1817.] On the Geolosy of South Wales. 117 
between the floetz and the older rocks is exhibited in a similar way. 
The strata of the old red sand-stone, and those of the clay-slate, dip 
in opposite directions, and consequently their outgoings abut 
against each other. Mr. Jameson has given a faithful drawing of 
this appearance in his Tour through the Western Islands. 
5. No other rock is seen on the road to Builth excepting clay- 
slate, which is constantly varying in its dip, but the hills generally 
present a precipitous front to the south. : 
6. The hills near Builth, after passing over the bridge on the 
Wye, are highly curious; but incessant rain for four days hardly 
allowed me an hour’s walking. About half a mile from the town 
on the road to Presteign, there is a small chapel at the foot of a 
hill. In this hill ] found several varieties of rock, viz. of clay- 
slate, of a kind of clay-slate porphyry, and an amygdaloid. In thé 
space of a few yards the strata seemed to dip to every point in the 
compass; and in some places were considerably inclined, and at 
others vertical. 
7. A collier had lately found interstratified with the clay-slate a 
lime-stone full of shells, and sometimes containing the remains of 
the encrinus. The beds are very thin; otherwise in that country 
the discovery would be a very valuable one. From the representa- 
tions of this man, some adventurers were trying for coal; and, 
from every thing I saw of the country, without any prospect of 
success. It is in such circumstances that a knowledge of mineralogy 
and geology is so eminently useful, as the mere practical miner may 
be deceived by a thousand illusory appearances. The collier, for 
instance, and the inhabitants, showed me a stone of a black colour, 
upon which they seemed to place great reliance as an indication of 
coal. ‘The stone, however, appeared to me the common clay-slate 
impregnated with bituminous matter, being ia fact a variety of 
black chalk or drawing slate. 
8. Between Builth and Rhyader I observed little else than clay- 
slate, and a very hard kind of sand-stone, till I came within three 
miles of Rhyader, when 1 found myself under a very high and 
beautiful cliff of greywacke* in strata, dipping to the north. This 
rock was associated with the transition sand-stone just now spoken 
of. Tothe greywacke clay-slate again succeeded, which I think 
is the prevailing rock round Rhyader. 
* By greywacke, I understand a rock answering in description to Mr, 
Jameson’s definition of it, as consisting of a basis of clay-slate, containing frag- 
ments of clay-slate, portions of quartz, and scales of mica, 
