Britain and the North Sea, or the ice fields of Greenland with the ample life of Scandinavia, if we wonder at the small part which latitude seems to have with opposite shores of the Atlantic, we are at once led back to the atmosphere for light upon our problem.
Likewise for all life, the atmospheric gases are essential as a heat regulator and in the employment of their substances for organic structures. Through these structures, in turn, all soil-making is profoundly conditioned, and those organic accumulations are made possible, which we find in the mineral fuels and in most limestones.
If we turn to the direct work of the atmosphere, we shall find a group of facts, not so conspicuous, perhaps, but worldwide and important. This is one of the newer subjects in geology, and to consult any text-book written twenty years ago would yield a scant result in information.
The atmosphere is the agent of incessant chemical change, the world over, and down to the level of the ground water. Whether the water table be found close to the surface or far down, the atmosphere finds access to some parts of the rocks down to that horizon. The depth below the surface depends on the water supply, on the character of the rocks and on the surface topography, and it is different in the same place at different times, but always and everywhere the gases that surround and surmount us, are pursuing their underground activities as well. And it is scarcely proper to limit them to zones above the water table, for they communicate some measure of their efficiency to the ground waters, which in turn may go far down, and may make long journeys before they emerge, thus giving the atmosphere some share in the segregation of those metallic substances which belong in the inventory of the world's wealth.
The making of soil is a complicated process, or bundle of processes. Fundamentally it is due to the breaking down of rocks, and this is effected by change of temperature, by organisms, by the wear of running streams, glaciers and ocean waves. But when the rocks are broken down, processes more intimate and essential must be added. These more intimate agencies are the water, the atmosphere and decaying organisms. The water will accomplish solution and thus make certain minerals available for the nutrition of crops. But the water falling as rain has gathered from the atmosphere minute portions of its carbon dioxide, and has become thereby an effective dissolving agent. Most rocks contain more or less iron in a disseminated condition. The oxygen of the air combines readily with this metal, promoting the decay of the rock mass, and coloring not only the rock, but the soils that ultimately come into being. When the farmer selects a field to lie fallow he stirs the soil, gives it all possible exposure to air, water and heat, and thus speeds these silent processes which go on in some measure everywhere, with or without his ken. It is the time required to produce
