composition of the lifeless and rigid masses around us, may be formed from the following list of strata by Dr. Mantell, which are wholly, or in great part, composed of animal remains:

In addition to this amount of animal matter, the vegetable kingdom has contributed, in an inferior degree, yet in no unimportant manner, to the composition of strata, as the anthracite, coal, and lignite formations attest, which occupy extensive areas of country. The thickness of the fossiliferous rocks in Great Britain is thus given by Professor Phillips :

The total, according to this estimate, is 34,080 feet, or about 6.5 miles: but it appears from more recent investigations, that the Cambrian group and Silurian system far exceed in thickness the entire aggregate of the other strata.

The division of rocks into fossiliferous and non-fossiliferous, into stratified and unstratified, is founded upon obvious and important natural characters, and accordingly all geologists are here agreed. Nor is there any difference of opinion as to the order of succession and relative age of the larger formations, however, in the details of classification, the grouping of strata, and the distinctive terms employed, some diversity prevails, perplexing to the student. The first arrangement was made by Lehman, who divided all rocks into — Primitive. - Those of hard and slaty structure, containing no fossil organic remains. Secondary.-Rocks of comminuted fragments, and containing organic remains. Local. Those of partial occurrence in different districts.

Werner introduced another class between the primary and secondary, to which he applied the term Transition, from the indications they exhibited of a transition state from the one to the other as to mineral structure, and of having been formed when the world was passing from an uninhabitable to a habitable condition. Afterwards the word Local gave place to Tertiary, which became the generic title of all regularly stratified beds above the chalk up to the superficial formations; and the transition class was incorporated with the secondary. But as geology advanced, this arrangement was soon found to be far too general. Each of these grand divisions has since been subdivided into systems and groups of strata, which are so distinguished from each other by mineral or fossil peculiarities, as to justify the idea that they were formed independently, under different conditions, during successive epochs. Thus, beginning at the surface, we have, in descending order, the following systems, distributed into great life periods, according to natural history distinctions :

This arrangement is more clearly exhibited on the next page.

This synopsis exhibits a view of the succession of strata as developed in Great Britain, and, excepting some circumscribed continental tertiaries, we have, within our own island, the representative or the equivalent of almost every formation whose existence has been ascertained. This order of succession is invariably maintained; for though many members of a group may be wanting, and also many groups, those that occur are found universally to occupy the same relative position. We may suppose the series to occur,

Thus, a system low in the diagram may be found immediately under the surface soil of a district, or quite naked to inspection, all the intermediate groups being absent; as the magnesian limestone in Durham, the coal in Fifeshire, the old red sandstone in Herefordshire, and the slate in North Wales; but if these lower groups in the diagram appear at the surface, then, however deeply we might pierce the strata there, we should never come to any of the systems higher in the scale. It would be vain to sink a shaft in search for chalk in Durham, or for magnesian limestone in Fife, or for coal in Hereford, or for the old red sandstone in the neighbourhood of Plinlimmon, because the formations sought are superior in the series to those which occupy the surface in the localities named. Hence a practical knowledge of the succession of strata would have prevented the expenditure of thousands in the search for coal, in situations geologically beneath it, or so far overlying it as not to be accessible. The absence of groups of strata, to which we have referred, may have arisen from the action of water, subsequent to deposition, denuding and washing them away, diffusing their materials upon the floor of the ocean, or entering into fresh formations; or other modifying circumstances may have prevented their deposition altogether in such sites.

The first nine of the systems named contain, in a varying degree, the remains of plants and animals. The other three exhibit no traces of them, except the upper part of the clay-slate system, in which they are occasionally found. Upon comparing the remains exhibited by formations of different ages together, decisive evidence appears of the progressive development of organic life, from the more simple and imperfect structures that obtain in the older strata, to the higher organisations that are found in those depositions which belong to æras immediately antecedent to the existing epoch. In the course of our planet's history, the less complex tribes of animals and plants were the first to appear, the more perfect species becoming more and more numerous up to the creation of the present races. This is not generally held by geologists as inferring a gradual perfectionnement of species. It is true that fishes, which of all vertebrata rank the lowest, appear first in geological history; but then they are not imperfect formations-they have no mark of inferior organisation, but occur in their highest state of approximation to the reptile, and not in their lowest condition of affinity to the worm, It is also true, that reptiles precede mammalia, but they are reptiles belonging to the higher grades of that class. The earliest zoophytes and mollusca likewise display no inferiority in their organisation when compared with their living representatives. Hence it is concluded, that while there has been a progressive development of organic life upon the face of the globe, it has not been by an improvement of species, but by an addition of fresh organisations, a new dramatis persona, to meet new physical conditions of the earth.

The dawn of organic life-the morning twilight of the era of living beings-appears in certain slaty rocks of North Wales, which are variously regarded as upper beds of the clay-slate system, as lower Silurian, and as a distinct formation under the name of Cambrian. Zoophytes, molluscs, and traces of annelidans, are occasionally detected. In the following table, the more important remains of animal and vegetable life, characteristic of great geological eras, are indicated:

Ammonite Catena.

UPPER NEW RED SANDSTONE, OR TRIASSIC PERIOD.
A limited flora indicated, both as to species and individuals,
but more allied to that of the present era than of the
preceding epochs. Frog-like reptiles, of huge size, appear,
and new families of saurians. Ichnolites (foot-prints on
stone), those of reptiles; and Ornithichnites (stony bird
tracks), the imprints of birds, some of gigantic size, are
characteristic memorials of the period.

OOLITIC PERIOD.

The

Organic remains extremely numerous and well-preserved.
most remarkable are reptiles, many of enormous size, as the
Ichthyosaurus (fish-like saurian), Plesiosaurus (akin to the saurian),
and the intermediate Pliosaurus, inhabitants of the ocean; the
Pterodactyle (wing-fingered reptile), a flying saurian; the Hylaosaurus
(forest-saurian); the Megalosaurus (great saurian), a gigantic carni-
vorous terrestrial reptile; and the Iguanodon, a huge herbivorous
reptile, allied to the existing iguana. Among molluscs, Ammonites
occur in prodigious numbers, of many species, and varying size.
Insects also appear, beetle-like and dragon-fly-like; and a specimen
or so of birds. Fragments of small marsupial animals proclaim the
existence of warm-blooded terrestrial mammalia.