u ae cS 
ei a) 
~ ¥- by : Oa 
=) Seen 
"© a 
668 | STRATIGRAPHICAL GEOLOGY.  [Boox VI. _ 
of these organisms gives a distinctive aspect to the Arenig rocks. Grap- 
tolites continue abundant in the overlying Llandeilo group, so that they 
form in Britain a convenient character by which to mark off the 
Cambrian from the Lower Silurian fauna. 
A remarkable feature in the history of the Arenig rocks in Wales 
was the volcanic action during their formation, whereby vast piles 
of various felsitic or rhyolitic lavas and tuffs were erupted to the surface 
and interstratified with the contemporaneously deposited sediments. 
Some of the more important Welsh mountains consist mainly of these 
ancient volcanic materials—Cader Idris, the Arans, Arenig Mountain, 
and others. 
2. Llandeilo Group—These dark argillaceous and occasionally calca- 
reous flagstones, sandstones, and shales were first described by Murchison 
as occurring at Llandeilo, in Carmarthenshire. They reappear near 
St. David’s, on the coast of Pembrokeshire, and at Builth, in Radnor- 
shire. Up to the present time they have yielded 80 genera and 
175 species of fossils. Of these eight genera and nine species are 
common to the Arenig below, 38 genera and 73 species to the Caradoc and 
Bala above, while 34 genera and 93 species are peculiar. The hydrozoa 
are still abundant forms, certain dark shales being copiously charged | 
with graptolites. Of crustacea 45 species belonging to 18 or 20 genera 
have been obtained. ‘These include characteristic trilobites which do 
not range beyond this group—Asaphus tyrannus, Barrandea Cordai, Caly- 
mene cambrensis, Cheirurus Sedgwicku, Ogygia Buchu, Trinucleus concentricus, 
T. Lloydii, T. favus. The phyllopod Peltocaris aptychoides is also peculiar. 
The brachiopods number 34 species, including the genera Acrotreta, - 
Crania, Lepteena, Rhynchonella, and Strophomena, which here make their 
first appearance. The Jamellibranchs are represented by six species, the 
gasteropods by 12 (Murchisonia, Cyclonema, Loxonema), the heteropods by 
seven (Bellerophon), the pteropods by two (Conularia, Theca), the cephalo- 
pods by seven (Orthoceras, Piloceras, Endoceras). 
3. Caradoc and Bala Grouwp.—Under this name are placed the thick 
yellowish and grey sandstones of Caer Caradoc in Shropshire, and the 
grey and dark slates, grits, and sandstones round Bala in Merionethshire. 
In the Shropshire area some of the rocks are so shelly as to become 
strongly calcareous. In the Bala district the strata contain two lime- 
stones separated by a sandy and slaty group of rocks 1400 feet thick. 
The lower or Bala limestone (25 feet thick) has been traced as a 
variable band over a large area in North Wales. It is usually 
identified with the Coniston limestone of the Westmoreland region. 
The upper or Hirnant limestone (10 feet) is more local. Bands of 
volcanic tuff and large beds of various felsitic lavas occur among the 
Bala beds, and prove the contemporaneous ejection of volcanic products. 
These attain a thickness of several thousand feet in the Snowdon 
region, 
"A large suite of fossils, including 179 genera and 614 species, has been 
obtained from this group. ‘The sponges are represented by Spheerospongia 
and other genera; the graptolites by Diplograptus pristis, Monograptus 
(Graptolithus) priodon, M. Sedgwickit, &c. ; the corals by 40 species belonging 
to Heliolites, Favosites, Monticulipora, Halysites, Petraia, &c.; the echino- 
derms by encrinites of the genera Cyathocrinus and Glyptocrinus, by no 
fewer than 23 species of cystideans (ehinospheerites, Spheeronites, &e.), and 
