LESSON V.-THE TRANSITION PERIOD.

GEOLOGICAL REMAINS OF ANIMALS OF THE TRANSITION PERIOD.

1, 6, and 8 are Coral Zoophytes of the Lower Silurian. 2, 10, and 12 are Lower Silurian Trilobites, from one to three inches in length. 3, 4, and 5 are the earliest Molluscs or Shellfish. 7, a Silurian Crinoidea-an animal having a radiated lily-shaped disk supported on a jointed stem. 9, a Placoid Fish of the Upper Silurian. 13 and 15 are Ammonites; and 14, a section of No. 13, showing the interior chambers. 16, a Star-fish. 17, one of the earliest Polypes, or plant-like Zoophytes.

1. IN entering upon the second age of the world's history, which is called the transition period, the evidences of stratification, which began to be dimly discerned in the uppermost of the primary rocks, are quite decisive, and layer follows upon layer, mostly of a slaty character, until the mass accumulates to the supposed average depth of five or six miles. All of these layers appear to have been gradually deposited at the bottom of the ocean during myriads of years by the slow wearing away of the mountains of the primary rocks by the action of water.

2. In this transition period, of incalculable vastness, we discern, in a few scattered fossils, the first faint traces of the beginnings of vegetable and animal life. In the lower, or Cambrian portion, a few sea-weeds have left their imprints in the rocks; and a few shells and corals, and a few trilobites -most singular species of Crustaceans-have been transformed into stone. In the upper, or Silurian portions, seaweeds are more numerous, and the fragmentary remains of a few terrestrial vegetables are discernible; but marine shells and corals abound, and the trilobites receive their fullest development, both in size and number. Here a few fishes first appear, of the Placoid order, as perfect in their kind as those of later ages, but their forms are not well known. In this period, myriads of ages ago, life appeared in fashions pecul

iarly antique, and nearly all of its types have long since become obsolete.

3. It is, however, interesting to notice here that shells of the family called Ammonites, which are among the earliest traces of the animal kingdom, appear in this early period; and it is a curious fact, that while all other families and orders of shells of this period-and, indeed, of many subsequent eras -have entirely died out, and now form vast layers of rocky strata of limestone and marble, some species allied to the ancient family of the Ammonites are found in every succeeding period of geological history, and kindred species exist in our seas at the present day. Among these is the Nautilus, whose geological history has been written in the following appropriate lines:

Thou didst laugh at sun and breeze
In the new created seas;

Thou wast with the reptile broods
In the old sea solitudes,

Sailing in the new-made light

With the curled-up ammonite.

Thou surviv'dst the awful shock

Which turned the ocean bed to rock,

And changed its myriad living swarms
To the marble's veined forms.

Thou wast there, thy little boat,
Airy voyager, kept afloat

O'er the waters wild and dismal,

O'er the yawning gulfs abysmal;
Amid wreck and overturning,

Rock-imbedding, heaving, burning.
Mid the tumult and the stir,

Thou, most ancient mariner,

In that pearly boat of thine,

Sail'dst upon the troubled brine.-MRS. HOWITT.

6. It should be remarked that thus far in the world's history no traces of any reptile, bird, or mammal have been discovered, which may be considered evidence conclusive that none of these animals were in existence at this epoch; but when, at length, after countless ages, fishes appeared, perfect in their kind, at the same time are presented the first evidences of a diminutive, yet highly organized tree vegetation. Vertebrated animals and land vegetation were new and distinct creations; and upward, from life's beginnings, through all its ascending stages, we constantly meet with evidences of new creations, but none whatever of any devel opment of higher grades from lower. The first fiat of creation doubtless insured the perfect adaptation of animals to the surrounding media; and thus, while the geologist recognizes a beginning, he sees the same evidences of Omniscience in the lower Crustaceans as in the completion of the higher Vertebrate form.

LESSON VI.-THE SECONDARY PERIOD.

GEOLOGICAL REMAINS OF ANIMALS OF THE MIDDLE SECONDARY PERIOD.

1, 3, and 4, remains of curious Fish, from six to ten inches in length. 6, a Ganoid Fish. 2 and 8, fossil Corals. 5 and 7, Trilobites, five or six inches in length. 9, the Plesiosaurus, a lizard-like marine reptile, from ten to fifteen feet in length. 10, the Ichthyosaurus, or fish-lizard, a kind of reptile whale, from twenty to thirty feet in length. [The relative proportions could not be preserved in the drawing.]

1. IN ascending from the Transition to the Secondary period, after passing the Devonian, which in North America exhibits no less than eleven distinct eras, we arrive at the Carboniferous system of rocks, which is so called from being the great depository of that important substance called coal. A new creation is here opened to view in the luxuriant tropical vegetation which distinguishes the Carboniferous epoch of our globe. The various kinds of coal are simply vegetable matter the remains of ancient forests deposited in vast ravines or ocean beds, and deeply buried there, and changed to their present forms by chemical processes in Nature's own laboratory. The coal is often covered by layers of shale, or slaty coal, which consists of masses of leaves and stems closely pressed together, and indicating an intermediate stage in the coal formation. The appearance of the roof of one of the coal-mines of Bohemia having this shale or partially formed coal for its covering, is thus described by Dr. Buckland:

2. "The most elaborate imitations of living foliage on the painted ceilings of the Italian palaces bear no comparison with the beauteous profusion of extinct vegetable forms with which the galleries of these instructive coal-mines are overhung. The roof is covered as with a canopy of gorgeous tapestry, enriched with festoons of most graceful foliage, flung

This seems to have been effected by exposure to heat and moisture, probably under great pressure, and in circumstances that excluded the air, and prevented the escape of the more volatile principles. Not only the various coals, but bitumen, amber, mineral oils, and even the diamond, were probably produced under various modifications of these circumstances.

in wild, irregular profusion over every portion of its surface. The effect is heightened by the contrast of the coal-black color of these vegetables with the light groundwork of the rock to which they are attached.

3. "The spectator feels transported, as if by enchantment, into the forests of another world; he beholds trees of form and character now unknown upon the surface of the earth, presented to his senses almost in the beauty and vigor of their primeval life; their scaly stems and bending branches, with their delicate apparatus of foliage, are all spread forth before him, little impaired by the lapse of indefinite ages, and bearing faithful records of extinct systems of vegetation, which began and terminated in times of which these relics are the infallible historians. Such are the grand natural herbaria wherein these most ancient remains of the vegetable kingdom are preserved in a state of integrity little short of their living perfection, under conditions of our planet which exist no more."

4. It is not only known that coal is of vegetable origin, but the kinds of plants which formed it have been accurately determined, to the number of more than three hundred species, but all different from any of the present age, although allied to existing types by common principles of organization. Of these fossil species, two thirds are related to the tree ferns and the higher orders of cryptogamous plants. The coniferous, or cone-bearing species, are also prominent; and there is little doubt that petroleum, and naphtha, and other mineral oils of coal regions, are nothing more than the turpentine oil of the pines of former ages. The internal heat of the earth has distilled it; and, after being buried for thousands of years, it is now discovered, to supply the wants of man. Remains of corals, shell-fish, a few insects, among which are several species of beetle, fishes of peculiar construction, the king-crab among Crustaceans, and in Pennsylvania the tracks of some Batrachian reptiles, have been found in the Carboniferous rocks. Here, also, are the last of the trilobites, which appear to have become extinct after the coal formations.

5. Ascending above the Carboniferous epoch, we pass successively, in this Secondary period, through three groups or systems of rocky strata, known as the Saliferous, or Red Sandstone, the Oolitic, and the Cretaceous. The first of these is comparatively scanty in organic remains; but in the other two, fossils are exceedingly abundant. Our existing islands and continents are principally composed of the spoils of this

period, whose history opens to us the fathomless depths of ancient seas, and vast marshes, with the remains of myriads of beings which lived and died in their waters.

6. The ocean then swarmed with sponges and other zoophytes, sea-weeds, and corals, and Crustaceans; even oysters were abundant, but different from existing species; remains of a shark-like fish are found here; smaller fish were numerous; and in almost every fragment of some of the flint formations their minute scales have been detected by the aid of the microscope. On the land were several species of spiders, and insects in considerable numbers. The tracks of gigantic birds have been detected-"footprints in the sands of time" -in the rocks of this period; but of the existence of any mammalia, the sole indications are the jaws of some small animals related to the opossum.

GEOLOGICAL REMAINS OF ANIMALS OF THE UPPER SECONDARY PERIOD.

1, a Crinoidea. These are sparingly found in this period. 2, 6, and 9 are remains of Echinites, or Sea-urchins. 3, 8, and 12 are Cretacean shells. The fish here represented are from one to three feet in length. 5 is the Pterodactyl, or flying reptile, having the head and neck of a bird, the jaws and teeth of a crocodile, the wings of a bat, and the body and tail of a mammal. It is believed that the spread of its wings was not less than twenty-five feet. 13 is the restored figure of the Iguanodon, as drawn by Martin, and found in Mantell's Geology. In making a complete drawing of such an animal from its fossil remains, much of its external appearance must be left to the imagination. It is certain, however, that the iguanodon was a monster reptile, thirty or forty feet in length. From the form of its teeth, and the vegetable matter found in connection with its skeleton, it is known to have been herbivorous.

7. The remains of turtles, the earliest clear indications of the reptile tribe, occur in the Saliferous period; and above them, and later in point of time, but still in this Secondary era, are the remains of the crocodile. But what especially mark this as the Age of Reptiles are the numerous species of monster Saurians, bearing such uncouth names as the ichthyosau'rus, plesiosau'rus, megalosaurus, and the iguan'odon, with the pterodac'tyls, or flying reptiles. In the island or peninsula of Portland, England, a petrified forest has been discovered in the upper formations of the Secondary period, and therefore contemporary with the monster reptiles whose