50 
SILURIA. 
[Chap. III. 
containing many lead-mines which have been worked from the time of the 
Romans. Snch mineral features require more illustration than can be given 
to them in the present volume ; and on this head, and for all local details 
respecting the igneous rocks, the reader is referred to the ' Silurian System.' 
From these lower strata the observer ascends into a thick succession of nag- 
stones of dark-grey and light-blue colours, in parts calcareous, such as those 
which compose the chief mass of the Llandeilo formation (Section, p. 24). 
These beds abound in Trilobites — Asaphus tyrannus, Ogygia Buchii, 
and Trinucleus being the most frequent, — together with Lingulse, Grapto- 
lites, &c. These and other characteristic fossils will be figured in a wood- 
cut ; they are also seen in Plates I., II., III., IV., V., taken from the 
' Silurian System.' 
The characteristic Trilobites also occur abundantly in flagstones around 
the town of Llandeilo, and in the adjacent parts of Carmarthenshire and 
Pembrokeshire, as well as at Builth in Radnorshire, and near Llanrhaiadr 
in Denbighshire ; but, as before said, the beds in which they there lie do 
not exhibit the relations to underlying rocks which are so clearly exposed 
in this district of Shropshire. 
Now all the strata which are exposed, from the shale or schist beneath 
the Stiper Stones inclusive to the western edge of this Shelve tract, or to 
the villages of Chirbury and Meadowtown (Section, p. 38), and which, with 
the interstratified volcanic ashes, have been ascertained by the Surveyors 
to have a thickness of about 14,000 feet, were laid down as Lower Silurian 
in my earliest publications, from the year 1833 onwards. On the other 
hand, the fossils of North Wales, subsequently shown to be of the same 
age, had not then even been detected, much less described, although the 
strata of the tract we have been considering had been paralleled, by means 
of their principal organic remains, with the Llandeilo formation of Car- 
marthenshire. 
The Llandeilo Formation in Wales .—Having indicated the relations of 
these ' Black Trilobite Flags ' to the subjacent rocks in the typical district 
of the Silurian Region, let us next consider them in those parts of "Wales 
where the formation is laden with the same characteristic fossils *, and 
surmounted by other strata which are now ascertained to be the equiva- 
lents of the Caradoc Sandstone of Shropshire. In North Wales, strata 
similar to those which have been described on the west flank of the Stiper 
Stones (differing only by being in a slaty condition) exhibit a like passage 
from the Lingula-flags into the mass of the Lower Silurian rocks. There 
the latter, with many slaty ash-beds, occur beneath the great bands of the 
porphyritic rocks of Arenig and Ffestiniog, near Tremadoc f , and at Llan- 
faelrhys, Carnarvonshire, at which places they contain the well-known 
* See Memoirs G-eol. Survey, vol. iii. Appendix, f Termed 4 Tremadoc Slates ' by Professor 
pp. 256 & 258, for Mr. Salter's complete list of the Sedgwick in 1847. None of their fossils, however, 
fossils of the Llandeilo rocks of Wales and Shrop- were described until the year 1851 ; indeed the 
ghire. greater part were found by Mr. Salter in 1853. 
