SILURIA. 
[Chap. XI. 
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I seen such a mass of red rocks (esti- 
mated at a thickness of not less than 
10,000 feet) so clearly intercalated be- 
tween the Silurian and the Carboni- 
ferous strata ; for, whilst (as repre- 
sented in the diagram, p. 58) the fore 
and middle grounds are occupied by 
Lower and Upper Silurian rocks, de- 
scribed in Chapters III. to VII., the 
lofty distant mountains are entirely 
composed of Old Eed Sandstone. The 
observer has only to repair to the 
southern slopes of those mountains be- 
yond the line of vision in that sketch, 
and he will there see the uppermost 
beds of the red rocks conformably over- 
lain by the Carboniferous Limestone of 
the Great South-Welsh coal-basin. 
(See annexed diagram.) 
If, then, the pictorial sketch given 
at p. 58 represents the whole region 
from the Lower Silurian slates to the 
summit of the Old Red, this diagram, 
taken from one of the coloured sections ! 
of the Silurian System,' indicates the 
copious succession of red strata, which 
are exposed between the Upper Si- 
lurian, a, of the Trewerne Hills, and 
the Carboniferous rocks, g, h, i, of the 
Great South-Welsh coal-field near 
Abergavenny. 
Consisting in its lower parts of red 
and green shale and flagstone, b, with 
some small ' cornstones ' and whitish 
sandstone, c, the central and largest 
portion of the deposit is composed of 
spotted green and red clays and marls, 
d, which afford, on decomposition, the 
soil of the richest tracts of the counties 
of Brecknock, Monmouth, Hereford, a 
large portion of Salop (Shropshire), and 
small parts of Gloucester and Wor- 
cester. These argillaceous beds alter- 
nate, indeed, with sandstones, occasion- 
