[4] Powrie, Geol. Quart. Journ., vol. xx, p. 417.

[5] Quart. Geol. Journ., vol. xxiii., 1867.

[6] Murchison’s Siluria, p. 329.

[7] Quart. Geol. Journ., vol. xv, p. 477, 1859; also vol. xviii, p. 296, 1862.

CHAPTER XXVI.
SILURIAN GROUP.

Classification of the Silurian Rocks. — Ludlow Formation and Fossils. — Bone-bed of the Upper Ludlow. — Lower Ludlow Shales with Pentamerus. — Oldest known Remains of fossil Fish. — Table of the progressive Discovery of Vertebrata in older Rocks. — Wenlock Formation, Corals, Cystideans and Trilobites. — Llandovery Group or Beds of Passage. — Lower Silurian Rocks. — Caradoc and Bala Beds. — Brachiopoda. — Trilobites. — Cystideæ. — Graptolites. — Llandeilo Flags. — Arenig or Stiper-stones Group. — Foreign Silurian Equivalents in Europe. — Silurian Strata of the United States. — Canadian Equivalents. — Amount of specific Agreement of Fossils with those of Europe.

Classification of the Silurian Rocks.—We come next in descending order to that division of Primary or Palæozoic rocks which immediately underlie the Devonian group or Old Red Sandstone. For these strata Sir Roderick Murchison first proposed the name of Silurian when he had studied and classified them in that part of Wales and some of the contiguous counties of England which once constituted the kingdom of the Silures, a tribe of ancient Britons. The following table will explain the two principal divisions, Upper and Lower, of the Silurian rocks, and the minor subdivisions usually adopted, comprehending all the strata originally embraced in the Silurian system by Sir Roderick Murchison. The formations below the Arenig or Stiper-stones group are treated of in the next chapter, when the “Primordial” or Cambrian group is described.

UPPER SILURIAN ROCKS.
Thickness
in feet
1. LUDLOWFORMATION:
a. Upper Ludlowbeds		780
b. Lower Ludlow beds:1,050
2. WENLOCKFORMATION:
a. Wenlock limestone andshale		above 4,000
b. Woolhope limestone and shale, and Denbighshire grits:
3. LLANDOVERYFORMATION
(Beds of passage between Upper andLower Silurian):
a. Upper Llandovery(May-Hill beds):		800
b. Lower Llandovery:600–1,000
LOWER SILURIAN ROCKS.
1. BALA AND CARADOCBEDS, including volcanic rocks:12,000
2. LLANDEILO FLAGS,including volcanic rocks:4,500
3. ARENIG ORSTIPER-STONES GROUP, includingvolcanic rocks:above 10,000

UPPER SILURIAN ROCKS.

1. Ludlow Formation.—This member of the Upper Silurian group, as will be seen by above table, is of great thickness, and subdivided into two parts—the Upper Ludlow and the Lower Ludlow. Each of these may be distinguished near the town of Ludlow, and at other places in Shropshire and Herefordshire, by peculiar organic remains; but out of more than 500 species found in the Ludlow formation as a whole, not more than five species per hundred are common to the overlying Devonian. The student may refer to the excellent tables given in the last edition of Sir R. Murchison’s Siluria for a list of the organic remains of all classes distributed through the different subdivisions of the Upper and Lower Silurian.