for it is too porous to retain the moisture. Sandy deserts are examples of tracts of land composed almost wholly of silicious earth. Calcareous earth is too dry and hot for vegetation, and argillaceous earth is too wet and cold. When these earths, however, are mixed together in due proportions, they correct and improve each other, and form the fertile soil of our gardens and fields. Sand corrects the stiffness of clayey land, and lime adds to its warmth.

Without sand, no glass could be made; nor could houses of brick or stone be built, for sand is a necessary ingredient in mortar. Without clay, we should have no springs; for beds of clay, or clayey rocks, alone arrest the downward progress of the water which falls in rain, thus forming the reservoirs from which springs flow.

The wearing away of solid rocks, by the influences already enumerated, is not confined exclusively to the particles which form mineral earths. Large fragments are frequently broken off, from which are formed the gravel, pebbles, and rounded stones that are seen on the sea-shore, and in the beds of rivers. At first, these fragments are rough, but when subjected to the rolling of waves on the beach, or to the action of running water in rivers, by rubbing against each other they become smooth and rounded.

After digging through the different earths which lie at the surface, we come to hard or consolidated materials, which are called rocks. These rocks form what is called "the crust of the earth;" and, generally speaking, they are of the same materials as the earths we have just described: the only difference being, that in the earths the materials are loose, or unconsolidated; and in the rocks, hard, or consolidated.

Popularly, the term "rock" is applied only to the more compact and solid portions of the globe; but geologically it extends to every formation, to the loose sands, gravels, and clays, as well as to the limestones and granites.

Our positive knowledge of the formations constituting the interior of the earth is very limited, the labours of the miner having

extended to but comparatively a short way below the surface of the earth, and scarcely two thousand feet below the level of the

sea.

All rocks may be classified,

1. As Stratified, or Unstratified;

2. As Fossiliferous, or Non-fossiliferous;

3. As Igneous, Metamorphic, or Aqueous.

Stratified rocks are found in the regular form of beds or layers, varying in depth from the thickness of a sheet of paper to many feet. These beds are sometimes arranged horizontally, but oftener inclined at various angles to the horizon. This class of rocks is estimated to occupy about nine-tenths of the land surface of the earth, and to have an average depth of eight or ten miles.

Unstratified rocks are irregular masses, the lowest of all rocks, forming the basis or bed on which the others rest. But while they thus form this basis or bed, they are frequently pressed up through the stratified rocks, constituting in many places the summits of lofty mountains. They do not, probably, occupy more than one-tenth part of the earth's surface; but we have reason to suppose they constitute the internal parts of the globe to a great depth.

Fossiliferous rocks contain, in a petrified state, the remains of plants and animals, sometimes in very small, but often in enormous quantities. The fossiliferous rocks are stratified. Some of the non-fossiliferous rocks are stratified-some are unstratified.

It is estimated that two-thirds of the surface of the existing continents are composed of fossiliferous rocks, and they are often several thousand feet in thickness. The quantity of microscopic shells discovered by the great Prussian naturalist, M. Ehrenberg, in rocks of this formation, is most wonderful. Shells not larger than a grain of sand form entire mountains! In one place in Germany he discovered a bed fourteen feet thick, made up of the shells of minute animals, so small that he esti

mated that forty millions of them would be required to form a cubic inch.

The quantity of fossil remains is so great, that with the excep tion of the metals, and some of the older rocks, probably not a particle of matter exists on the surface of the earth that has not at some time formed part of a living creature.

Igneous rocks, or those which are supposed to owe their origin to fire, are subdivided into Plutonic and Volcanic rocks. They are unstratified and non-fossiliferous.

Plutonic rocks, it is supposed, were formed of melted matter, cooled and consolidated at a great depth, and under an enormous pressure, and then thrown up by the elastic force of internal heat. Volcanic rocks are the products of ancient volcanic eruptions.

Granite and its varieties constitute the principal Plutonic rocks. Basalts and greenstone are among the principal volcanic rocks. From their frequent arrangement in the form of steps, they are often called "trap rocks," from the Swedish trappa, “a stair." Fingal's Cave and the Giant's Causeway are familiar examples.

The theory of the igneous formation of rocks, of which we have positive proof by observation in the case of volcanic rocks, presupposes the earth to have been originally in a melted state, and that its centre now, excluding a crust only from twenty to fifty miles in thickness, is a sea of fire. Additional evidence of this is found in the fact that the temperature of the earth regularly increases one degree for every fifty-four feet of descent beneath its surface. At this rate of increase, a heat sufficient to melt all known rocks would be reached at a depth of between forty and fifty miles.

Metamorphic rocks are supposed to have been formed in regular beds or layers, of the sediment of water, but having been deposited near the place where Plutonic rocks were generated, their character has been changed by the immense heat, and they have become as highly crystallized as granite itself, without losing their regular form. They are stratified, and non-fossiliferous.

Gneiss is a metamorphic rock. It so nearly resembles granite, as hardly to be distinguishable from it even by a practised eye. Aqueous rocks seem to have been formed by the gradual deposit of sedimentary matter in water, which has become more or less hardened into solid rocks. They are stratified and fossiliferous.

Aqueous rocks are variously subdivided by different geologists, with reference to their age, and the depth at which they are found from the surface, into numerous groups and orders. Groups found at the greatest depth, and containing the remains of the earliest formed animals, are regarded as the oldest; those containing the remains of animals similar to those now living, are deemed to be of the most recent formation. Aqueous rocks constitute by far the greatest portion of the exposed crust of the earth. The various kinds of soils, gravels, sands, clays, limestone, coal, sandstone, and some slates, are the principal rocks of this class.

All stratified rocks maintain a regular order of succession; that is, if an older rock is at the surface, we may be assured none of later formation is underneath it. Thus, no geologist would expect to find beds of coal underneath strata of talcose slate, the latter being an older formation; yet this slate has been bored into, at great labour and with much expense, in search for coal.

The crust of the earth is undergoing incessant change. The atmosphere, the ocean, and rivers, are agents constantly acting upon the land, and removing its particles into the sea; while, as if to compensate for this gradual wearing away of the land, at intervals of time volcanic eruptions elevate enormous masses of matter, sometimes forming new islands in the midst of the ocean. Yet these changes are trivial, compared with those which geology teaches us must have taken place to fit the earth for the abode of man,

ACTION OF CLIMATE UPON MAN.

SINCE man is made to acquire the full possession and mastery of his faculties by toil, and by the exercise of all his energies, no climate could so well minister to his progress in this work as the climate of the temperate regions.

Excessive heat enfeebles man; it invites to repose and inaction. In the tropical regions the power of life in nature is carried to its highest degree; thus, with the tropical man, the life of the body overmasters that of the soul; the physical instincts of our nature eclipse those of the higher faculties; passion predominates over intellect and reason; the passive faculties over the active faculties. A Nature too rich, too prodigal of her gifts, does not compel man to wrest from her his daily bread by his daily toil. A regular climate, and the absence of a dormant season, render forethought of little use to him. Nothing invites him to that struggle of intelligence against Nature which raises the powers of man to their highest pitch. Thus, he never dreams of resisting physical Nature; he is conquered by her; he submits to the yoke, and becomes again the animal man, in proportion as he abandons himself to external influences, forgetful of his high moral destination.

In the temperate climates all is activity and movement. The alternations of heat and cold, the changes of the seasons, a fresher and more bracing air, incite man to a constant struggle, to forethought, and to the vigorous employment of all his faculties. A more economical Nature yields nothing, except to the sweat of his brow; every gift on her part is a recompense for effort on his. Nature here, even while challenging man to the conflict, gives him the hope of victory; and, if she does not show herself prodigal, she grants to his active and intelligent labour more than his necessities require; while she calls out his energy, she thus gives him ease and leisure, which permit him to cultivate all the lofty faculties of his higher nature. Here, physical Nature is not a tyrant, but a useful helper; the active faculties, the understanding