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structure of any one stratified rock, that entitles us absolutely to say, that other rocks and living bodies could not have existed prior to its formation. But as there are large tracts of country occupied by strata, in which neither fragments of pre-existing rocks nor organic remains have yet been discovered, geologists are justified in designating them the primary strata; to call them primitive, as they used to be, and indeed still are called by some geologists, is to employ a term, which expresses much more than we are entitled to assert.
The unstratified rock, most usually associated with the primary strata, is granite, of different varieties of composition, usually lying under them in great masses, and bursting through, forming lofty pinnacles, as in the Alps, and sometimes sending forth shoots or veins, which penetrate the superincumbent strata in all directions.
Immediately above the primary strata there commences another series, very like many of the rocks below them, in respect of mineral composition, but containing the remains of shells, and some pebbles, and interstratified with thick beds of limestone, including shells and corals. These rocks are penetrated also by granite, and, in common with the primary strata, form the great depository of the metallic ores. They are, for want of a better term by which the class can be distinguished, usually called the transition strata, a name given by the elder geologists, because they were supposed to form a step or transition from the primitive state of the globe to the condition in which it began to be inhabited by living bodies; in strictness, they form the lowest members of the next great division of the strata, which is distinguished by the name of the Secondary Rocks. These will be treated of in our next section. lbid.
VI.- MINERAL KINGDOM.
The SECONDARY Rocks comprehend a great variety of different beds of stone, extending from the primary strata to the chalk, which forms the upper or most recent member of the division.
These rocks consist of an extensive series of strata, of limestones, sandstones, and clays, all of which contain either rounded fragments of pre-existing rocks, or organic remains, or both; and each group, and all the subordinate members of the groups, are distinguishable by characters of great constancy and certainty, derived from the peculiar nature of the included fossils. They must all have been deposited in a horizontal position; but there are parts of them, which have undergone greater or less disturbance, being often thrown into a vertical position, and broken, twisted, and disturbed in the most extraordinary manner. Many of the disturbances of the lower groups took place prior to the deposition of the upper; for the latter are found lying in unconformable stratification on the ends of the former, as represented in the diagram, in page 52. They are traversed by veins, or dykes, as they are often termed, of whinstone and other unstratified rocks; and there is usually great disturbance of the strata, when these occur. The dykes are often of great magnitude, and the rock is frequently thrust in huge wedgeshaped masses, of miles in superficial dimensions and, some hundred feet thick, between the regular strata. After the deposit of the secondary rocks, a remarkable change took place; for all the strata that lie above the chalk, have a totally different character from that rock, and all below it.
These have been classed together in one great division, and have been designated the TERTIARY Rocks. Thus the whole series of strata, of which the crust of the globe is composed, is divided into the Primary, the Secondary, and the Tertiary. It is evident, that at the time the secondary rocks were deposited, a great
part of the present continent of Europe must have been considerably lower than the present level of the sea; that when the oldest or lowest members of the series were forming, the summits of the mountain ridges of primary rocks rose as islands of different magnitudes from the bosom of the deep ; that at several successive periods these islands were more elevated, and attained consequently a greater superficial extent, the newer formed strata occupying the lower levels. In the progress of this series of changes of the surface of the globe, when there were evidently occasional depressions of the land as well as elevations, there appear to have been formed basin-shaped cavities or troughs, not entirely cut off from communication with the sea, and vast estuaries, in which the tertiary strata were deposited. While the secondary strata stretch continuously for hundreds of leagues, the tertiary are found only in detached insulated spots of comparatively limited extent. In this state of the earth's surface there must have been vast inland freshwater lakes ; for we find regularly stratified deposits of great thickness full of organic remains, which exclusively belong to animals, that lived in fresh water, and to terrestrial animals and plants. Like the secondary, the tertiary rocks consist of a great variety of strata of limestones, sandstones, clays, and sands which have distinct characters, and have been united in several groups. In them we first discover the remains of land quadrupeds and birds; and bones of mammalia are most abundant in the beds nearest to the surface. Among all the various remains of animals and plants, that are found in the secondary rocks, from the chalk downwards, not one has been found, which is identical with any living species. Although they have characters agreeing with those, by which existing animals have been grouped together in the greater divisions of genera, families and classes; the living individuals of the same divisions have forms of structure distinct from
ny found in a fossil state in the secondary rocks. But, with the tertiary strata, a new order of things commences; for, in the lowest of these, a small propor
tion,-about three and a half per cent. of the fossil shells cannot be distinguished from species that now exist: as we approach the higher beds the proportion always increases; and in the most recent stratum, it amounts to nine-tenths of the whole. It is not more than twenty-one years since the great division of the tertiary rocks was established. Prior to that time the peculiar characters, which separate them from the secondary strata, had been entirely overlooked,-a circumstance which marks very strongly that geology is the youngest of the sciences. The discovery was made by the celebrated Cuvier and his associate M. Brongniart, who found that the city of Paris was built in a hollow basin of chalk, that had been subsequently partially filled by vast deposits of clays, limestones, sands, and sandstones, and that there were alternations of beds, containing remains of fresh-water and terrestrial animals and plants, with others containing only the remains of marine animals.
The publication of the work of the French naturalists led to a similar discovery in our own island, and sin. gularly enough in the valley of the Thames ; so that the capitals of France and England are both built upon these strata, so strangely neglected for so long a time, although occurring in the very spots, where the greatest numbers of scientific men are collected together in both countries. A series of tertiary 'strata was discovered by Mr. Webster in the Isle of Wight, having strong points of resemblance with that of the environs of Paris ; and these, with some partial deposits on the coasts of Suffolk and Lancashire, constitute the whole of the tertiary rocks found in Great Britain. It was for some time supposed, that these newer strata, which were soon found not to be confined to the neighbourhood of Paris and London, extended like the secondary rocks over great tracts of country; and that there was such a degree of uniformity in their characters, that deposits widely distant from each other could be recognised as belonging to the same period in the chronological order of succession of the strata. Later observations, however, have shown, that, although possessing a
general character of resemblance, they have been so much modified in their formation by local circumstances, that no two tertiary deposits, even of the same era, are alike. The discoveries of the last few years have led geologists to establish distinct subordinate groups, as in the case of the secondary rocks; and the upper stratum of the Paris basin, which was at one time considered the most recent of stratified rocks, has been found to be inferior in the order of succession to many others, some thousand feet thick.
We have already stated, that the stratified rocks contain the remains of animals and plants: and that beds of stone, situated many miles distant from each other, may be proved to belong to the same place, in the order of succession of the strata, by remains of organized bodies, or FOSSILS, of identical species, being found in the stone at both places. The word Fossil, which means anything that may be dug out of the earth, used to be applied to all minerals; but modern geologists have conveniently restricted its application to organized bodies contained in the loose or solid beds composing the crust of the globe, and for the most part petrified; that is, converted into stone. Fossils are now always understood to be petrified remains of animals or plants, and we say, fossil shells, fossil bones, fossil trees, &c. We are enabled to make out, by the aid of those bodies, that a bed of limestone on the coast of Dorsetshire, another on the coast of Yorkshire, a third in the western islands of Scotland, and a fourth in the interior of Germany, although differing, perhaps in appearance, as far as the mere limestone is concerned,