CLASSIFICATION OF THE SILURIAN ROCKS. 31 
( Hard sandstones, conglomerates, and flaggy shaly beds; the fossils 
Lower Llandovery. | closely allied to those of the Upper Llandovery series ; but 
distinguished by Pentamerus lens, and by unconformity. 
f Shelly sandstones, shales, and slaty beds, with quartzose grits, 
conglomerates, and occasional limestone or calcareous bands ; 
| fossils numerous, especially Echinodermata, Crustacea (Tri- 
lobites), Polyzoa, and Brachiopoda; of the last nearly fifty 
Caradoc or Bala ; 
| species, including Orthis Actonie and O. flabellulum, Loc. 
L 
Rocks. 
Caradoc, Horderley, Norbury, Bala, Snowdon, Wexford, 
Kildare. 
Dark-grey flagstones, occasionally calcareous, with black slates con- 
taining Graptolites, &e. The Llandeilo Flags constitute two 
groups, Upper and Lower. Lingule, Obolelle, Diseine, and 
Orthide occur in, and characterise this group of rocks, with 
Ogygia Buchii, Asaphus tyrannus, and Trinucleus concen- 
tricus. oc. Llandeilo, Builth, Shelve district, Cader Idris, 
&e. 
Llandeilo Flags 
Dark-grey and ferruginous slates, sandy shales, and hard bluish 
flags, with occasional beds of pisolitic iron-ore, much felspathic 
matter in lines and thin beds; the Lower and Upper Tremadoc 
Slates contain numerous genera of Trilobites, Ogygia scu- 
tatrix, Asaphus Homfrayi, Aiglina, Trinucleus, &c., also Hetero- 
pod and Pteropod Mollusca, &c., with Graptolites (Diplograpsus 
pristis and D. folium), and Orthis alata and O. testudinaria. 
Loc. Stiper Stones, Tremadoc, Portmadoce, Ffestiniog, Dolgelly, 
Dudreath, &c. 
Group. 
Lower Llandeilo 
or 
Tremadoc Slates 
[ 
| 
4 
| 
ie 
LLANDEILO r 
| 
| 
| 
a 
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A great series of black and dark shales, grey and brown 
thin slaty flagstones and sandstones, with siliceous grits and 
quartzites (Stiper Stones) ; occasionally crowded with fossils, and 
always having the characteristic Lingula (Lingulella) Davisii. 
It is equally remarkable for its Crustacea (Paradozides, Oleni, 
Agnosti, Hymenocaris, &e.). Loc. Dolgelly, Barmouth, Ffesti- 
niog, St. David’s, &c. 
Lingula-flags 
LOWER SILURIAN. 
— ee ee 
-—_-*+-— 
CAMBRIAN ROCKS Dark-green, grey, black, and brown flags, also grits and con- 
glomerates, occurring at Harlech, Anglesea, Llanberris, St. David’s, 
the Longmynd, &c., with few fossils. 
LAURENTIAN ROCKS “Fundamental Gneiss” of Murchison. Western promontories of 
Ross and Sutherland, the Lewis, Lews, or Long Island, 
including Harris, and other parts of the Hebrides. 
low bone-bed in the Old Red Sandstone or Devonian. But I have shown that the presence of the 
peculiar small Ichthyolites, Plectrodus mirabilis and Onchus Murchisoni, ‘ Sil. Sys.’ (which have never 
been found in the Old Red Sandstone), afford no reason for separating this fish-bed from the Ludlow 
Rocks, seeing that it is filled with unequivocal Silurian marine shells, some of which occur even low down 
in the system (‘Siluria, ed. 1859, p. 157, note). ; 
Fossil Fishes have also been found in the highest bed of the Upper Silurian of the Baltic island of Osel 
(Thyestes verrucosus, Hichwald, and Cephalaspis Schrenkii, Pander ; see the memoir by F. Schmidt, ‘ Imper. 
Mineral. Soc. St. Petersburg,’ 1865-6). There also, as at Ludlow, the Ichthyolites are intermixed with 
marine Upper Silurian forms, such as Platyschisma helicites, Orthis, Ptilodictya, Chonetes, Pterinea, 
Orthoceras, Calymene. Again, just as in Britain, none of the true Old Red Sandstone fishes, so common 
in Russia, occur with these peculiar Ichthyolites of the youngest Silurian strata. 
