86 
SILURIA. 
[Chap. V. 
up into the lower member of these deposits, and from the latter into the 
npper band of the formation. The table of colours attached to the Map, 
and the accompanying brackets, explain the nature of this link ; and the 
geologist who may examine the various tracts of country described in these 
pages will readily understand this classification. 
Reverting to the natural section where the flags of Llandeilo (p. 73), to 
the north of the town of that name, are overlain by the equivalents of the 
Caradoc formation, it has been observed that there the strata of the latter 
graduate upwards into coarser sandstones, occasionally conglomerates, in 
which some Pentameri have been found. These last-mentioned strata, 
though containing few fossils, extend, as already mentioned, over consi- 
derable tracts of South and North Wales, where they really form the sum- 
mit of the Lower Silurian rocks. 
On the left bank of the Towy, and particularly in all the tract extending 
from the River Sowdde to ISToeth Griig, north of Llandovery, these sandstones 
and shales, with a conglomerate at their base, and also resting on Caradoc 
rocks, prevail in a range of hills whence many characteristic fossils, pub- 
lished in the ' Silurian System/ were procured *. It was formerly stated 
that in the environs of Llandovery this sandstone had not the durability 
and firmness of the Caradoc Sandstone of Shropshire, and was not so easily 
separable from those Upper Silurian rocks from beneath which it emerged 
in the wooded hills near that town. The formation, however, was shown 
to be loaded with many fossils of true Lower Silurian character, derived 
from Rhiwfelig, Blaen-y-cwm, Cefn Rhyddan, and Goleugoed, the last- 
mentioned locality, whence the Llandovery building- stone is extracted, 
having afforded most of the species. 
The following fossils, figured in the ' Silurian System/ are repeated from that 
work in Plates VIII., IX., X., XI. : viz., Orthoceras bisiphonatum ; 0. approxinia- 
tuni ; Lituites undosus ; L. cornu-arietis ; Pleurotornaria Pryceae ; P. angulata ; 
Holopella cancellata ; Rhynchonella pusilla j R. -neglecta ; R. tripartita ; Orthis 
lata (including O. protensa) ; 0. calligramma ; 0. Actoniae ; 0. elegantula ; Atrypa 
crassa ; A. reticularis ; Pentanierus lens j P. globosus ; P. undatus ; P. oblongus, 
or lsevis ; Strophomena compressa ; Leptaena sericea, &c. &c.f 
In following these rocks to the north-east, by Llandovery, through the 
elevated moory grounds of Mwmfre, they are seen to become more 
quartzose, and to rise up into the bare and stony hills of Noeth Griig and 
Cefn-y-garreg about 1500 feet above the sea, where the whole forma- 
tion is admirably exposed, as in this diagram 
* The collections were chiefly made by my re- J I revisited this locality since the first edition 
spected friend the late Mr. Williams, Surgeon, of of this work, in company with Earl Ducie, Prof. 
Llandovery, and his son, now the Kev. Stewart Eamsay, and Mr. Aveline. The latter furnished 
Williams. me with a section more accurate than any one 
t Several names of fossils given in the ' Silurian previously published, although it does not differ 
System,' p. 351 &c„ are here omitted, as more essentially from the original section given in the 
perfect specimens have shown that those forms ' Silurian System.' 
belong to some of the species here named. The 
names of some genera have also been changed; 
but the t3 r pes remain the same. 
