60 
SILUKIA. 
[Chap. III. 
On the other hand, the annexed diagram * represents how the Llandeilo 
Llandeilo and Caradoc Eocks on the East Flank of the Berwyns. 
N.W. S.E. 
Craig-y-glyn. Llanrhaiadr. Tract watered by the Tanat. 
(From a coloured Section, Sil. Syst. pi, 32. fig. 9.) 
b. Black slates, b'. Llandeilo limestones and schists, c. Caradoc sandstones. 
formation, consisting of black slates (5) below, and of overlying calcareous 
flagstones (6', occasionally burnt for lime), occupies the eastern slopes of 
the Berwyn Mountains, as exposed in the gorge of the rapid river Twrch 
at and below Graig-y-Glyn, and above the small town of Llanrhaiadr. The 
masses more or less calcareous have a thickness of 400 or 500 feet, and are 
laden in their lower part with Asaphus tyrannus, Encrinites, and Corals, 
and in their higher portion with Trinucleus concentricus (T. Caractaci, Sil. 
Syst.), Acidaspis, Leptaena sericea, Orthis turgida, &c. 
All these strata, perfectly identical by their fossils with those of the 
Llandeilo formation elsewhere, dip at an angle of about 25° to the S.E., 
and pass with perfect uniformity under that series of shelly sandstones 
which undulate over this picturesque northern portion of Montgomeryshire. 
The latter were identified during my early researches with the mass of the 
Caradoc Sandstones. 
This view has been confirmed, the Bala limestone and associated beds 
being now further known to be identical with these overlying shelly sand- 
stones of the Yales of Meifod and the Tanat f. 
The distinctions of this overlying or Caradoc formation, both lithological 
and zoological, will be described in the next Chapter. It should here, how- 
ever, be stated that on the flanks of the Berwyn Mountains, as in the 
valleys of the Ffyrnwy and Tanat, the Lower Silurian rocks have been 
affected by a transverse slaty cleavage, from which they are usually exempt 
in the typical tracts wherein the Silurian classification was established. 
In thus collating all the results of those explorations of "Wales which 
commenced with the labours of Sedgwick and myself, and have been since 
extensively and accurately developed by the Government Geological Sur- 
veyors, we learn how local elevations have, here and there only, raised to 
the surface the strata which connect the broken succession of former life. 
When the Upper Silurian rocks shall have been described, the omissions 
of certain deposits in some parts, and their attenuation in others, will be 
placed in tabular apposition to the complete scries. 
Having spoken of the tracts where the Llandeilo schists and flags are 
* This diagram is like the section in the ' Silurian System,' pi 32. f. 9. 
t Sil. Syst. p. 306 et seq. 
