406                                                  SILURIA.                                    [Ciiap. XVIL
 

CHAPTEE XYII.

SILURIAN AND OVERLYINa PALEOZOIC 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 Ogygiae, with Illaenus giganteus <fec.,
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 Yerneuil 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 Dufrenoy.    Their 'Explication de la Carte Ge'ologique de France,'
2 vols. 4to, is a rich storehouse of valuable observations.