Talc, a very soft mineral, of pearly luster, and greasy feel; the characteristic ingredient of steatite or soapstone, French chalk, and chlorite.

Serpentine, a green, opake mineral, becoming yellowish gray on exposure; it sometimes presents a mottled ap

pearance.

Carbonate of Lime, marble, chalk, limestone; effervescing with acids, and reduced by heat to caustic lime.

Dolomite, magnesian limestone; a compound of the carbonate of lime with the carbonate of magnesia, infusible, and effervescing slowly with acids; used in the manufacture of Epsom salts.

Sulphate of Lime, gypsum, or plaster of Paris, alabaster.
Chloride of Sodium, common salt.

Tourmaline-sometimes called schorl—a black, or dark brown mineral; brittle, becoming electrical when heated. Iron Pyrites, a yellow, hard, cubical mineral, often mistaken for gold; composed of sulphur and iron.

Iron Ores, and Coal.

While an intimate knowledge of Mineralogy is not indispensable to the Geological student, an acquaintance with those minerals which enter largely into the composition of rocks is necessary, since the structure of the rocks is determined by them.

AGENCIES USED IN EFFECTING GEOLOGICAL CHANGES.

12. The study of Nature in all its departments, impresses upon the mind the idea of incessant change, as well in the solid strata of the earth, as in the more fleeting forms of animal and vegetable organization. Matter and motion seem to be inseparably connected. Changes, however, whether slight and momentary, or so grand as to task the imagination in their conception, and require ages for their

completion, evince the most enduring permanence in the laws which produce and govern them; and while our belief in the constancy of natural laws is intuitive, we perceive that incessant change is the means of effecting stability in the whole system.

13. While Geologists agree respecting the nature of the agents employed in effecting former geological changes —that they were identical with those now in operationthey are divided in opinion with reference to the intensity with which these agents operated. One class contends that the phenomena require, for their explanation, much more extensive and violent action than we witness at present; while others claim that existing causes have operated through all periods with the same degree of energy, and when long continued are adequate to the production of all the phenomena. The latter opinion is maintained by Sir Charles Lyell in his Principles of Geology.

The agencies of geological changes may be classified as atmospheric, aqueous, igneous, and organic.

ATMOSPHERIC AGENCIES.

14. Atmospheric agents produce both chemical and mechanical effects. The atmosphere consists of nitrogen seventy-nine parts, and of oxygen twenty-one parts in a hundred; of carbonic acid about one part in a thousand; and of watery vapor a variable quantity.

Many minerals are acted upon chemically by oxygen and carbonic acid. The sulphuret of iron (iron pyrites) when exposed to moist air, undergoes decomposition; the sulphur unites with a portion of oxygen, forming sulphuric acid, and the iron with another portion of the oxygen, forming oxide of iron; the two new bodies, thus formed, unite

ATMOSPHERIC AGENCIES.

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with each other, and the sulphuret of iron is converted into the sulphate of iron, (copperas.) Carbonic acid unites with the oxide of iron, producing the carbonate of iron. It also renders water a solvent of limestone, so that water charged a with it, falling upon a limestone rock dissolves, and thus removes portions of it, leaving fissures and caverns in the rock. These chemical changes are usually followed by a sta presse crumbling-disintegration—of the rocks. It is, however, 2 lat are di sometimes the means of consolidation, as when the carbonIate of lime in solution infiltrates a mass of loose sand, ita q cements it, producing a calciferous sandstone.

15. Winds modify the surface of the earth by drifting sand, gravel, shells, &c., from exposed to sheltered positions," in deserts and on the sea coasts.

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The sands of the Lybian desert have buried ancient cities and temples in Egypt. Many parts of the coasts of England, Holland and France are partially or wholly submerged by them. In Brittany, of a whole village overwhelmed by drifting sands, nothing is visible above their surface but the spire of a church. In Cornwall and Suffolk, England, are sand hills composed partly of comminuted ouldnt of shells, several hundred feet above the level of the sea, vancing upon the cultivated land at the rate of five miles in a century. Ansted states that on the coast of Spain, at Cape Finisterre, they have advanced sixteen miles in half a century-five hundred and sixty yards per annum. These sand hills are called in England dunes or downs; in Scotland, links. They are sometimes arrested in their progress by the roots of plants, especially of the arundo arenaria, which bind them into a firm mass. They are also sometimes indurated by the carbonate of iron, or lime, or by silica, cementing them, thus forming sandstones. The

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phenomena of drifting sands are exhibited in our country on the sandy capes-as at Cape Cod.

16. Frost exerts a destructive influence upon rocks. When water which has entered the fissures or pores of rocks freezes, it expands with a force adequate to rend the rocks; each fragment is again subjected to the same process until it is reduced to powder. This is most conspicuous in rocks of loose texture, but the densest and firmest do not escape. Blocks and ledges of granite, by this process of exfoliation, lose their angles and edges, and are oftentimes converted into rounded and grotesque forms. The accumulation of fragments at the bases of precipices is called a talus, usually presenting a slope of about 40°.

AQUEOUS AGENCIES.

17. Aqueous agencies are exerted either chemically, dissolving and decomposing rocks, or mechanically, abrading them and removing their particles. Water is the most general solvent known, and few rocks at the surface escape its influence. Its solvent power is enhanced by acids and by heat.

The amount of common salt, carbonate of lime, and other salts, held in solution in the waters of the Earth, is enormous. The amount of solid matter dissolved in the ocean is estimated to be more than thirty-nine parts in a thousand of water. From this source, shell fishes, coralpolyps and other animals inhabiting the sea, obtain their supplies for their skeletons-shells, coral-reefs, &c. The amount thus withdrawn is replaced by the solvent power of the water exerted upon the rocks to which the ocean has access. Water charged with carbonic acid, having dissolved the carbonate of lime, or the oxide of iron, may lose its solvent power over these bodies by the removal of

the acid; in such case the contained salt is precipitated. Such deposites of carbonate of lime are called tufa or travertin; or, if they occur in water trickling from the crevices of caverns, they are called stalactites. The deposits made on the floor beneath the stalactites, are called stalagmites. Similar deposites of the salts of iron previously held in solution constitute the hydrate of iron, bog iron ore. The waters of hot springs often hold silica in solution, which they deposite on cooling.

18. The mechanical agency of water is still more manifest, and is either gradual, as in the wearing down rocks by the rain, or rapid, by torrents, and ocean storms.

Rivers are most efficient agents in transporting mineral masses. When their fall is considerable and their motion rapid, they wear their beds, deepening their channels. They deposit the materials suspended in them along their banks, upon their bottoms, or at their mouths.

Their eroding power is increased by the friction of the ice, sand, and pebbles conveyed by them, and is exhibited on a grand scale at large waterfalls. The falls of Niagara are one hundred and fifty feet in height, and the average amount of water passing over each minute, is estimated at six hundred and seventy thousand tons. The position of the falls is not stationary; they have receded about fifty yards in forty years, and it is difficult to avoid the conclusion, that they were originally at Queenstown, seven miles below their present position. The length of time required to wear through this space, can not be satisfactorily determined, since they must have passed through several rocks varying in texture, in which the rate of wearing can not now be ascertained. For the same reason their future progress can not be predicted. Their present situation is favorable