Parr IL. Szcr, ii. §1.] SILURIAN. | 661 
Section II. Silurian. 
The important system of rocks next to be described was first 
investigated by the late Sir R. I. Murchison in Wales and the bor- 
dering counties of England. He found it to be characteristically 
developed over the tract once inhabited by the Silures, an ancient 
_ British tribe, and he thence chose the name of Silurian as a 
convenient designation. Passing down conformably into the Tre- 
madoe slates at the top of the Cambrian series, and being covered 
conformably by the base of the Old Red Sandstone, it there repre- 
sents a somewhat better defined section of Paleozoic time than the 
_Cambrian system, and offers a more satisfactory base for comparison 
in other countries. No geological suite of deposits has been traced 
over a wider extent of the earth’s surface, or presents, on the whole, 
so uniform a series of lithological and paleontological characters. 
§1.—General Characters. 
Rocks.—The Silurian system consists usually of a massive series 
of greywackes, sandstones, grits, shales, or slates, with occasional bands 
of limestone. The arenaceous strata include pebbly grits and con- 
glomerates, which are specially apt to occur at or near any local base 
of the formation, where they rest unconformably on older rocks. 
Occasional zones of massive conglomerate occur, as among the 
Llandovery rocks of Britain. The argillaceous strata are in some 
regions (Livonia, &c.) mere soft clays: most commonly they are hard 
fissile shales, but in some regions, (Wales, &c)., where they have 
been subjected to intense compression, they appear as hard cleaved 
slates or even as schist and gneiss (Scotland, Ireland). In Europe 
the limestones are, as a rule, lenticular, as in the examples of the 
Bala, Aymestry, and Dudley bands, though in the basin of the Baltic 
some of the limestones have a greater continuity. In North America, 
on the other hand, the Trenton limestone in the Lower, and the Niagara 
limestone in the Upper Silurian system are among the most persistent 
formations of the United States. Hasily recognizable bands in many 
Silurian tracts, especially in the north-west of HKurope, are certain 
dark anthracitic shales or schists, which, though sometimes only 
a few feet thick, can be followed for many leagues. As they usually 
contain much decomposing iron disulphide which produces an efflores- 
cence of alum, they are known in Scandinavia as the alum-schists. 
In Scotland they are the chief repositories of the Lower Silurian 
graptolites. Their black, coal-like aspect has led to much fruitless 
mining in them for coal. In the northern part of the State of New 
York, a series of beds of red marl with salt and gypsum occurs in the 
Upper Silurian series, and in the Salt Range of the Punjaub a group 
of saliferous strata belongs to a still older period. These salt-bearing 
