This system is of great thickness, amounting, in places where it is well developed, to twenty thousand feet.

The Champlain division commences with a quartzose sandstone, passing gradually into limestone, which is succeeded by a very thick argillaceous deposit, the Utica slate and Hudson River group. The Ontario division in the lower part is a mass of sandstone. Above this is the Clinton group, consisting of shales and sandstones. The most important part of this group, in an economical point of view, is a fossiliferous, argillaceous iron ore, coextensive with the group in this country, and is worked to supply a large number of furnaces. The last of the division is the Niagara group, which commences with a mass of shale, and becoming at length calcareous, it terminates in a firm compact limestone. This limestone has withstood the action of denuding causes better than the shales either above or below it. It therefore presents a bold escarpment at its outcrop, and occasions waterfalls wherever streams of water cross it. The falls of Niagara are formed by this rock. The Niagara limestone, in its extension westward, becomes the lead-bearing rock of Missouri, Iowa and Wisconsin. The Helderberg division is a succession of highly fossiliferous limestones, with the intervention of only occasional beds of grits and shales. One member of the series is the Onondaga Salt group. The water obtained from this group in New York annually furnishes immense quantities of salt. The Erie division consists of a thick mass of shales and sandstones.

Fig. 8.

Fig. 9.

Fig. 10.

Fig. 11.

Fig. 12.

The fossils of this system are very numerous, but consist mostly of the lower forms of animal life. Corals (Fig. [8.] and 9) are abundant, and constitute in some places a large proportion of the limestones. The Crinoidea, or lily-shaped animals, consist of a jointed stem permanently attached, and bearing at the free extremity of the stem an expanded portion, which is the pelvis, or digestive cavity. The mouth is surrounded with a series of leaf-like tentacula, which serve the purpose of seizing and holding food. [Fig. 10] represents the pelvis of one of the silurian fossils. The general character of the animal is better represented by [Fig. 30]. The most abundant fossils of this period are the lowest orders of bivalve mollusca ([Fig. 11]). The Cephalopoda are characterized by having the organs of locomotion attached to the head. The shell of several species is peculiar in being divided into distinct cells, or chambers ([Fig. 12], b d), perforated by a tube (siphuncle a). These fossil shells are sometimes straight, as the Orthoceras ([Fig. 13]), or curved, as shown in the several forms of [Fig. 14]. The Trilobite was an articulated, crustaceous animal, having two lines along the back dividing it into three lobes, from which circumstance its name is derived. It is found in great numbers in the Silurian rocks ([Fig. 15]). In a few instances remains of fishes have been found, but they by no means characterize the system.

Fig. 13. Fig. 14.

Fig. 15.

The geographical range of this system is probably greater than that of any system of rocks above it. It is found occupying a large part of the territory west of the Alleghany Mountains, from Canada, through New York, and the other states, to Alabama; and extending westward to and beyond the Mississippi river. It occupies a large district in the west of England, and is found in great force in the north and east of Europe.