258 Dr Gilby jiin. on the Trap and Clay-slate 
nally occupied by the soft slate, which has been subsequently 
washed away by the flowing of the stream. 
I have said that most of the ridges projecting into the bed 
of tlie current, are composed of trap. This, however, is not al- 
ways the case, for there are several composed of an exceedingly 
hard stratified limestone, occurring in beds in the clay-slate, 
and wliich is frequently so hard as not to yield to the knife. 
Two of these limestone ridges are seen between the first and 
second great ledge of trap. The limestone is also sometimes 
mixed with the trap, as is the case at the third great ledge, 
where the stream runs in a narrow chasm in the trap, near an 
oak which stands in the middle of the stream. On the north 
side of this tongue of rock, the slate is mixed with the lime- 
stone in a very curious way, the limestone having apparently 
been disrupted from a regular bed, and appearing in detached 
lumps in the slate. Higher up, the small island above alluded 
to, is composed of trap mixed with limestone, sometimes afford- 
ing specimens somewhat resembling Verde Antique. 
I have now a few words to say upon the subject of or- 
ganic remains. The trap is, as usual, entirely destitute of 
them. In no instance did I observe any thing like a fossil re- 
main in the trap. In different situations we found the clay- 
slate containing orthoceratites, trilobites, and curious impres- 
sions of a small shell like a limpet. I have a very fine or- 
thoceratite which I obtained from the clay-slate between The 
Three Cocks and Builth. In the quarry in Mr Thomas’s 
grounds, we found the hardened slate full of trilobite im- 
pressions, which are sometimes very delicate and beautiful. 
The fossil is quite different from the Dudley one, and precisely 
the same as that which I have seen from the clay-state in other 
parts of W ales. The limestone and sandstone which occur as 
beds in the slate, likewise contain organic remains. In the 
sandstone I have frequently seen nests of a peculiar kind of 
beaked anomia ; and we generally found the limestone crowded 
with madrepores, tubipores, small branches of coral, and seve- 
ral kinds of shells, most of which might readily be identified 
with those found in the Dudley limestone. In fact, from the 
nature of the organic remains alone, it is impossible for a mo- 
ment to hesitate in pronouncing the formations above described, 
as belonging to the Transition series. 
