Page:Popular Science Monthly Volume 76.djvu/578

574 these fine results, that makes wanton destruction of soils criminal and points the rebuke to public authorities and legislative bodies, when they halt at reasonable measures of conservation. We can not grind rocks in a mill and make soil. The operation is at once too large and too delicate, demanding the silent intervention of mechanical and chemical forces, of atmosphere, water, heat and life, through long periods of time.

When the land has suffered a glacial invasion, much of its ancient soil has been lost in the sea, and such as remains is moved from its place and mixed with a large body of drift, mechanically broken from fresh bed rock. This latter material is not soil until it has been subjected to the atmospheric and vital processes which fit it for its function of mediation between the rocky planet and the plant life of the world.

In a non-glacial region, all the soils, save along rivers, or on steep slopes, have been formed by the decay of the bed rocks in place, and this decay does indeed, and fortunately, in favored regions, proceed swiftly. It may be some compensation for people subject to disaster on the slopes of Vesuvius or Etna, that the friable lavas and ash, in that genial climate, speedily become soil. On a lava stream still warm, at the foot of Vesuvius, baskets of earth, suitably spaced for vines, have been deposited, and the lava itself in a year or two will be hospitable to the roots.

Stone from the quarry is popularly thought to be a durable building material, but only the most compact and resistant varieties, and these in a favorable climate, can make any approach to permanence. Granites are regarded as indestructible, but the title of granite to serve as the standard and symbol of strength is clouded when we remember that many beds of soft clay owe their accumulation to the decay of one of the chief mineral constituents of this rock, a decay in which the atmosphere has been a powerful agent. Granite that can be excavated with pick and shovel betokens the ceaseless activity of the gases and waters of the earth's surface.

Some of you will recall the promptness of atmospheric attack upon the obelisk of Central Park after it was transplanted from its arid habitat of millenniums. Many of the beautiful structures of the Oxford colleges, boasting not three centuries of antiquity, are under restoration piece by piece, showing an apparent hoary age through the solvent work of the atmosphere upon their unstable calcareous material. No marble monument has stood in the open air for half a century and retained its polish, and it must have been an exceptional piece of monumental stone if it does not now crack and scale and take on the look of age. Every humid climate with large temperature range introduces a ceaseless struggle with the destructive forces of the atmosphere, whose sum of hostility to the structures of man is far greater than that of flood, fire or earthquake.

The energies of the atmosphere mechanically applied, bring before