1.  The Arenig Group.—­This group derives its name from the Arenig mountains, where it is extensively developed.  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.

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):—­