Emit Truog 25 



in a climate like that of Wisconsin. Under this action, cracks, fissures, 

 and flaws develop in all rocks, into which seeds of plants carried by 

 wind and birds may lodge, germinate, and grow. All of us have seen 

 examples of this in a stone quarry, stone wall, or right in our rock 

 garden, and have marveled at the ability of certain plants to anchor 

 themselves in minute cracks and then flourish on virtually bare rock. 

 Lichens and moss, which will grow on practically bare rock, are often 

 the forerunners of the higher plants. The dense and compact growth 

 of moss provides favorable conditions for the accumulation of both 

 mineral and organic matter. Having gained a foothold, the plants help 

 to hold the rock powder being formed in place against removal by wind 

 and water, and in addition exert a powerful influence in accelerating 

 the further disintegration of the rock. Once established, the plants start 

 to shed and cast off leaves, roots, and other parts which become mixed 

 with the rock powder and provide organic matter which serves as food 

 for microorganisms brought in by wind, water, and other agencies. 

 Thus the embryonic soil receives the touch of life which enables it to 

 grow to maturity. 



However, even after the beginning just described has been made, it 

 may take 1,000 years to produce an inch of soil material. After a layer, 

 usually eight to twelve inches deep, has developed, the further produc- 

 tion of soil material by weathering of rock material produces a second 

 layer which is quite different from the one above; it receives very little 

 organic matter, being somewhat distant from the main source of such 

 material, and its population of microorganisms is thus much lower; 

 also, it is less subject to leaching and may, in fact, receive fine clay and 

 other material from above. In time this layer becomes as thick as or 

 thicker than the layer above, but, because of a lack of organic matter 

 which gives all or a portion of the layer above a dark color, it usually 

 has a yellowish, brownish, or mottled brownish to bluish color, de- 

 pending upon the state of oxidation of the iron as determined by 

 drainage and aeration. 



Of course, the bedrock beneath keeps on disintegrating under the 

 solvent action of water and the action of frost and other agencies. The 

 weathering rock material exists first largely as a layer of loose gravel 

 which receives additions continually from below as the bedrock dis- 



