The three primary geological life-periods are arranged into eight systems, each distinctly marked lithologically and palæontologically. These systems represent many long epochs of our earth's history; and though in some instances long intervals may have elapsed between them, yet each may be expected to afford us a very correct knowledge of a vast continuous length of time. The systems are again divided; and those divisions, most important and uniform in character, have been introduced into the illustrative table. To have detailed all such subdivisions would have extended the subject beyond due limits. The classification of the vegetable kingdom into five classes, and that of the animal kingdom into nineteen classes, includes all existing plants and animals. All these classes have been recognized in the Cainozoic period; and all fossil organisms that have yet come to light, can, with propriety, be placed under some one of the twenty-four classes adopted. The table is arranged so as to indicate the classes present in each system; and the diagram conveys an idea of the relative thickness of each of the representative rocks, and perhaps a very accurate one of the time occupied in formation, supposing, of course, the rate of deposition to have been uniform.

The earliest Flora we are acquainted with is that of the Lower Silurian; it consists of marine aquatic plants,* Thallogens; that humble class being absent in the Cambrian system. The first trace of a land plant is a Lepidostrobus-an Acrogen, at the very top of the Upper Silurian. We may, therefore, conclude that there were no land plants during the long Silurian epoch, a vast interval, far exceeding in duration any other system. In the Devonian system, in addition to Thallogens and Acrogens, are found the first trace of Gymnogens; but the remains of

*Algæ,

vegetable life are very rare, though affording undoubted evidence of the existence of these three classes. In the Carboniferous system, the Acrogens and Gymnogens, unite and form its magnificent Flora, "the most splendid the world ever saw"-Palms, Endogens, are comparatively rare, being noticed for the first time. Of true Exogens*

no trace has been discovered. In the Permian a similar Flora prevailed, though very much reduced in quantity. This system closes the Palæozoic period, Acrogenous plants having predominated Lepidodendron and Sigillaria forming thick beds of coal over large areas towards the close of the period.

Ascending to the Mesozoic period, we find in the Trias of this country all organic remains to be exceedingly rare, the plants are few in number, but, throughout the whole period Gymnogens are the prevailing type. Conifers and Cycadaceæ are abundant, and give a peculiar character to the vegetation. The Flora of the Oolite differs from that of the Carboniferous system, in the substitution of Gymnogens for Acrogens. In the Cretaceous system, plants are rare, but the former class is frequent in the green sand. In the Lower Cretaceous deposits of Aix-laChapelle there are abundant evidences of Angiospermous dicotyledons, or true Exogens; but nothing of this type has been observed in this country. With the close of the system terminates the Mesozoic period.

In the Cainozoic or Tertiary period, we find evidences of a very different vegetation, Exogens being decidedly the most abundant, the sub-class Angiospermæ far exceeding the others. The Endogens, especially the Palms, are numerous, but the Acrogens are comparatively rare. The Flora of the latter part of the period approximates nearer to that of recent times, yet differing much from our present European vegetation. The climate must have

* Trigonocarpum, Cyperaceæ, &c.

been much warmer, probably the result of a different distribution of land and water, that only assumed its present configuration towards the dawn of the modern period.

Adopting the classification of Sir Roderick J. Murchison and the government geologists, the Cambrian system, represented by an immense thickness of deposits, contains only the remains of Bryozoa and traces of Annelida, the former found in Ireland only. If we adopt the nomenclature of Professor Sedgwick, the Cambrian system yields Zoophyta, Annelida, Crustacea, Bryozoa, and Brachiopoda; but, according to the former arrangement, that most generally accepted, these classes appertain to Lower Silurian rocks, in which are also found Amoryshozoa, Echinodermata, Monomyaria, Dimyaria, Pteropoda, Gasteropoda, and Cephalopoda. The Crustacea and Brachiopoda are the most numerous. In the great Silurian system all the marine classes of animals, excepting Cirrhipedia, are undoubtedly present. The Cephalopoda,* the highest class of invertebrata, existed in the Lower Silurian, though not in the lowest zone; and at the latter part of the epoch, judging from their great size, they appear to have performed the duties of fishes. The Silurian was a long period abounding with invertebrata, when all those classes of life were spread over the earth: we find their remains by millions, yet the most careful searches have failed to produce even a trace of a vertebrated animal.

During the deposition of the Silurian rocks collectively there appears to have been no fishes. The Cephalopoda were then the most highly organized beings that existed, until just at the close of the period, in the upper Ludlow shales, we find the first traces of fishes.† They are exceedingly rare, and consist of teeth and fragments of about seven species of Placoids.

Lituites and Orthoceras.

Onchus, Sphagotus, &c.

It is, of course, impossible for us to calculate the time occupied by the Silurian era; but from the immense depth of the deposits representing the system, their constancy over such vast regions, and the evidence of the organic remains, we may safely conclude that its duration must have far exceeded that of any other; yet each succeeding layer, if only a few inches, tells us of the order and regularity that prevailed, and often leaves us faithful and imperishable remains of the earliest life that existed upon the earth.

Ascending to the Devonian system, as with each succeeding advance, we find a continual substitution of genera and species, but never a repetition of one that had previously died out or become extinct. Species seem subject to some such natural law of life and death as individuals.

In this era the number of Crustacea is much reduced. Fishes are found in so perfect a state of preservation, and in numbers and variety so remarkable, that though they were all very much unlike those of existing nature, we have obtained a very correct knowledge of their structure. It is very interesting to observe, in the upper part of this system, the discovery of the first reptile* (of the Lacertian order), a step higher in the vertebrated series. Of the invertebrata no class has been found that is not also in Silurian strata.

In the Carboniferous system fishes were abundant; the Ganoidst of this era are very interesting, for they appear to have occupied the place of the large reptiles that were destined soon after to replace them. In this system only one true reptile has yet come to light; it is of the Batrachian order, so that we may conclude they were but rare; and the sauroid character of the

fishes lead us to the conclusion that this rarity was real, and that air-breathing animals formed but a small proportion of the entire fauna.

The organisms of the mountain limestone and of the coal-measures, differ essentially to the extent that might be expected of marine and fresh-water deposits. In the latter we find the earliest Insecta of the orders Coleoptera and Neuroptera.

In the Permian system the reptiles are more numerous, though still rare. No class of the animal kingdom is present that is not also in the Carboniferous system, and no invertebrated class not also found in Silurian strata, while in each the number of observed species is considerably less. This system closes the Palæozoic period. Afterwards the Flora and Fauna become so changed, that most of the generic types, and certainly all the species, of both plants and animals appear to have been replaced by new ones. All nature assumes a new aspect, and reveals as it were a new creation. In the fishes this change is very striking, in the well-known heterocercal and homocercal forms, as they emerge from the Permian to the Triassic systems.

There can be little doubt but that these great organic changes were intimately connected with the comprehensive disturbances that marked the close of the Paleozoic period. The complicated upheavals of the Carboniferous and Permian strata seem to have chiefly occurred at this time; and when we consider the gigantic nature of these convulsions, we cannot be surprised at the scanty traces of ancient life afforded to our view in the succeeding Triassic system. The tract of country in England occupied by the Trias or New Red Sandstone is as extensive as that of any other; yet the paucity, and often total absence, of organic remains is remarkable. The diffusion of per-oxide of iron through the waters, and the unstable