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1822.] Geology of Snowdon, and the surrounding Country. 421 
about a mile south of the town, an abrupt cliff of considerable 
height attracted observation from the nature of the rock, which 
is in pretty thick beds or layers, dipping to the NW about 35°, 
and consists apparently of minute portions of crystalline quartz 
firmly adhering, and presenting innumerable small ochreous 
specks, as though some one of its constituents had suffered 
decomposition, and thus constituting a paste which included 
somewhat round (perhaps rolled) masses of granular quartz and 
of hornstone (?) of considerable size, and here and there small 
transparent crystals of felspar. This rock altogether greatly 
resembles some varieties of the more compact and quartzose 
beds of the old red sandstone. 
The appearance of a rock possessing characters so greatly 
differmg from every thing that we had seen in North Wales, 
indicated a complete change in the geological features of the 
country. This, however, did not altogether prove to be the 
fact, for we afterwards observed repeated instances of the same 
slates, and included rocks, as had been noticed before; but 
still it appears to us that an investigation of this part of the 
country, and particularly of the immediate neighbourhood of 
Rhyader, would prove of great interest to the geologist, as 
affording him the opportunity of observing rocks of very differ- 
ent characters in a very short compass, in such a manner, as to 
prove their connexion and possible transition from the one into 
the other. 
Between Rhyader and Built we also observed a rock most 
essentially differmg from any of the preceding. It has the 
appearance of an indurated clay, which sometimes appears in 
layers, including masses of the same substance, which, by expo- 
sure, open concentrically, and finally break down into a clay ; 
and it is only on the assumption of the prevalence of this indu- 
rated clay to a considerable extent, that we are enabled to 
account for the appearance of some large and high commons in 
this route, having pretty level surfaces. 
About a mile on the north of Built, we observed in the bed of 
the river a rock which may, perhaps, be the greenstone noted 
in Mr. Greenough’s map as belonging to the coal formation. It 
is an extremely fine-grained rock, nearly black, traversed by 
veins of quartz, but the component materials of the rock itself 
are not discoverable by the help of a glass. 
In the bed of the river on the left of the bridge, on entering 
Built, we observed a shale much resembling that of the coal 
formation, containing large spherical masses, often in the form 
of septa, of a substance which is very ponderous, and consider- 
ably resembling that of the septaria enclosed in the London 
clay. It also contained impressions of vegetables. 
t 
After leaving Built towards Brecon, the country still continues 
to improve infertility, and the hills are lower, but still few open- 
ings appear, and scarcely a rock is visible above the surface. - 
