GEOGNOSY AND GEOLOGY. 147 
The strata of the Silurian system, in Brittany, exhibit a peculiar 
character: they are bent in an undulating manner, so that the land consists 
of small flat hills, whose heights are formed by sandstone, and the valleys by 
slate. Asection of the country between Rennes and Nantes ( pl. 46, fig. 3) 
will make this sufficiently evident. . The undulating layers, @, consist of the 
sandstones ; the slates, b, occupying the troughs of the valleys. Each wave 
appears to have been produced by the action of the granite, d, in proof of 
which may be mentioned the metamorphism experienced by the slates 
lying nearest to it. Many members, occurring in England, in the Silurian 
system, are entirely wanting in Brittany, such as the Llandeilo flags and the 
Ludlow rocks. Upon the Cambrian system, A ( pl. 46, fig. 2), there rests 
immediately a coarse silicious conglomerate, of a red color (1), which, 
from the accurate investigations of May, appears to belong to the Caradoc 
rocks, with the incumbent greenish quartz sandstone strata (2), the 
non-fossiliferous limestones (3), and the fossiliferous quartz sandstones. 
The strata of a bituminous limestone (5), which alternates with black 
clay slate, and is known as F'iguelles limestone, correspond to the limestone 
masses which occur in England in the Wenlock group. 
c. The Devonian System (Old Red Sandstone, Terrain anthraxifere, 
Systeme Dévonien). This has likewise been ably investigated by Mur- 
chison, who distinguishes three principal subdivisions in England. | Fig. 4 
is a section of this system as found in England. 
The first division, 1, the tile-stone, lies immediately upon the upper strata 
of the Silurian system. it contains a fine-grained sandstone of decided 
stratification, so as to admit of being split into fine lamine, serving the 
purpose of tiles. It passes gradually into the Ludlow rocks, upon which it 
immediately rests, and with which it has a similar stratification: it contains 
very many ichthyolites. The second division forms the cornstone, 2, an 
alternation of variegated marls with sandstones and impure limestones, in 
which are scattered small concretions resembling grains of corn. The 
upper division, 3, consists of quartz sandstone, alternating with coarse- 
grained conglomerates and marls, through which pass inconsiderable beds 
of coal. The Devonian system attains a thickness of 10,000 feet in the 
southwest of England. 
The transition slate rocks are very rich in springs, both mineral and hot. 
Their distribution is very extended, being found in Sweden, Norway. 
Great Britain, in the Hartz, in Thuringia, Hesse, Wallachia, and West- 
phalia: in the Rhine mountains, in the Taunus, West Forest, and in part im 
the Eifel: in Upper Saxony, Bohemia, Silesia, and in the Black Forest, in the 
chain of the Alps, in Brittany, and other parts of France; in the Pyrenees, 
the Apennines, in Turkey, Greece, Siebenbtirgen, Poland, Russia, and 
in Africa, as well as America, both North and South. Their rich 
development in North America has been ably investigated by the New 
York geologists. Murchison, in connexion with M. de Verneuil, has 
published very copious investigations upon these rocks as they occur im 
Eastern Europe. 
ICONOGRAPHIC ENCYCLOP£DIA.—VOL. I. 37 577 
