Of the physical agencies, _temperature changes _are first in order of time, and perhaps of first importance. As the heat of the day increases, the rock expands, and as the cold night approaches, contracts. This alternate expansion and contraction, in time, cracks the surfaces of the rocks. Into the tiny crevices thus formed water enters from the falling snow or rain. When winter comes, the water in these cracks freezes to ice, and in so doing expands and widens each of the cracks. As these processes are repeated from day to day, from year to year, and from generation to generation, the surfaces of the rocks crumble. The smaller rocks so formed are acted upon by the same agencies, in the same manner, and thus the process of pulverization goes on. It is clear, then, that the second great agency of soil formation, which always acts in conjunction with temperature changes, is _freezing water. _The rock particles formed in this manner are often washed down into the mountain valleys, there caught by great rivers, ground into finer dust, and at length deposited in the lower valleys. _Moving water _thus becomes another physical agency of soil production. Most of the soils covering the great dry-farm territory of the United States and other countries have been formed in this way. In places, glaciers moving slowly down the canons crush and grind into powder the rock over which they pass and deposit it lower down as soils. In other places, where strong winds blow with frequent regularity, sharp soil grains are picked up by the air and hurled against the rocks, which, under this action, are carved into fantastic forms. In still other places, the strong winds carry soil over long distances to be mixed with other soils. Finally, on the seashore the great waves dashing against the rocks of the coast line, and rolling the mass of pebbles back and forth, break and pulverize the rock until soil is formed._ Glaciers, winds, _and _waves _are also, therefore, physical agencies of soil formation. It may be noted that the result of the action of all these agencies is to form a rock powder, each particle of which preserves the composition that it had while it was a constituent part of the rock. It may further be noted that the chief of these soil-forming agencies act more vigorously in arid than in humid sections. Under the cloudless sky and dry atmosphere of regions of limited rainfall, the daily and seasonal temperature changes are much greater than in sections of greater rainfall. Consequently the pulverization of rocks goes on most rapidly in dry-farm districts. Constant heavy winds, which as soil formers are second only to temperature changes and freezing water, are also usually more common in arid than in humid countries. This is strikingly shown, for instance, on the Colorado desert and the Great Plains.