FOSSILS OF THE LLANDEILO FLAGS. 
473 
black mud abounded. The formation, when traced into South 
Wales and Ireland, assumes a greatly altered mineral aspect, 
but still retains its characteristic fossils. The known fauna of 
the Bala group comprises 565 species, 352 of which are pe¬ 
culiar, and 93, as before stated, are common to the overlying 
Llandovery rocks. It is worthy of remark that, when it oc¬ 
curs under the form of trajDpean tutf (volcanic ashes of De la 
Beche), as in the crest of-Snow don, the peculiar species which 
distinguish it from the Llandeilo beds are still observable. 
The formation generally appears to be of shallow-water ori¬ 
gin, and in that respect is contrasted with the group next 
to be described. Professor Ramsay estimates the thickness 
of the Bala Beds, including the contemporaneous volcanic 
rocks, stratified and unstratified, as Jbeing from 10,000 to 
12,000 feet. 
Llandeilo Flags. — The Lower Silurian strata were origi¬ 
nally divided by Sir R. Murchison into the upper group al¬ 
ready described, under the name of Caradoc Sandstone, and 
a lower one, called, from a town in Carmarthenshire, the 
Llandeilo flags. The last mentioned 
strata consist of dark-colored micaceous 
flags, frequently calcareous, with a great 
thickness of shales, generally black, be- 
low them._ The same beds are also seen nidymo„rapsus (Oraptouus) 
at Builth, in Radnorshire, where they are M^rcMsonn, Beck. Liau- 
interstratified with volcanic matter. deiio flags, Wales. 
A still lower part of the Llandeilo rocks consists of a black 
carbonaceous slate of great thickness, frequently containing 
sulphate of alumina, and sometimes, as in Dumfriesshire, beds 
of anthracite. It has been conjectured that this carbona¬ 
ceous matter may be due in great measure to large quanti¬ 
ties of imbedded animal remains, for the number of Grapto- 
lites included in these slates was certainly very great. In 
Fig. 556. 
Fig. 557. 
Fig. 558. 
Diplograpsus pristis, Hisiiiger. Llandeilo 
beds, Waterford. 
Rastrites peregrinus, Barrande. 
Scotland; Bohemia; Saxony. 
Llandeilo flags. 
