Characteristics of Soils. 7 



is found in liollow places where this soil prevails, and the sage-bush 

 (Artemisiatridentata) and grease-wood {Sarcobatus vermieulatus) form 

 the principal vegetation. 



27. Although none of these soils can alone be .called fertile, their 

 proper mixture, and especially of the first three, with humus, af- 

 fords conditions highly favorable to success. 



28. Besides the qualities resulting from their chemical composi- 

 tion, and their relations to moisture, soils differ greatly in their ca- 

 pacity for absorbing, retaining, and radiating heat. A soil covered 

 with siliceous pebbles retains the heat better than fine sand, and 

 hence it is one of the circumstances that favor the growth of the 

 vine. In the wine districts of France, differences in the time of 

 maturing the fruit have been traced directly to this cause. A sandy- 

 soil radiates heat very readily in clear summer nights, and frosts in- 

 jurious to vegetation are more apt to occur upon these soils, where 

 exposed. 



29. The color of a soil has much efiect in determining absorption 

 of solar heat. If dark colored, it becomes warm sooner and to a 

 greater depth and degree than if light. 



30. The character of the underlying rock itself has also its influ- 

 ence upon the growth of plants and trees. If siliceous and solid, 

 their roots get no nourishment and no hold. If friable and com- 

 posed of mineral elements that may be taken up in solution by the 

 roots, their fibers will insinuate themselves into the fissures and as- 

 sist the disintegration, especially where moisture is present, and 

 where the frost can act. This operation takes place more readily 

 where the rock, is stratified in thin layers that are highly inclined. 



31. It is a fact familiar to geologists that certain forms of vege- 

 tation, and especially of trees, are characteristic of the rock forma- 

 tion that underlies them. We have examples of this in shales rich 

 with potash, that are congenial to the elms. A limestone soil is 

 favorable to the maples but not to the pines, while the latter flour- 

 ish best on siliceous soils, if suitably mixed with other ingredients. 

 A line of outcrop of some rock formation upon a hillside may be 

 made known in some cases conspicuously by the color of the foliage 

 of the trees that grow upon it, especially when they are colored in 

 autumn or remain green in winter. 



32. The soil upon a sloping surface is generally deeper, more hu- 

 mid, and richer near the foot, and tends gradually to become thiu, 



