t ^53 ) 
Art. VII. — Account ^the Trap and Clay-slate Formation esc^ 
lending from Llandegly to Builth in BrecTcnochshire. In a 
Letter from Dr Gilby jun. of Bristol, to Professor Jameson. 
XIappening to pass through Builth three years ago, I was 
much surprised to find that the hills in front of the town are 
composed of trap-rocks. Incessant rain at that time prevented 
me from making any particular examination of the country ; 
hut in the summer of last year, I had another opportunity of ex- 
ploring it in company with a friend, to whose activity and acute 
observations I am much indebted; and we soon found, that 
these trap-hills form part of a long range, running north-east 
and south-west, through an extent of eight or nine miles. The 
form of this chain is irregular, and it is difficult to describe it so 
as to give an adequate idea of its shape.. The northern extre- 
mity is at Llandegly, where it is seen at what are called Llande- 
gly Rocks. The appearance of the trap here, and in every other 
part of its course, is extremely characteristic. The hills which 
it forms are frequently of considerable height ; and every where 
presenting an uniform roundness of contour, their outline is 
generally heavy and ungraceful. Upon ascending the highest 
part of Llandegly Rocks, the trap may be followed for a mile or 
more, forming a high crest or ridge; after which, it seems to 
spread itself into several parallel ridges, separated from each 
other by very distinct valleys, and finally terminates in a hill of 
considerable extent, called Carnethy, which comes down to 
Llanelwyth, ,and upon part of which Mr Thomas‘’s house of 
Llanelwyth is built. The trap, as far as this part of its course, 
preserves pretty nearly the direction of north-east and south- 
west ; but from this spot it seems to run out more to the west, 
and, crossing the river about half a mile above the bridge at 
Builth, spends itself in the ground about Park Wells. I have 
thus described the range of the trap from north to south, which 
must certainly be a distance of eight or nine miles; and its breadth, 
where it is spread out into separate parallel ridges, reckoning 
from Llandrindod Wells its western, to Betty s its eastern dank, 
cannot be less than between four and five miles. 
Ill general, it is considered interesting in the description of a 
formation, to mention the different varieties of rock of which it 
