Chap. III.] 
SLATY LLANDEILO FLAGS. 
57 
thickness of fossiliferous schists and sandstones with some calcareous courses, 
which clearly represent the original Caradoc Shelly Sandstones, to be de- 
scribed in the next Chapter f. 
The Llandeilo nags and schists of Pembrokeshire and Carmarthenshire, 
charged with the same characteristic Trilobites and other fossils, emerge 
everywhere from beneath overlying deposits, often of a more arenaceous 
type, which are of the age of the Caradoc formation. In Carmarthenshire 
the Llandeilo Flags are associated, as elsewhere, with igneous rocks, both 
stratified and eruptive, though not on the same scale or of that clear and 
demonstrative character which is exhibited in the tract of Shelve in Shrop- 
shire, and in the district between Builth and Llandegly. By reference to 
the Map, indeed, it will be seen that it is just where such igneous rocks 
abound, as marked by bright red colour (*), whether in Shropshire or in 
Wales, that the Llandeilo formation (2 b ) rises prominently to the surface, 
through the surrounding deposits. 
Thus, in proceeding from the Llandeilo tract to the north-east, we first 
S.E. 
Llanwrtyd Wells. 
(From Sil. Syst. p. 336. See also woodcut, Sil. Syst. p. 343.) 
N.W. 
Lower Silurian slaty rocks. 
View in a combe below the Baths of Llanwrtyd. (From a drawing by Mrs. Traherne.) 
meet with decisive eruptive masses of porphyry and other rocks at and 
near the Wells of Llanwrtyd in Brecknockshire, and their black schists and 
I believed it to be of older date than the Lower 
Silurian. Hence I mapped it all as Cambrian. The 
first rectification of this erroneous view was made 
in 1842, by Professor Kamsay, who observed that, 
instead of being succeeded by lower rocks to the 
north and west, the Llandeilo Flags folded over in 
those directions and passed under superior strata 
charged with fossils which Mr. Salter recognized 
as well-known types of the Caradoc or Bala beds. 
Hence the whole of those districts of South Wales 
came to be coloured as Lower Silurian by the Greo- 
logical Surveyors. (See Map.) 
t To the north and west of the Llandeilo dis- 
trict, and all along the western frontier of the 
Builth country, the boundary-line of my first map 
of the Silurian Kegion, which separated the Silu- 
rian rocks from the so-called Cambrian of that 
day, was purely arbitrary. It is merely to be 
regarded as a demarcation between rocks on the 
east and south , with which I was acquainted, and 
a vast slaty region on the west and north — which I 
had not examined, and with whose order and fossil 
contents I was unacquainted, though in common 
with Sedgwick and other geologists of the day 
