volcanoes, explains to him how some rocks are "on edge," some "inclined" to each other, and some resting at right angles upon others. Again he visits the sea-side, and he sees that the incessant action of the waves upon the coast originates the numerous hollows and caverns that there abound. All these are operations of nature, which are going on during every moment of the day; they can be watched by any of us, if we will only use the eyes and the understanding that God has given us. And by these operations all geological facts can be explained.

By the term "earth's crust" is meant "that portion of the earth's surface which comes within human research." It is of small extent, compared with the entire thickness of the earth. But still the information thus gained is of the most interesting and most useful nature. Unaided by the geologist, gold mines could only accidentally be found, and none would know where to seek for coal. Unassisted by the geologist, the builder would not know where to find suitable stone, while to the researches of geologists the engineer owes much of the success that attends his undertakings.

The atmosphere, by means of the gases of which it is composed, greatly influences the rocks composing the earth's crust. The two gases most instrumental in these changes are oxygen and carbonic acid, the latter acts upon those containing lime, the former upon those containing iron. Stand at the foot of any great rock, and you will easily trace the results of atmospheric influence upon it. See its top shaped most grotesquely, here forming a natural grotto, there forming a curiouslyshaped castle, here with sharp points shooting heavenwards, there with peaks whose sides are completely jagged and broken. Again look at the foot of the rock, and you will notice directly the broken fragments of the rock itself, the portions which have been detached by the wearing influence of the atmosphere and which now lie carelessly strewn along its base.

Low plains also suffer from atmospheric influences. Across deserts blow violent winds, which increase in

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intensity as they proceed, and which carry with them the sand of the desert. Ofttimes the sand thus carried away brings destruction of crops and future barrenness of soil to the fertile lands that bound the desert. Frost splits rocks and loosens the surface soil.

Both fresh and salt water effect changes. The river pursues its course heavily laden with sand, mud, and gravel, and these burdens it deposits at its mouth, and thus forms a delta. The waves of the ocean dashing against a rock-bound coast, slowly, but gradually and surely, wear away the coast, and wash away the softer portions of the rocks. The English coast from Yorkshire to Kent has suffered in this way during the last eight hundred years. The Goodwin Sands once formed the estate of Earl Goodwin, the powerful opponent of Norman influence at Edward the Confessor's Court. Ravenspur, the place where both Henry IV. and Edward IV. landed on the Yorkshire coast, has disappeared, and its exact locality is not known.

Volcanoes, earthquakes, and, alterations of level, are the manifestations of the force of the fire in the earth's centre. These forces are compensating agents; the action of the atmosphere and water would speedily degrade the earth, they would quickly lessen the surface in the ways that have just been described, but the volcanic forces raise up again the parts that have fallen, so that although individual parts of the earth may from year to year suffer change, yet the entire area of the land above the water's surface always remains the same. The alterations of level of various parts are remarkable. Upon the tops of the highest mountains are found marine shells, the proof that once those mountains formed a portion of the ocean's bed. In the high plains which exist in many of the continents at immense distances from the sea, there the same proof exists, and their surfaces are covered with sea sand, and shells. The alterations take place very gradually; years elapse before any remarkable height is attained, but at length the wonderful working of these forces is seen. The coast of South America continues to rise in this manner.

The island of Greenland is gradually sinking, while the shores of the Baltic are rising.

All classes of life, both plants and animals, contribute to effect changes. Vegetation constantly adds to the soil. Leaves fall, decay, and become changed into "leafmould." Large forests become submerged, and change after the lapse of many centuries into beds of coal. The excretions of animals continually produce changes. Some living creatures spend the period of their existence in increasing the amount of evil possessed by the earth. The little coral insects congregate together in thousands, they elaborate lime from the sea-water, and build therewith the numerous coral islands which stud the ocean. Sponges, &c., elaborate silex or flint from the water, and form at the bottom of the ocean huge beds of flint similar to those that distinguish past ages.

THE VARIETIES OF ROCKS.

Careful study of portions of the earth's surface situated in all parts of the globe, has resulted in the conclusion that all kinds of rocks may be arranged in two great groups. These two groups are, the stratified and the unstratified; the former arranged, the latter dis arranged; the former shewing conclusively the manner in which they were formed, the latter consisting of all kinds of matter, the method of whose formation can only be conjectured. Between these two great groups there is an intermediate series. It possesses neither the confusion of particles which distinguishes unstratified rocks, nor the traces of life which characterize each of the stratified systems. The series partially belongs to the one set of rocks, and partially to the other. Hence the name applied to these rocks of transitional.

the

The unstratified rocks form the base upon which all the other rock systems rest; they are "the foundations of the earth." They have been formed by the agency of fire. The unstratified frequently appear associated with the stratified: either they have upheaved the stra

tified, broken them, or become intermixed with them. Granite is the most ancient, lava the most modern, of this group of rocks.

The transition rocks occur in beds horizontally arranged. This would shew, that originally their particles were deposited by water. But, besides this, they possess no single trace of their origin and history. No fossil forms are found within them. Some explain their present appearance by stating that it is the pressure they have sustained from overlying rocks that has so materially altered them, and they assert that if a sufficient time were allowed to elapse, and the pressure allowed to continue, the transition rocks would assume the appearance of granite.

The stratified rocks extend over a vast area of the earth's surface. Were they all placed one upon another, we should never be able to discover the numerous classes into which geologists have divided them. But in few parts of the world are the stratified systems, as a whole, arranged horizontally. In nearly every country, volcanic forces have been at work, here lifting up, there breaking them asunder. It is the apparently unnatural positions assumed by strata, when influenced by these forces, that enables us to learn the relative positions of the various systems. By crossing England and Wales, from Kent on the one side to the Welsh coast on the other, we should pass over all the geological systems. England, in this matter, possesses a great advantage over other countries. The volcanic forces have been working with such force in past ages beneath our country, that all the strata have lost their horizontal position, and have become set" on edge:" by this term geologists mean tilted up. From this circumstance England derives such varieties of soil and such valuable mines. The edge of a stratum, that part which lies along the surface, is called its outcrop, and the line of outcrop forms the strike. The manner in which a stratum is inclined to the horizon is called its dip. The angle of the dip may be measured by holding one of your hands upright, and then placing the other hand at

right angles to it. When we walk along the upturned part of a stratum, we follow the strike; when we dig, following the stratum, we are guided by the dip.

Four circumstances distinguish the stratified systems, and by these four circumstances the relative age of each can be ascertained. These are,-position, mineral constituents, imbedded contents, and the traces of life.

Position is very important. If one rock is found to underlie another, the necessary deduction is, that the lower rock is older than the upper. The Ganges and the Nile wash down to their mouths tons of matter daily; but the layers at the bottom will always be older than those at the top. Hence, if coal-containing strata be found beneath beds of red-sand, we at once conjecture that the former are of a greater age than the latter. The slates of Wales underlie the sandstones of Devonshire; the sandstones of Devonshire underlie the coal formations of Northern England; and the chalk-beds of Kent underlie the clay and sands of London.

The true means of ascertaining a rock's age are the traces of life it possesses. Each system has a distinctive form of life, and those forms still exist as "fossils." Fossils are petrified remains of once living beings; the word means anything dug up: but it is advisable to restrict its meaning. Dr. Page, in his Geology, thus describes an ordinary process of fossilization:-"A shell, like the common cockle, may be buried in a mass of mud, and when so enclosed, it is of itself composed of carbonate of lime and a little animal matter. As it remains imbedded, chemical changes take place, the animal matter decomposes and passes off in gas, and its place is supplied by an additional deposit of lime from the mass. As the mass becomes consolidated into limestone rock, the shell will also become hard and stony, but still preserving its form to the minutest ridge of its surface. By-and-bye, carbonated waters may filter through the pores of the limestone; the shell may be dissolved entirely, and leave only a hollow cast of its form. Another charge may now take place; water holding flinty matter may run through the rock, and