It consists
of about 4000 feet of slates, shales, and flags, and is divisible
into a lower, middle, and upper division, of which the former
is often regarded as Cambrian under the name of "Upper Tremadoc
Slates."
2. The _Llandeilo Group_.--The thickness of this group varies
from about 4000 to as much as 10,000 feet; but in this latter
case a great amount of the thickness is made up of volcanic ashes
and interbedded traps. The sedimentary beds of this group are
principally slates and flags, the latter occasionally with calcareous
bands; and the whole series can be divided into a lower, middle,
and upper Llandeilo division, of which the last is the most
important. The name of "Llandeilo" is derived from the town of
the same name in Wales, where strata of this age were described
by Murchison.
3. The _Caradoc_ or _Bala Group_.--The alternative names of this
group are also of local origin, and are derived, the one from
Caer Caradoc in Shropshire, the other from Bala in Wales, strata
of this age occurring in both localities. The series is divided
into a lower and upper group, the latter chiefly composed of
shales and flags, and the former of sandstones and shales, together
with the important and interesting calcareous band known as the
"Bala Limestone." The thickness of the entire series varies from
4000 to as much as 12,000 feet, according as it contains more
or less of interstratified igneous rocks.
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