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Upper. | 
Black Slates, Flags, and Flaggy Sandstones, with Ogygia 
Buchii , &c. 
Lower. 
Calcareous Shales and Flags, &c., with Asaphus tyr annus, 
&c. 
Bala group . — The beds of this group are particularly well 
developed in the neighbourhood of Bala, Wales, therefore the 
name suggested by Sedgwick is peculiarly appropriate. The rocks 
have also been called by other authors by the name Caradoc, from 
the Caradoc mountain range in Shropshire, where they are also 
exposed. The group consists of sandstones, shales, and flags, 
alternating with calcareous bands and contemporaneous volcanic 
ash. It attains to a great thickness, but varies considerably in 
different areas in consequence of the presence of so much volcanic 
material. Of late years but little work has been done in this 
group in Wales, but in Scotland the researches of Prof. 
Lapworth have added considerably to our knowledge of the 
fauna. The fauna of the Bala group is an exceedingly rich one, 
especially in trilobites. Graptolites also occur in abundance, 
chiefly Diplograptidse and Dicranograptidae. Prof. Lapworth 
calls special attention* to the ‘total distinction in palaeonto- 
logical features between the graptolite faunas of the Bala and 
Arenig 9 groups. ‘ In the true Bala beds not a single example 
of the families of the Dichograptidae or Phyllograptidae has 
hitherto been detected. So far as our present information en- 
ables us to judge, they appear to have become wholly extinct, 
and their place is occupied by the very distinct families of the 
Diplograptidae and Dicranograptidae/ It is important also to 
notice, as bearing on the physical history of this epoch, that the 
‘ mortality in families, genera, and species of Bhabdophora in 
the Upper Caradoc (Bala) beds/ is very great. And that the 
palaeontological break between this last member of the Ordo- 
vian formation and the overlying Silurian, so far as the Bhabdo- 
phora are concerned, is almost complete. The Hartfell shales 
of Scotland belong to this horizon. 
Upper. Shales, Flags, and Limestones. 
Lower. Sandstones and Shales, with Calcareous Bands. 
Annals and Mag. of Natural History , series 5, vol. iii. 
