410 
SILUKIA. 
[Chap. XVII. 
In general, this division exhibits, in ascending order, conglomerates, white and 
reddish siliceous sandstones, with some subordinate ampelitic schists containing 
Graptolites, chiefly G. colonus and G. testis of Barrande. The position of these 
beds is probably the same as that of the Graptolite -schists of Dalecarlia and 
parts of Sweden. They agree also, according to de Verneuil, with the great 
schistose member of the American Silurians called the ' Hudson-river Group,' 
which is equally characterized by Graptolites, and overlies the Trenton Lime- 
stone or Llandeilo Flags, the equivalents, as above said, of the Angers Slates. 
As to the siliceous sandstones wherein the schists are enveloped, they are iden- 
tical with the rocks of Jurques, Gahard, and May in Normandy, placed by 
French geologists without hesitation on the parallel of the Caradoc Sandstone 
of Britain. 
In some districts the sandstone is so ferruginous as to be the seat of iron- 
mines, and the ore in parts is of an oolitic structure : Trilobites are found in it. 
The sandstones of May, Jurques, Gahard, &c. are not rich in fossils; but, 
where I have examined them in company with M. de Verneuil, they contain the 
British species Bellerophon bilobatus, with Conularia pyramid ata, some species 
of Dalmania, Homalonotus Brongniarti, and H. (Plesiocoma) rarus,- the last- 
mentioned fossil occurs also in the Lower Silurian of Bohemia and Spain. 
Like the lower and larger portion of the Caradoc formation in Britain, with 
which the Bala rocks were identified, this group of sandstones and schists is 
also connected by its fossils with the inferior slates. 
Overlying the above-mentioned strata, there is in France an upper course of 
ampelite-schist, which, though often confounded with the lower, must be sepa- 
rated from it. These upper ampelitic schists (11 of the section), being black and 
occasionally bituminous, have, as usual, given rise to expensive and futile searches 
after coal, especially at St.-Sauveur-le-Vicomte (Manche), Feuguerolles near 
Caen, and St.-Jean-sur-Ervein the Department of la Sarthe. They are distin- 
guishable from the inferior masses by containing concretions of black limestone 
with a brilliant fracture, and by certain imbedded fossils. The most characteristic 
remains are Graptolithus priodon, with some Orthocerata, including Orthoceras 
pelagicum, 0. gregarium, and the Cardiola interrupta *. As this last-mentioned 
species is chiefly an Upper-Silurian fossil in Britain, and occurs in Bohemia at 
the base of that division, it might seem fair, in the first instance, to regard this 
French deposit as of like age. At the same time we must observe that Cardiola 
interrupta, on the presence of which the comparison was drawn, is occasionally, 
though rarely, found in the Lower Silurian both of Britain and Bohemia. 
Whilst, therefore, this zone is clearly recognized through extensive districts of 
France as the highest of the rocks which are referable to the Silurian epoch, it 
is by no means an equivalent of the great British formations of Wenlock and 
Ludlow. We must, indeed, recollect that in Britain, as in Bohemia and Sweden, 
such schists with Cardiola interrupta and Graptolites underlie limestones with 
the well-known fossils of Wenlock, Dudley, and Gothland. 
In France, therefore, the ascending series of strata is very different from that 
of Britain. The Upper Silurian, as a whole, is wanting, — the succession being 
analogous to that of parts of Russia and Germany, where the Lower Silurian is 
succeeded by Devonian rocks. (See Chapters XIV., XV., XVI.) 
Devonian Bocks. — France is by no means poor in Devonian equivalents. Numerous 
species of fossils derived from the strata 12 and 13 of the foregoing diagram, p. 408, 
* See an account of the extension of this zone of ampelite-schists and its intercalated minerals, by 
M. de Boblaye, Bull. Soc. Geol. France, 1st ser. vol. x. p. 227. 
