es, 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.

4. The _Llandovery Group_ (Lower Llandovery of Murchison).--This
series, as developed near the town of Llandovery, in
Caermarthenshire, consists of less than 1000 feet of conglomerates,
sandstones, and shales. It is probable, however, that the little
calcareous band known as the "Hirnant Limestone," together with
certain pale-coloured slates which lie above the Bala Limestone,
though usually referred to the Caradoc series, should in reality
be regarded as belonging to the Llandovery group.

The general succession of the Lower Silurian strata of Wales
and its borders, attaining a maximum thickness (along with
contemporaneous igneous matter) of nearly 30,000 feet, is
diagramatically represented in the annexed sketch-section (fig.
34):--

[Illustration: Fig 34. GENERALIZED SECTION OF THE LOWER SILURIAN
ROCKS OF WALES.]

In North America, both in the United States and in Canada, the
Silurian rocks are very largely developed, and may be regarded
as constituting an exceedingly full and typical series of the
deposits of this period. The chief groups of the Silurian rocks
of North America are as follows, beginning, as before, with the
lowest strata, and proceeding upwards (fig. 35):--

1. _Quebec Group_.--This group is typically developed in the
vicinity of Quebec, where it consists of about 5000 feet of strata,
chiefly variously-coloured shales, together with some sandstones
and a few calcareous bands. It contains a number of peculiar
Graptolites, by which it can be identified without question with
the Arenig group of Wales and the corresponding Skiddaw Slates
of the North of England. It is also to be noted that numerous
Trilobites of a distinct Cambrian _facies_ have been obtained in
the limestones of the Quebec group, near Quebec. These fossils,
however, have been exclusively obtained from the limestones of
the group; and as these limestones are principally calcareous
breccias or conglomerates, there is room for believing that these
primordial fossils are really derived, in part at any