AIR CONTAINED IN SNOW.

41

cent.; while atmospheric air, as we have seen, contains 21 per cent. Hence it was long supposed that the air, always present in snow, naturally contained this small proportion. of oxygen, and that snow, therefore, possessed some peculiar property of absorbing the gases of the atmosphere in this new proportion. But the explanation is, that the snow, in melting into water, takes up a larger proportionate quantity of the oxygen than it does of the nitrogen of the air which was contained in its pores, and consequently leaves a smaller proportion behind.

Thus the water we drink, like the air we breathe, is a substance of much chemical interest. Both are indispensable to the existence of life; both are mixed in nature with many substances not essential to their composition; and both, in their most important properties, exhibit many direct relations to the growth of plants and to the wants and comforts of living animals.

CHAPTER III.

THE SOIL WE CULTIVATE.

General origin of soils; natural differences in their quality; how it arises.-Stratified and unstratified rocks.-Soils of the stratified rocks.-Improved soils where different rocks intermix.-Soils of the granites, traps, and lavas.-Agency of rains, winds, and vegetable accumulations in producing diversities of soil.-General chemical composition of soils.-Illustrations afforded by the Atlantic border of the United States.--Some plants affect sandy soils, others clay soils, and yet do not always flourish upon them.-Cause of this.-Minute chemical composition of the soil; its mineral and organic parts-Chemical difference between granite and trap soils.Dependence of fertility on chemical composition.-Influence of rain and moisture, and of the degree of warmth, on comparative fertility.-District floras and crops.— Influence of man in modifying geological, chemical, and climatic tendencies.--Progress of exhausting culture in new regions; example of North America.--Reclaiming influences of human exertion; example of Great Britain.

IN immediate importance to man, the soil he cultivates is scarcely inferior to the air he breathes, or the water he drinks. Upon the plants which the soil produces he and all other animals depend for their daily sustenance. Hence, where the soil is fruitful, animal life is abundant; where it yields only sparingly, animals are few, and human inhabitants, as a general rule, but sparsely scattered.

The soil is formed, for the most part, from the rocks of which the crust of the earth is composed. By the action of air and water these rocks crumble, and their surface be

DIFFERENT QUALITIES OF SOILS.

43

comes covered with loose materials. The seeds of plants are sprinkled over them by the winds; they germinate and grow up; animals come to feed upon them; both plants and animals die; and thus a mixture of decayed rock, with the remains of animals and plants, gradually overspreads the entire surface of the dry land. It is to this mixture that we apply the name of soil.

But the soil thus naturally formed differs in quality, from various causes. The rocks which crumble differ in chemical composition; their crumbled fragments are spread over the surface, and sorted by wind and water in different ways; and the kind and quantity of the animal and vegetable matters they are mixed with differ much. Through the agency of these and similar causes of diversity, many varieties of soil are produced, which are not only unlike to each other in their sensible properties, but very different also in their agricultural value.

If we examine with a little attention the numerous rocks we meet with in travelling over a country like our own, an important difference in their physical structure will early strike us. Some are seen to form hills, cliffs, or mountains, which consist each of a single huge lump or mass, cracked here and there, perhaps irregularly, but exhibiting no continuous division into distinct parts or portions. Others again are as clearly divided into layers or beds, spread over each other like vast flagstones of different thicknesses, sometimes extending horizontally for distances of many miles. The following section (fig. 10) exhibits these differences of physical appearance.

The rocks marked A and B are the undivided masses, those marked CD are the rocks which lie in beds. The numbers 1 2 3 indicate the groups into which the beds, when numerous on any spot, can usually be subdivided.

The most ignorant of science can observe differences of this kind-it requires only the use of the eyes; and yet this difference of structure is so important, that upon it is founded the division of all rocks into stratified and unstratified. Those which are composed of beds or strata are called stratified, those in which no such partings are visible are called unstratified.

The stratified rocks cover by far the largest portion of the earth's surface. They are not always quite horizontal, as represented in the above section; they are more often inclined, so as to dip into the earth at a greater or less angle. Sometimes they are even piled against each other like flagstones placed on edge. The following section (fig. 11) ex

Fig. 11.

B

D

hibits these three several modes in which the stratified rocks occur, A showing them on edge, B dipping at a considerable angle, and C D E perfectly horizontal. This disposition of the rocks, it will be seen, must materially affect the quality. of the soil, and especially the extent of surface over which any particular soil is to be found. If the quality of the soil depend in any degree upon the quality of the rock, the changes of soil must be very frequent where the surface is formed of the edges only of different rocks, as is seen at A and B.

These stratified rocks consist essentially of one or more of three different kinds of matter only: limestones, sand

stones, and clays, more or less hard, form the substance of them all. When a limestone crumbles, it forms a calcareous soil; a sandstone, a sandy soil; and a hard clay rock, a more or less tenacious clay soil. Hence, these are the three leading qualities of soil known and spoken of among practical men.

But many rocks do not consist altogether either of limestone, of sandstone, or of clay, but of a mixture of each in varied proportions. The crumbling of such rocks, therefore, gives rise to soils of various intermediate qualities, neither calcareous, properly speaking, nor sandy nor clayey; and these form, for the most part, those more open, fertile, and valuable loams, which the farmers of every country prefer to cultivate.

Similar mixed soils are also naturally produced where the edges of different rocks overlap each other, and mingle their mutual debris. Thus, when the fragments of a rock rich in lime naturally intermix with one poor in this ingredient, the soil produced is of a much better and more useful. quality than when the surface is formed by the fragments of one of the rocks only. This is illustrated in the south of England in many places, where the materials of the plastic clay, the chalk, and the green-sand, meet and intermingle, as seen in the following section, (fig. 12).

This woodcut represents the plastic clay as coming in contact with the chalk which lies below it, and the chalk again coming in contact with the upper green-sand, upon