SYNOPSIS OF THE BRITISH STRATA. ao 
Subdivisions :— 
UPPER SILURIAN. 
1. Ludlow rocks ;—slightly micaceous grey-coloured sandstones. 
Blue and grey argillaceous limestones. Dark-coloured shales and flag- 
stones, with concretions of earthy limestone, containing marine shells, 
Orthocerata, Spirifers, and Trilobites. Jishes. 
2. Wenlock, or Dudley limestone ;—sub-crystalline blue and grey 
limestone, abounding in Trilobites, Crinoidea, Polyparia, Spirifers, 
Orthocerata, &c. 
3. Wenlock shale ;—dark grey argillaceous shale, with nodules of 
sandstone. 
LOWER SILURIAN. 
1. Caradoc sandstone ;—shelly limestones, and finely laminated, 
slightly micaceous, greenish sandstones. Corals, Shells, Trilobites. 
2. Llandeilo flags and limestones. Freestone, conglomerates, grits, 
and limestones. Dark-coloured flags. Beds of schist with abundance 
of Trilobites and shells. The lowermost beds are full of small 
bivalves, termed Lingule. 
Obs.—The Silurian System, (so named by Sir R. Murchison, from 
the Silures, the ancient Britons who inhabited those parts of our island 
in which the geological relations of these strata were first recognised 
by him,) occupies the border counties of England and Wales, and 
spreads over a vast area of both North and South Wales, intervening 
between the Carboniferous series and the Cambrian or ancient slate- 
rocks of that country.* The strata are entirely of marine origin, and 
many of the beds (as the well-known Dudley or Wenlock limestone) 
are composed of shells, corals, crinoideans, and remains of that re- 
markable family of crustaceans termed T’rilobites, cemented together 
by carbonate of lime. A few remains of Fishes occur: Reptiles are 
* «Top SILURIAN System, founded on Geological Researches in the 
Border Counties of England and Wales.” In two parts, royal 4to., 
with map, sections, &c., by Sir R. I. Murchison, G.C. St.S. &. In 
studying the beautiful map which accompanies the work, it must be 
borne in mind that ten years have elapsed since Sir R. Murchison 
abrogated the boundary line that separates the Cambrian and Silurian 
rocks in this chart, from the conviction that those deposits constitute 
but one natural system (see Wond. p. 803). For an account of the 
Silurian rocks of other countries, see’ GEoLocy or Russra,” by the 
same Author. A summary of the characters of the Silurian System, by 
Sir R. Murchison, is given in Geolog. Journal, vol. viii. pp. 173—183. 
VOL. I. D 
