146 GEOGNOSY AND GEOLOGY. 
The veins occurring in the Silurian and Devonian systems, of most 
importanve, are: the auriferous; the argentiferous, with red silver ore and 
antimonial silver, accompanied by arsenic and galena; the hydrargyriferous, 
with native mercury and cinnabar ; the cupriferous, with copper oxydes and 
salts, combined with quartz and barytes; ‘the plumbiferous, with galena ; 
antimoniferous, with quartz; and pyriferous, with spathic iron. These 
veins, which are sometimes of great extent, occur principally in the 
grauwacke and clay slate. 
Elie de Beaumont classifies the older strata in quite a different manner. 
He divides them into a Cambrian, a Silurian, and a Devonian system. 
a. The Cambrian system (systeéme Cambrique), according to this eminent 
geologist, is composed of the strata which rest immediately on the abnormal 
masses, gneiss, &c., included. It derives its name from the Cambrian 
mountains in Wales, where it has been particularly studied by Sedgewick. 
No very conspicuous or decided characters distinguish this system from the 
Silurian; they are, however, deposited nonconformably, and consist of 
grauwacke, clay slate, and quartz rock. ‘The limestone, of very dark color 
and brittle fracture, is in inconsiderable amount, and the clay slate is the 
same which we have already referred to as containing chiastolite. The 
rocks rarely contain organic remains, and plainly exhibit the effects of an 
elevated temperature. 
b. The Silurian system (lower grauwacke, terrain ardoisier, systéme 
Silurien). We owe our knowledge of this system to the elaborate 
investigations of Murchison. The petrographical character of the rocks 
coincides with that of the Cambrian system, although the paleontological 
and stratification conditions are different. They are found in complete 
development in England, and have been divided by Murchison into many 
groups. Pl. 46, fig. 1, presents a section of the Silurian system. Below 
all the rest lie the strata of the Cambrian system, A, against which lean 
those of the Silurian. The lowest group forms the Llandeilo formation, a, 
consisting of beds of sandstone (10) and fine granular slaty grauwacke, 
sometimes containing lime concretions. Against this, and of course at a 
higher geognostical level, rests the Caradoc formation, 6, composed of deep 
red sandstones (7 and 9), penetrated by dirty yellow veins of quartz, 
alternating with limestone (6 and 8). Then follow the Wenlock 
strata, c, consisting of clay slate (5) as the principal mass, with richly 
fossiliferous, dark, and partly crystalline limestone (4). The addition 
of clay-slate substance carries this Wenlock lime gradually into clay slate. 
The boundary of this system is formed by the Ludlow formation, d. The 
lowest bed, consisting of clay slate (3), rests immediately on the 
Wenlock lime; upon this lies a subcrystalline clayey lime (2), called 
Aymestry limestone by Murchison, and, with the micaceous lime and clay 
sandstone containing an abundance of ichthyolites, closing the series. 
The thickness of the Llandeilo strata amounts to 1200 feet; of the 
Caradoc, to 2500; of the Wenlock, to 1000 (1800); and of the Ludlow, to 
1500 (2000 ?). The entire thickness of the Silurian system in England thus 
amounts 10 6200 feet (7500 ?). 
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