70 
SILURIA. 
[Chap. IV. 
where this zone appears at the surface, and chiefly at Gretton, the following 
copious list has been made out, nearly all the species being also known in the 
Welsh mountains around Bala, or on the summit of Snowdon &c. Those 
marked* are most abundant. 
Acidaspis Caractaci; Calymene Blumenbachii * ; Homalonotus bisulcatus*; 
Phacops truncato-caudatus * ; P. conophthalmus * ; Lichas laxatus ; Illaenus ; 
Tentaculites anglicus * ; Serpulites ; Cornulites serpularius ; Discina oblongata 
(Portl.) ; Orthis vespertilio * ; O. elegantula * - % 0. Actoniee ; O. biforata j Stro- 
phomena * resembling S. grandis ; S. tenuistriata * ; S. bipartita * ; Leptsena 
sericea * ; L. transversalis * ; Modiolopsis orbicularis * ; Ctenodonta varicosa * ; 
Murchisonia simplex ; Cyclonema crebristria ; Conularia Sowerbyi ; Belle- 
rophon bilobatus * ; B. acutus * ; an Orthoceras ; with a profusion of fragments 
of Encrinites, Polyzoa, and several Corals, of which the Stenopora fibrosa and 
Nebulipora favulosa (var. lens) are the most frequent. 
The uppermost band strictly belonging to this formation has been called the 
Trinucleus Shales. It is charged with the prevailing Trinucleus concentricus, 
and the Lower Silurian Brachiopod Leptaena sericea, together with Orthis ele- 
gantula, Ctenodonta, &c. 
In working out the classification first propounded in the ' Silurian 
System/ I also grouped with the Caradoc formation certain beds of pebbly 
grits and impure earthy limestone, which, overlying the great mass of the in- 
ferior Shelly Sandstone of Shropshire, are seen to pass under the "Wenlock 
formation. Whilst in the first edition of this work I pointed out a dis- 
covery made since my early survey, in regard to the physical relation of these 
strata to the inferior rocks, I still held to the opinion that they ought rather 
to be grouped with the Lower than the Upper Silurian rocks, and hence I 
spoke of them as ' Upper Caradoc' A reexamination of tracts in South 
Wales, where these deposits are more fully developed than in Shropshire, 
led me to consider them as a formation intermediate between the two 
great groups of Lower and Upper Silurian, and as connected through their 
organic remains with both. They were therefore described in the last 
edition under the name of ' Llandovery Bocks ' (see next Chapter). 
Caradoc (or Bala) Formation in Wales. — Adopting the belief prevalent 
among geologists when the Silurian classification was proposed, that the 
slaty and crystalline rocks of Wales were, as a whole, of older date than 
the softer mud-stone rocks on the east, the greater part of the former 
were referred, without proofs of infraposition, to an older class of deposits. 
That view was indeed abandoned very shortly after the publication of the 
' Silurian System,' or twenty-five years ago, — i. e. as soon as it was ascer- 
tained that the fossils of Snowdon and Bala were identical with those of 
previously described Caradoc rocks. 
A careful comparison of the fossils of North and South Wales with those 
of the Caradoc Sandstones of Shropshire, and a reference to certain natural 
sections, have in fact enabled us to go still further, and to make a separa- 
tion between the Llandeilo and Bala formations, which, for want of such 
detailed acquaintance, was not attempted even when the first edition of 
