44 GEOLOGICAL SURVEY 
New York Reports, and it is remarkable that the first appearance of life on our continent, as pre- 
sented in this rock, occurs in the form of the shell of a marine animal, the Lingula, a genus of 
which there are a few species living at the present day—a fossil which is also found in great 
abundance in Russia. Fucoids, or sea-weeds, are generally the first organic remains found in these 
rocks; and in Norway and Sweden, according to the researches ot Prof. Forchammer, they have 
played quite an important part in the formation of the oldest fossiliferous strata. As we ascend in 
the series fossils become more abundant: in the Trenton limestone there are more than twenty 
genera. : 
The Hudson River group Mr. Hall considers the true equivalent of the Cambrian system of 
Great Britain. It marks, in the New York system, a very important point, for scarcely any fossil 
species are common to it and the supermcumbent rocks in that State. 
The chief characteristic of the Fauna of the lower Silurian rocks is the great development of Bra- 
chiopod molluscs of the genera Terebratula, Orthis, Delthyris, Atrypa, &c. and a remarkable family 
of Crustaceans, the Trilobites, now extinct. It is supposed these ancient articulated animals stood 
in the same relation to the fucoidal plants of the Silurian seas that the numerous small crustacea 
of the present day do to the sea-weeds, among which they abound. Some corals and other remains 
are also found among these early inhabitants of the globe. They exhibit, however, as a group, 
animals of low organization—not extending higher in the scale of being than the class Articulata. 
And no vestige of land plants or of any vertebrate animal has yet been discovered in these rocks. 
Upper Silurian Rocks.—In America this division of the Paleozoic series is represented by the 
formations between the Niagara group and the Chemung group of the New York Reports; and by 
formations Nos. 7, 8, and 9 of the Pennsylvania and Virginia Surveys; in Ohjo, by the Cliff lime- 
stone and Waverly sandstone ; and in Michigan, by the Mackinac limestone, and the sandstones of 
Point aux Barques. 
In England they include the rocks known as the Ludlow and Wenlock formations, which con- 
sist of beds of great thickness of shales and limestones. On the continent of Europe they have 
been traced at intervals from Norway to Constantinople. 'They are found in South America and 
in the Polynesian Islands. 
These rocks abound in 'Trilobites, Crinoidea, and a peculiar chambered shell, called Orthocera. 
In the upper, or Wenlock, formation scales and teeth of fishes, and an ichthyodorulite, (a part of the 
fin of a fish,) have been found in the coniferous limestone of the New York system. These are 
the sole remains yet found of vertebrate animals in the Silurian rocks of this country. 
Land plants, such as ferns, make their appearance towards the uppermost groups of the series. 
The rocks of the New York system resemble the Silurian rocks of Russia, in reposing horizontally 
and without disturbance on the metamorphic rocks. , 
The absence of any disturbing cause for so long a period is remarkable. The Silurian system, 
as developed in New York, presents the enormous thickness of eight or nine thousand feet, and 
in England these rocks are equally extensive. The organic remains found entombed in them on 
the spot where the animals to which they belonged lived and died, show that they were deposited 
gradually and slowly. So that the period of time necessary for the deposition of so great a mass 
of sedimentary matter must be immense. 
Devonian or Old Red Sandstone System.—The rocks of this system consist of slates, limestones, 
