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ELEMENTS OF GEOLOGY. 
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,—Cystidege. 
“—Graptolites.—Llandeilo Flags.—Arenig or Stiper-stones Group.—For¬ 
eign 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 de¬ 
scending order to that division of Primary or Palaeozoic 
rocks which immediately underlie the Devonian group or 
Old Red Sandstone. For these strata Sir Roderick Murchi¬ 
son 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 fol¬ 
lowing 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 em¬ 
braced 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 
1. Ludlow Formation : in feet. 
a. Upper Ludlow beds . .. 780 
h. Lower Ludlow beds 1,050 
2. Wenlock Formation: 
a. Wenlock limestone and shale a 
h. Woolhope limestone and shale, and Denbighshire grits j ^ ■ 
3. Llandovery Formation (Beds of passage between Upper 
and Lower Silurian): 
a. Upper Llandovery (May-Hill beds) . ..800 
b. Lower Llandovery *. 600-1,000 
LOWER SILURIAN ROCKS. 
1. Bala and Caradoc Beds, including volcanic rocks . . . 12,000 
2 . Llandeilo Flags, including volcanic rocks.4,500 
3. Arenig or Stiper-stones Group, including volcanic 
rocks.above 10,000 
