Chap. XVII.] 
SILUEIAN OF BRITTANY. 
409 
ault ; Acidaspis Buchii, Barr. ; Phacopa (Dalmania) socialis, Barr. ; P. Dujardini, 
Rouault ; Placoparia Tournemini, Rouault ; Cheirurus claviger, Beyr. ; Lichas Heberti, 
Rouault ; Trinucleus Pongerardi, Rouault *, &c. 
Mollusks are scarcer than Trilobites ; the well-known British^species Bel- 
lerophon bilobatus, however, is not unfrequent ; but, as a rule, few of the species 
of Crustacea or of Shells are the same as those of Britain, while many are iden- 
tical with the fossils of this age in Spain and Bohemia. 
These Lower Silurian schists, in which also are Graptolites, occupy an ex- 
tensive tract (as may be seen by referring to the geological map of France), and 
crop out south of the harbour of Brest in the Bay of Crozon, where de Verneuil 
has detected in them the Calymene Tristani. The deposit affords the best roof- 
ing-slates at Angers, where the quarries are 300 to 400 feet deep. There the 
lines of original deposit and the laminae of cleavage coincide. In fact, the re- 
mains of the included Trilobites occur along the division by which the highly 
inclined slates are cleaved — a phenomenon rarely observed in Britain, where the 
cleavage is usually oblique to the bedding. (See p. 32.) 
The peninsula of the Cotentin also contains these Trilobite- schists, near Siou- 
ville, and in that tract the late M. de Gerville collected their fossils, including 
Calymene Tristani. 
That species and Placoparia Tournemini have recently been found on the Hill 
of le Roule near Cherbourg, in slates underlying the hard siliceous sandstones 
largely employed in the construction of the great breakwater of that port — 
rocks which, according to de Verneuil f, are the exact equivalents of the Caradoc 
Sandstone in age as well as in lithological structure. 
M. Dalimier, on the contrary, who has written several good papers on the 
Silurian rocks of Normandy and Brittany, considers the hard sandstones of the 
Roule to be older than the slates with Calymene Tristani, on account of the 
Scolithus he found in them. The apparent infraposition of the slates he explains 
by an overthrow or reversal of the beds. The following succession of the older 
deposits in Brittany, as admitted by Dalimier J, is indeed perfectly in accordance 
with the section by Triger and de Verneuil we have just given : — 
Lower Devonian... Limestone and fossiliferous greywacke (Ize, Gahard). 
Upper Silurian. ] 
(Trace of lowest I Slates with Cardiola interrupta. 
part only.) J 
( a. Sandstones with the fauna of May (Gahard, May). 
Middle Silurian. b. Slates with Graptolithus colonus (Mortain, Poligne). 
(Lower Silurian, \ c. White sandstone without fossils. 
Murchison.) j d. Roofing-slates with Calymene Tristani (Angers, St. Leonhard, 
^ Cherbourg). 
Lower Silurian, f White sandstone with Scolithus, Bilobites, and Lingulae ( =Stiper 
('Primordial' Silu--! Stones and Lingula-flags). 
rian, Murchison.) [ Red slates and conglomerates. 
^ , . f Green slates and sandstones. 
Cambrian i Gneiss. 
The lower slaty rocks of Llandeilo age are likewise succeeded in La Sarthe 
by an arenaceous group, represented by Nos. 8 and 9 of the previous woodcut. 
* De Verneuil and Kouault, Bull. Soc. Grdol. Fr. of M.Dalimier's Table does not represent the Llan- 
2nd ser. vol. iv. p. 320. dovery rocks. It is simply a natural Lower Silu- 
t Bull. Soc. Gre'ol. Fr. 2nd ser. vol. xiii. p. 303. rian or Caradoc and Llandeilo group, the lowest 
I Ibid. vol. xx. p. 126. part of which (d) is, as before shown, the unmis- 
It is to be understood that the ' Middle Silurian ' takeable equivalent of the Llandeilo Flags. 
