfy himself of this truth, by examining the hill near the village 
of Llanelwyth, where, from numberless projections, he will 
scarcely obtain two specimens resembling each other. In ob- 
serving, therefore, the mineralogy of this formation, I have se^ 
lected such specimens as were characteristic, or which presented 
any thing singular in their composition, and these I have tran- 
smitted as an accompaniment to the paper The rocks that I 
chiefly observed are. Greenstone of every variety. Felspar, Fel- 
spar passing into hornstone, Wacke, Porphyritic greenstone, 
and Clinkstone in a quarry near Llandrindod Wells. 
The next point of interest regarding the geology of this dis- 
trict, is the relation which the trap bears to the clay-slate for- 
mation ; for the whole of the country, comprehending a very ex^ 
tensive circuit, is composed of these two rocks. 
In ascending any part of the trap range, we at once perceive 
that it is quite shut in by a complete circuit of hills, so that the 
line of the trap formation appears like the diameter of a circle. 
These hills, as we fully ascertained, are composed entirely of 
clay-slate, with the exception of subordinate beds of sandstone, 
limestone, and grey- wacke. At the two extremities of the trap, 
VIZ. at Llandegly, and at the end of the range near Builth, the 
distance between the trap and the clay-slate is not very great, 
but on the sides of the range there is a considerable space of 
level country between them, 
In describing the course of the clay-slate hills, we may com- 
mence the line from the hills immediately behind Builth, and 
proceed with it onwards to the long continuous ridge running 
south of, and parallel Avith the road to Radnor, and which is 
knoAvn, I believe, by the name of the Aberedway Range. 
From Llanvihangel Nart Melan, the range is continued north- 
^ These have not been received, — E d, 
