fathoms; and to which other calculations have assigned a depth of no less than nine miles.

AVALANCHES (French, lavanches, avalanges)—are accumulations of snow, or of snow and ice, which descend from lofty mountains, like the Alps, into the valleys beneath. They originate in the higher regions of mountains, and begin to descend when the gravity of the mass becomes too great for the slope on which it rests, or when fresh weather destroys its adherence to the surface. Avalanches are generally distinguished as drift, rolling, sliding, and glacier or ice avalanches. Drift are those caused by the action of the wind on the snow while loose and powdery; rolling, when a detached piece of snow begins to roll down the steep-it licks up the snow over which it passes, and thus acquires bulk and force as it descends; sliding, when the mass looses its adherence to the surface, and descends, carrying everything before it which is unable to resist its pressure; and glacier or ice, when pieces of frozen snow and ice are loosened by the heat of summer, and precipitated into the plains below.

ICEBERG (German eis, ice, and berg, mountain)—the name given to the masses of ice resembling mountains, often found floating in the polar seas. They are sometimes formed in the sea itself by the accumulation of ice and snow; at other times they seem to be glaciers which have been piling up on a precipitous shore, till broken off and launched into the ocean by their own weight. Masses of this kind have been found in Baffin's Bay two miles long and half a mile in breadth, rising from 40 to 200 feet above the water, and loaded with beds of earth, gravel, and rocks. Some idea of the size of these icebergs may be formed from the fact, that the mass of ice below the level of the water is about eight times greater than that above. As they float towards warmer regions, they gradually dissolve, dropping their burden of rock debris, and thus strewing the bottom of the ocean with clay, gravel, and boulder stones, some of which are many tons in weight.

PROPORTION OF LAND AND SEA. The proportion of land to sea is, accurately, as 266 to 734. If, therefore, the whole superficies of the globe be taken at 196,816,658 square miles, it follows that the dry land occupies 52,353,231 square miles, and the ocean an area of 144,463,427 square miles.

PRESSURE OF THE ATMOSPHERE.-If the density of air at the surface of the earth be represented by one, at seven miles above the earth it will be 1-4th, at fourteen miles 1-16th, at twenty-one miles it will be 1-64th, and so on. This property of air would lead to the idea of an indefinite extension of the atmosphere, but there is evidently an appreciable limit to this; and hence, by calculations relative to the progress of the sun's light, and other astronomical phenomena, forty-five miles has been fixed as the altitude of the atmosphere. Air is ponderable-100 cubic inches at the temperature of 60 degrees weighing 30 grains. A perpendicular column of the whole atmosphere is balanced by one of mercury rising to 30 inches; hence the atmosphere presses on every cubic inch of surface with a weight equal to 16 pounds.

PRESSURE OF THE OCEAN.-Water being slightly compressible, it follows, as in the atmosphere, that water at great depths in the ocean will be denser than at the surface. According to calculations by Oersted, water at the depth of 1000 feet is compressed 1-340th part of its own bulk.

CAUSES MODIFYING THE STRUCTURE AND CONDITIONS OF THE GLOBE.

37. Had the general structure and conditions of the globe, as described in the foregoing section, been subjected to no modifying causes, they would have remained unchanged from the beginning of time, and the earth would have presented now the same appearance as at any former period. But these very conditions are themselves the causes of change, for they mutually act upon each other, and give rise to innumerable agents which have continued through all time to modify the face of nature. Thus, for example, the planetary relations of the earth enable it to derive heat from the sun; this heat vaporises the water of the ocean, the vapour produces rains, these rains form springs and rivers, the rivers wear down the land, and thus change the surface configuration; the matter borne down by the rivers forms new land along the sea-shore, altering the distribution of land and water; and this distribution of land and water materially affects the kind and distribution of plants and animals. This is a simple instance of the changes produced by the action and reaction which takes place among the general conditions of the globe; and the student would do well, at this stage of his progress, to familiarise himself with such trains of cause and effect, as it is only by the ready application of similar reasoning that he will be able to comprehend many of the phenomena hereafter described.

38. The modifying causes produced by the mutual influence of the general conditions already considered are exceedingly numerous and varied. At present, it is necessary to notice only such as seem to account for the principal facts connected with the solid materials which form the crust of the globe, and the order and manner of their arrangement. causes, or agents, may be divided into four great classes namely, ATMOSPHERIC, AQUEOUS, IGNEOUS, and ORGANIC; and their modes of action may be either mechanical, chemical, electrical, or vital.

These

:

ATMOSPHERIC AGENCIES.

39. ATMOSPHERIC AGENTS act either mechanically or chemically the action of wind in drifting loose sand is mechanical, the action of the air in weathering the surface of rocks is chemical. The atmosphere may either act directly, as in

the case of winds, or indirectly, as in the production of waves, the effects of which on the sea-coast are often destructive and extensive. The changes produced on the earth's crust by atmospheric agency are sometimes slow and gradual, such as in the crumbling down of rocks; or immediate, as in the uprooting of forests by tempests, and the covering of green valleys by barren sand-drift. The air, or atmosphere, is one of the most important elements, and is more or less connected with every operation in nature. By it the sun's light and heat are equally diffused; it is indispensable to the existence of plants and animals; and it is the great laboratory in which the waters of the ocean are purified and distributed over the face of the globe. These may be said to be universal functions of the atmosphere. It also acts peculiarly, and over limited extent, as in the production of winds, frost, heat, electricity, and gaseous admixtures.

40. Winds are aërial currents. When the air of one region becomes heated or rarefied, the colder and heavier air of the surrounding regions rushes in to restore the balance, and thus atmospheric currents are produced. These currents are extremely unstable, blowing without regard to time or direction, and modified and obstructed in a thousand ways by hills, valleys, and other surface irregularities. They are equally unstable in their velocity, varying from the gentlest breeze to the fiercest hurricane which overturns cities and uproots forests. But though characterised by these irregularities over the greater portion of the globe, there are regions over which they pass with wonderful steadiness for months together. The trade-winds which take place within the tropics possess this character, and blow from east to west with little variation of direction or force. The monsoons, which are connected with the trade-winds, are also pretty regular; and in most countries an east, a south, or a west wind, is found to prevail over other directions, and that at particular periods of the year. Of the phenomena of winds in the earlier eras of the world we have no knowledge; but we are warranted to conclude, that since the elevation of dry land, and the distribution of land and water, they have been analogous to what is now in daily

Occurrence.

41. Wind acts on all loose material, bearing it from exposed to sheltered places. Sand, gravel, and loose shells are most frequently shifted by its force, and blown into hillocks, or scooped out into hollows, without order or regularity. All those extensive tracts of sand found along the sea-coastknown in Scotland as links, and as downs in England-owe their surface formation to wind. The sand collected in bays

and creeks by the waves and tides of the ocean, is no sooner left dry by the tide, and exposed to the sun, than it becomes light, and easily acted upon by the wind, which raises it into knolls and ridges beyond the influence of the returning tide. By and by a scanty herbage gathers over the sand, and thus, in course of ages, extensive downs are formed. In a similar manner the wind acts upon the sandy deserts of Arabia and Egypt, continually shifting their surface; and if it sets in from any prevailing direction, these sands are carried forward, year after year, burying trees, fields, and villages, and thus converting fertile districts into barren wastes. When a river enters the sea through a sandy district, it has a tendency frequently to shift its channel; and this tendency is greatly increased by winds damming up the current with drifted sand. Volcanoes occasionally discharge showers of dust and ashes, which, during high winds, are carried over many leagues of surface, or borne out to the ocean. During calm weather volcanic dust and ashes would fall in the neighbourhood of the crater; during high winds they may be deposited at vast distances from their original sources. Such examples as the above are the ordinary actions of wind: the uprooting of forests, the destruction of cities, and the like, are extraordinary, and are caused by whirlwinds and hurricanes.

42. Frost exercises a slow but permanent influence in modifying the surface configuration of the globe. When the heat of the surrounding atmosphere falls below 32 degrees of the thermometer, water begins to freeze, and in this state expands. During winter or moist weather, water enters between the particles of all rocky matter at the surface of the earth, and also into the larger fissures; and the expansion of this water by frost separates these particles, and leaves them to fall asunder when the ice is dissolved. This takes place more or less every winter; and there is not a cliff or hill side but bears evidence of this kind of action. The effects of frost in crumbling down rocky material has been long observed: the farmer takes advantage of it to pulverise his soil; and in some districts slate and flagstone are split into thin lamina by being exposed to the frost.

43. Of the amount of change produced by frost, it would be difficult to form an estimate; but, taking it over a lapse of ages, there can be little doubt that it has been an important agent. In mountain regions, such as the Alps, its effects are strikingly apparent in the formation of avalanches; and in northern latitudes the iceberg is one of its familiar productions. The action of frost in crumbling down a rocky surface seems slow and insignificant, but when we look

upon the avalanche carrying rocks, gravel, trees, and houses before it, and burying them in one common ruin; when we look upon the iceberg laden with huge stones, and dropping them into the ocean as it dissolves, we are more impressed with its importance, and are enabled to account for certain geological appearances which no other agency could have produced. (See note, page 24.)

44. Solar heat and light may, without inuch impropriety, be classed as atmospheric agents, as the atmosphere is the medium through which they act, and by which they are modified. Water conducts heat faster than air, and air, at the surface of the earth, faster than highly rarefied air at great heights; hence different conditions of the atmosphere may have hitherto conducted more heat to the earth's surface. The quantity of light which reaches the earth depends upon the serenity of the atmosphere, and the height of the sun above the horizon; hence, also, a different condition of atmosphere would produce a different amount of light.

45. Heat and light are indispensable to vegetable and animal existence; and the kind and number of plants and animals depend, in a great measure, upon the degree and uniformity of their influence. Heat converts water into vapour, and vapour forms dews, rains, &c. The amount of vapour, and consequently the amount of rain, will depend upon the degree of heat; and hence the heavy periodical rains of the tropics. A higher degree of heat all over the earth would greatly increase the amount of rains, these rains would form more gigantic rivers, and geological effects of corresponding magnitude would follow.

46. Of the amount of solar heat received by the earth at any former period, we are left to infer from the kind of plants and animals which are found imbedded in the rocky strata; a scantiness and peculiar character of these remains indicating a temperature analogous to that of the polar regions, and numerical amount and external form indicating a climate similar to that of the tropics.

47. Electricity is also ranked among atmospheric agents, though electric, galvanic, and magnetic influences may be going on in the crust of the globe totally independent of the atmosphere. The effects of these subtle forces are not easily calculated; and what connexion they may have with earthquakes, with the formation of metallic veins, and similar phenomena, geology has not been able to determine. We know that the hardest and most untractable substances in nature can be artificially dissolved and reconstructed by the aid of electricity; this force sometimes acting slowly and