406 
SILUKIA. 
[Chap. XVII. 
CHAPTER XYII. 
SILURIAN AND OVERLYING PALAEOZOIC ROCKS OF FRANCE, SPAIN, 
PORTUGAL, AND SARDINIA. 
Although the Palaeozoic rocks (often, however, in a metamorphic state) 
occupy considerable areas in France, it is not possible, on this occasion, 
to offer more than a slight sketch of their features, even in the tracts 
where they are clearly exhibited. They are largely developed in Brittany ; 
and there the authors of that great work the Geological Map of France * 
divide them into two principal masses, — the inferior being composed of 
glossy schists (schistes satines luisans) of great thickness, in which a few 
thin courses of grit and shaly limestone occur. In their mineral aspect, 
and in their entire want of fossils, these strata remind the geologist of the 
rocks which underlie the lowest fossiliferous deposits of Bohemia; and 
they may not unaptly be compared with some of the crystalline and sub- 
crystalline rocks of Anglesea and the Longmynd, or hardest Cambrian 
rocks of the "Welsh and English series. Their mean direction in Brittany 
is, like that of the Longmynd in Shropshire, from east 20° north, to west 
20° south ; and they were, indeed, long ago termed ' Cambrian ' by Elie de 
Beaumont and Dufrenoy. 
Under the name of ' Silurian ' these eminent authors included a thick 
and complex series of fossiliferous strata, which they again divided into 
two groups. The lowest of these they thus arrange : — 1st, conglomerates 
and siliceous sandstones ; 2nd, bluish schists, which at Angers, Poligny, 
&c. furnish good slates, and correspond, by their fossils, to the Llandeilo 
formation of Britain, — Trinuclei and Ogygiee, with Illaenus giganteus &c, 
being abundant. So far the succession in Brittany, as dependent on 
organic remains, is in unison with that of Britain ; but the chief portion 
of the next division, consisting of compact limestones and schists, has 
been abstracted by de Verneuil from the Silurian, and shown to belong to 
the Devonian system. In this way, the order in Brittany is analogous to 
that of Cornwall and many parts of Germany, in which tracts there is, 
as already shown, an equally sudden succession from Lower Silurian to 
Devonian. 
The second group adopted by the authors of the Geological Map of 
France is made up, first, of siliceous conglomerates, coarse grits, and 
argillaceous schists, which old geologists would have termed ' greywacke,' 
then of beds with coal, and, lastly, of a limestone specially characterized by 
* MM. Elie de Beaumont and Dufre'noy. Their 'Explication de la Carte Gdologique de France,' 
2 vols. 4to, is a rich storehouse of valuable observations. 
