Chap. IV.] 
CAEADOC FOKMATION IN WALES. 
75 
undulate in gentle curves. There are, indeed, proofs, in several tracts, 
that these rocks, as in the district north of Llandeilo, graduate upwards 
into the Pentamerus zone of Llandovery, the inferior and larger portion of 
which belongs essentially to the Lower Silurian division. 
When the earliest edition of this work was published (1854), it was still 
the belief of geologists that the Llandeilo and the Bala formations were 
one and the same ; and, although the inferences which have since been 
arrived at rest greatly upon the true order of superposition as worked out 
by Ramsay and Aveline, we are also greatly indebted to the palaeontologist 
(Mr. Salter) who has cooperated in establishing the true sequence. On my 
own part, I simply claim the credit of having originally placed the Llan- 
deilo formation beneath all the deposits of the age of the shelly sandstones 
of Caer Caradoc, with which the Bala beds were long after identified. This 
order was also first pointed out by me in parts of Montgomeryshire and 
Denbighshire ; and it is now applied to the whole of Wales*. 
Throughout large portions of North Wales there are, indeed, other 
evidences of the age of the rocks in question, besides those already adduced 
on the east flank of the Berwyns. At and near Bala, for example, the 
sandstones, slates, and limestones are the absolute equivalents of the 
Caradoc shelly sandstones in Shropshire. The chief limestone of this 
group, exposed in low hills near the town and lake of Bala, is so impure 
that it is now never used for burning, and, dwindling away to the S.S.W., 
is lost among the slaty strata. Another calcareous course, near Hirnant, 
is simply one of the shelly sandstones of Shropshire. Thus, with the ex- 
ception of having been affected by a slaty cleavage unknown in the English 
types of these rocks, the North- Welsh strata are, even lithologically, to a 
great extent similar to the Caradoc formation of Shropshire. 
The fossils of the Bala beds are, indeed, as already said, identical with 
those of the flanks of Caer Caradoc, there being scarcely a species found 
in Wales which does not occur in Shropshire. 
Sufficient proofs having been now offered of the superposition of the 
Caradoc to the Llandeilo rocks, and the fossil characters of the two forma- 
tions having been generally indicated, we might at once proceed to con- 
sider the distinctions of the next overlying beds, all of which I once 
grouped with the Lower Silurian rocks. But, having for some time re- 
garded those strata as an intermediate deposit, the larger and inferior por- 
tion of which is naturally connected with the Lower, and the higher mem- 
bers with the Upper Silurian, a separate chapter is devoted to them. In 
the meantime, however, a few pages must be occupied by a sketch of 
the igneous or volcanic masses which especially abound in the Lower 
Silurian formations. 
* In the first edition of this work I endeavoured, and descriptions given in the 'Silurian System,' 
in spite of my old convictions, to accommodate which are clear and demonstrative, as before 
my views to those of my successors, who had re- stated, in identifying the great undulating masses 
garded ' Llandeilo ' and ' Bala ' as synonyms, and of strata in the Welsh region watered by the 
therefore suppressed my old section from the Tanat and Ffyrnwy rivers, as well as the Welsh 
flank of the Berwyns to Welsh Pool. But in the Pool ridge, with the shelly sandstones of Caer 
second edition I reverted to the original details Caradoc. 
