116 On the Geology of South Wales. (Fer. 
locum tenens of the mountain lime-stone, lies upon the old red 
sand-stone and its accompanying conglomerate. In walking from 
Myrthyr Tydvil to Brecon, the mountain lime-stone makes its ap- 
pearance for the first time about three miles from Myrthyr, and 
forms on each side of the road a cliff of considerable height. About 
a mile and a half further it ceases; and then succeeds the red sand- 
stone conglomerate; and shortly afterwards the old red sand-stone 
in its true character. Jt is seen on each side of the road, as far as 
the little river Grue. A mile or two onwards I found myself at the 
foot of a high mountain, called the Van or Brecon Beacon, which 
I ascended, and descended on the opposite side, and found that the 
whote of it, from top to bottom, is composed of the old red sand- 
stone, in strata dipping S.S.W. under the mountain lime-stone. 
The summit of this mountain commands a very extensive view. To 
the west are seen other lofty hills, presenting the same kind of 
contour as the Van, which are, therefore, probably composed of 
the same kind of rock. The country to the north of the Van pre- 
sents a hilly appearance; but the height of the hills seems very 
inconsiderable when compared with that of the Van and of those on 
the west. In fact, the configuration of the country at once shows 
an alteration in its geological structure. 
3. The coal measures on South Wales are almost identically the 
same with those in Gloucestershire and Somersetshire. They are 
micaceous sand-stone, clay-stone, slate-clay, quartzy sand-stone, 
besides which they have many beds of clay-iron-stone ; and I have 
also seen a curious kind of conglomerate very like that which occurs 
in the Dudley coal-field, 
4, A little beyond Llanvibangel between Brecon and Builth, I 
took the old road over a hill, where the geology of the country. 
assumes a new character, as the hill is almost entirely formed of 
clay-slate. Some of the strata abound with impressions of shells, 
and of a very singular minute body, which I had never seen before, 
resembling somewhat a limpet. ‘The dip of the strata in this hill is 
contrariwise to that of the Van, and its escarpement, together with 
that of the adjoining eminences, is turned towards the S.E., 
whereas the grand and precipitous facades of the Van are opposed 
to the N.E. These circumstances are highly interesting to the 
geologist, as it is clear that they could only have arisen from some 
remarkable difference in the formations themselves, as well as in 
the periods at which they were deposited ; and they become doubly 
interesting when we find them in accordance with a system which 
we have been accustomed to follow, of the truth of which they 
may then be taken as a kind of natural evidence. Thus we have 
seen that the rocks tothe south of Brecon are composed of the 
earliest members of the floetz formation, while those to the north 
of it consist of clay-slate containing organic remains, which is more 
to the north (as hereafter to be shown) associated with graywacke, 
and they are, therefore, to be considered as belonging to the transi- 
tion class. in the Island of Arran, near Loch Ranza, the difference . 
