latter are the foot, the acclivity, and the summit. Lowland comprises those extensive flat tracts which are almost entirely destitute of small mountain groups. To the bottom of the sea belong the flat, the rocky bottom, shoals, reefs, and islands. It is only after a diligent study of the inequalities just pointed out, that we can with advantage undertake to explore the means employed by nature to produce them; and the first step is to proceed to the examination of the physical causes of the slow, but unceasing changes of the globe. Observation teaches us, that most of the elevations and hollows we meet with on the surface of the earth owe their origin to the action of the atmosphere, to that of the ocean, and to volcanic fire. These powerful agents may be considered with regard to their destroying, and, in consequence of this destruction, with regard to their forming effects. All geologists are agreed that our present continents were once covered with water. This is proved by the remains of marine animals imbedded in the strata which lie on the summits of the highest mountains. The structure of the globe, as far as we are acquainted with it from the intersections made by rivers, by the action of the sea upon the coast, and by mining operations, consists of beds of different kinds of stone, which generally increase in thickness as we descend deeper. Stratification, in its simplest form, may easily be conceived, by placing a closed book with the back resting upon the table, and raising the opposite edges a little; the book may represent a thick mineral bed, and the leaves a series of strata. In nature we frequently find the strata much broken, and thrown out of the original position. Where any series of strata are wanting, a question naturally arises, have they been carried away by some sudden inundation, before the upper strata were deposited, or have they never extended to that place? In some instances it is certain that the strata have been carried away from particular situations, as in some of the excavations which have formed valleys, in which the strata that terminated on one side of the valley may be discovered again in the hills on the opposite side. The substances of which the strata are composed, are argillaceous, calcareous, or siliceous earth, which are generally more or less intermixed or combined. The strata of clay, or argillaceous strata, being water-tight, give rise to springs, as they arrest the water that runs through the porous strata, and convey it to other situations. The inclinations of the strata, with the breaks and inequalities, render the globe habitable, by distributing the waters over the surface. The strata to a great depth are generally characterized by the remains of animals or vegetables, in what is called a petrified state, the organic structure being distinctly visible, although the animal or vegetable matter is almost entirely removed, and its place generally supplied by calcareous or siliceous earth. These organic remains are more abundant in the upper than the lower strata; and in the lowest beds of rock which have yet been explored, no traces of organic existence have been found. These remains make us acquainted with the great changes which must have taken place in the condition of our planet in remote ages. The uppermost stratum in England and in various parts of Europe, is formed of alluvial soil. In this soil, the remains of quadrupeds of vast size, such as the elephant, the rhinoceros, and mammoth or mastodon, are frequently found. Many of these are different from any existing species, and they prove that dry land existed in the vicinity, and that Europe was then inhabited by species of animals at present unknown. The researches of modern geologists have given abundant confirmation to the sacred history, not only with respect to the general deluge, but also with regard to the age of the earth. Until very lately several geological phenomena were considered, by superficial inquirers, as indicating that the creation of the globe we inhabit was an event much more remote than the sacred history represents. This opinion was kept in countenance only as long as geology was in its infancy. Every successive step which has been lately taken in the improvement of this science has served to show its fallacy. The investigations of the latest and most accurate philosophers have afforded the strongest proofs, that the earth, in its present form, cannot have existed longer than appears from the Mosaic account. QUESTIONS.-1. How is the surface of the globe divided? 2. What does the highland comprise ?-lowland? bottom of the sea? 3. What does observation teach us? 4. In what are all geologists agreed? 5. How is this proved? 6. What is said of the structure of the globe? 7. How may stratification be conceived? 8. What are the substances of which the strata are composed? 9. What is said of organic remains? 10. What have modern geological researches confirmed? ROCKS. LESSON 85. Relative Situation of Rocks. 189 Pseu'do, a prefix, which, put before words, signifies false, counter- Presented to the cultured eye of taste, No rock is barren, and no wild is waste. ROCKY masses, variously placed over each other, compose the whole crust of the earth, to the greatest depth that the industry of man has been able to penetrate. Now these rocks, with respect to each other, occupy a determinate situation, which holds invariably in every part of the earth. Thus limestone is no where found under granite, but always above it. Werner has chosen this relative situation as the basis of his classification of rocks. He divides them into five classes which are called formations; as primitive, transition, fletz, alluvial, and volcanic. The primitive formations are of course the lowest of all, and the alluvial constitute the very surface of the earth; for the volcanic, as is obvious, are confined to particular points. Not that the primitive are always at a great depth under the surface, very often they are at the surface and constitute mountains. In such cases the other classes of formations are wanting altogether. In like manner the transition and other formations may, each in its turn, occupy the surface, or constitute the mass of a mountain. In some cases all the subsequent formations which ought to cover them are wanting in that particular spot. Each of these grand classes of formations consists of a greater or smaller number of rocks, which occupy a determinate position with respect to each other, and which like the great formations may often be wanting in particular places. The rocks which constitute the primitive formations are very numerous. They have been divided into several sets, such as granite, gniess, mica-slate, and others. It deserves. attention, that the rocks constituting them are all chemical combinations, and generally crystallized; that they contain no petrifactions; and that the oldest formations contain no carbonaceous matter. Transition rocks are not so nu. merous. In these, petrifactions first make their appearance, and they usually consist of species of corals and zoophytes, which do not at present exist, and are therefore supposed to be extinct. Fletz rocks are disposed in flat or horizontal strata. They contain abundance of petrifactions; and these much more various in their nature than those which occur in the transition formations, consisting of shells, fish, and plants. The alluvial formations constitute the great mass of the earth's surface. They have been formed by the gradual action of rain and river water upon the other formations. They consist of the component parts of previously existing rocks, separated by the influence of air, moisture, and change of temperature, and deposited in beds. Sand, gravel, loam, and petrifactions of animals and vegetables, are often found in this class. Volcanic formations are pseudovolcanic, or such minerals as are altered in consequence of the burning of beds of coal situated in their neighbourhood; and true volcanic, or such as are actually thrown from the crater of a volcano. The expansion of water in the pores or fissures of rocks by heat, or congelation, is a physical cause of the separation of their parts. The solvent power of moisture exerted upon alkaline or calcareous matter, in rocks, is another cause of their decomposition. Electricity, which is shown, by experiments with the voltaic apparatus, to be a most powerful. agent of decomposition, seems to assist in all these changes; electrical powers being almost constantly exhibited in the atmosphere. The production of a bed for vegetation is effected by the decomposition of rocks. As soon as the rock begins to be softened, the seeds of lichens, which are constantly floating in the air, make it their resting-place. Their generations occupy it, till a finely-divided earth is formed, which becomes capable of supporting mosses and heath: acted upon by light and heat, these plants imbibe the dew, and convert constituent parts of the air into nourishment. Their death and decay afford food for a more perfect species of vegetable; and, at length, a mould is formed, in which even the trees of the forest can fix their roots, and which is capable of rewarding the labours of the cultivator. The decomposition of rocks tends to the renovation of soils, as well as their cultivation. Finely-divided matter is carried by rivers from the higher districts to the low countries, and alluvial lands are usually extremely fertile. The quantity of habitable surface is constantly increased by these operations; precipitous cliffs are gradually made gentle slopes, lakes are filled up, and islands are formed at the mouths of great rivers. In these series of changes, connected with the beauty and fertility of the surface of the globe, small quantities of solid matter are carried into the sea; but this seems fully compensated for by the effects of vegetation in absorbing matter from the atmosphere, by the production of coral rocks and islands in the ocean, and by the operation of volcanic fires. What does not fade? the tower, that long had stood The sun himself shall die, and ancient night QUESTIONS.-1. What is the basis of Werner's classification of rocks? 2. Into what five classes does he divide them? 3. What is said of primitive rocks? 4. Transition? 5. Fletz? 6. Alluvial? 7. Volcanic? 8. How does the decomposition of rocks produce a bed for vegetation? 9. Tend to the renovation of soils? [NOTE. Some knowledge of geology is daily becoming more necessary, for without it, scarce a volume of travels or topography, a review or a journal, can be read with all the interest it demands. The structure of the country and the stratification of its mountains, are now as often and as minutely described, as the plants and the animals which are found upon their acclivities.] LESSON 86. Biographical Sketch of Linnæus. CHARLES LINNEUS, the founder of modern botany, was born in 1707, at a small village in Sweden, where his father |