which Paleontology forms an essential part. A group of strata extending over a certain geographical extent, all of which contain some fossils in common, no matter what may be the chemical character of the rock, whether it be limestone, sand, or clay, is termed a geological Formation. Thus, the coal beds, with the intervening slates and grits, and the masses of limestone, between which they often lie, constitute but one formation the carboniferous formation. 461. Among the stratified rocks we distinguish ten principal Formations, each of which indicates an entirely new era in the earth's history; while each of the layers which compose a formation indicates but some partial revolution. Proceeding from below upwards, they are as follows, as indicated in the cut, and also in the lower diagram on the Frontispiece. 1st. The Lower Silurian. This is a most extensive formation, no less than eight stages of which have been made out by Geologists in North America, composed of various limestones and sandstones.* *1. Potsdam Sandstone; 2. Calciferous Sandstone; 3. Chazy Lime stone; 4. Bird's-eye Limestone; 5. Black River Limestone; 6. Trentou Limestone; 7. Utica Slate; 8. Hudson River Group; being all found in the western parts of the United States. 2d. The Upper Silurian. It is also a very extensive for mation, since about ten stages of it are found in the State of New York.* 3d. The Devonian, including in North America no less than eleven stages. It occurs also in Russia and Scotland, where it was first made out as a peculiar formation. 4th. The Carboniferous Formation, consisting of three grand divisions.‡ 5th. The Trias, or Saliferous Formation, which, containing the richest deposits of Salt on the continent of Europe, comprises three stages, to one of which the Sandstone of the Connecticut valley belongs. It comprises at least four 6th. The Oölitic Formation, only faint traces of which exist on the continent of America. distinct stages.|| 7th. The Cretaceous, or Chalk Formation, of which three principal stages have been recognized, two of which are feebly represented in this country, in the Southern and Middle States. 8th. The Lower Tertiary, or Eocene, very abundant in the Southern States of the Union, and to which belong the coarse limestone of Paris, and the London clay in England. 1. Oneida Conglomerate; 2. Medina Sandstone; 3. Clinton Group; 4. Niagara Group; 5. Onondaga Salt Group; 6. Water Limestone; 7. Pentamerus Limestone; 8. Delthyris Shaly Limestone; 9. Encrinal Limestone; 10. Upper Pentamerus Limestone. 1. Oriskany Sandstone; 2. Cauda-Galli Grit; 3. Onondaga Limestone; 4. Corniferous Limestone; 5. Marcellus Shale; 6. Hamilton Group; 7. Tully Limestone; 8. Genesee Slate; 9. Portage Group; 10. Chemung Group; 11. Old Red Sandstone. 1. The Permian, extensively developed in Russia, especially in the government of Perm; 2. The coal measures, containing the rich deposits of coal in the Old and New World; 3. The Magnesian Limestone of England. § 1. New Red Sandstone; 2. Muschelkalk; 3. Keuper. 1. The Lias; 2. The Lower Oolite; 3. The Middle Oʊlite; 4 The Upper Oŏlite. 9th. The Upper Tertiary, or Miocene and Pleiocene, found also in the United States, as far north as Martha s Vineyard and Nantucket, and very extensive in Southern Europe, as well as in South America. 10th. The Drift, forming the most superficial deposits, and extending over a large portion of the northern countries in both hemispheres. We have thus more than forty distinct layers already made out, each of which marks a distinct epoch in the earth's history, indicating a more or less extensive and important change in the condition of its surface. 462. All the formations are not every where found, or are not developed to the same extent, in all places. So it is with the several strata of which they are composed. In other words, the layers of the earth's crust are not continuous throughout, like the coats of an onion. There is no place on the globe where, if it were possible to bore down to its centre, all the strata would be found. It is easy to understand how this must be so. Since irregularities in the distribution of water upon the solid crust have, necessarily, always existed to a certain extent, portions of the earth's surface must have been left dry at every epoch of its history, gradually forming large islands and continents, as the changes were multiplied. And since the rocks were formed by the subsidence of sediment in water, no rocks would be formed except in regions covered by water; they would be thickest at the parts where most sediment was deposited, and gradually thin out towards their circumference. We may therefore infer, that all those portions of the earth's surface which are destitute of a certain formation were dry land, during that epoch of the earth's history to which such formation relates, excepting, indeed, where the rocks have been subsequently removed by the denuding action of wat or other causes. 463. Each formation represents an immense period of time, during which the earth was inhabited by successive races of animals and plants, whose remains are often found in their natural position, in the places where they lived and died, not scattered at random, though sometimes mingled together by currents of water, or other influences, subsequent to the time of their interment. From the manner in which the remains of various species are found associated in the rock, it is easy to determine whether the animals to which these remains belonged lived in the water, or on land, on the beach or in the depths of the ocean, in a warm or in a cold climate. They will be found associated in just the same way as animals are that live under similar influences at the present day. 464. In most geological formations, the number of species of animals and plants found in any locality of given extent, is not below that of the species now living in an area of equal extent and of a similar character; for though, in some deposits, the variety of the animals contained may be less, in others it is greater than that on the present surface. Thus, the coarse limestone in the neighborhood of Paris, which is only one stage of the lower tertiary, contains not less than 1200 species of shells; whereas the species now living in the Mediterranean do not amount to half that number. Similar relations may be pointed out in America. Mr. Hall, one of the geologists of the New York Survey, has described, from the Trenton limestone, (one of the ten stages of the lower Silurian,) 170 species of shells, a number almost equal to that of all the species found now living on the coast of Massachusetts. 465. Nor was the number of individuals less than at present. Whole rocks are entirely formed of animal remains, particularly of corals and shells. So, also, coal is composed of the remains of plants. If we consider the slow ness with which corals and shells are formed, it will give us some fant notion of the vast series of ages that must have elapsed in order to allow the formation of those rocks, and their regular deposition, under the water, to so great a thickness. If, as all things combine to prove, this deposition took place in a slow and gradual manner in each formation, we must conclude, that the successive species of animals found in them followed each other at long intervals, and are not the work of a single epoch. 466. It was once believed that animals were successively created in the order of their relative perfection; so that the most ancient formations contained only animals of the lowest grade, such as the Polyps, the Echinoderms, to which succeeded the Mollusks, then the Articulated Animals, and, last of all, the Vertebrates. This theory, however, is now untenable; since fossils belonging to each of the four depart ments have been found in the fossiliferous deposits of every age. Indeed, we shall see that even in the lower Silurian formation there exist not only Polyps and other Radiata, but also numerous Mollusks, Trilobites, (belonging to the Articulata,) and even Fishes. SECTION II. AGES OF NATURE. 467. Each formation, as has been before stated, (460,) contains remains peculiar to itself, which do not extend into the neighboring deposits above or below it. Still there is a connection between the different formations, proportion to their proximity to each other. mal remains of the Chalk, while they differ from those of all other formations, are, nevertheless, much more nearly related more strong in Thus, the ani |