XI] BRECKNOCKSHIRE 165 



shown in the sketches opposite, showing a plan of the summits 

 and a section through them to explain how accurately the two 

 coincide in their slope with that of the original plateau. 

 Every year the frost loosens the rock on the northern pre- 

 cipices, every heavy rain washes down earth from the ridge, 

 while the gentler showers and mists penetrate the soil to the 

 rock surface, which they slowly decompose. Thus, year by 

 year, the flat portion of the summits becomes smaller, and a 

 few thousand years will probably suffice to eat them away 

 altogether, and leave rocky peaks more like that of Snowdon. 

 The formation, as we now find it, is, in my experience, unique 

 — that is, a mountain-top presenting two small patches of 

 almost level ground, evidently being the last remnant of the 

 great rolling plateau, out of which the whole range has been 

 excavated. Double-headed mountains are by no means un- 

 common, but they are usually peaked or irregular, and carved 

 out of inclined or twisted strata. The peculiarity of the 

 Beacons consists in the strata being nearly horizontal and 

 undisturbed, while the rock formation is not such as usually 

 to break away into vertical precipices. The original surface 

 must have had a very easy slope, while there were no meteoro- 

 logical conditions leading to great inequalities of weathering. 

 The thick covering of peat has also aided in the result by 

 preserving the original surface from being scored into gullies, 

 and thus more rapidly denuded. 



After we had completed most of our work at Trallong we 

 had to go further up the valley to Devynock. This is an 

 enormous parish of more than twenty thousand acres, divided 

 into four townships or chapelries, the two eastern of which, 

 Maescar and Senni, we had to survey. In these mountain 

 districts, however, we only surveyed those small portions 

 where the new roads or new enclosures had been made, 

 the older maps being accepted as sufficiently accurate for 

 the large unenclosed areas of mountain land. We first went 

 to Senni Bridge, where both districts terminate in the Usk 

 valley ; but after a short time I went to stay in a little public- 

 house at Senni in the midst of my work, while my brother 



