. Soil physics and management. abundant in rooks?0. What is the chief value of zeolites? 7. What is the importance of non-silicates as soil formers? S. Which are the principal non-silicate minerals in soils of luuuid regions? 9. How are igneous rocks formed : 10. Give distinctions between granites, svenites. diorites, and diabases. 11. Distinguish between chemical precipitates and seilimentary How are soils formed from calcareous rocks? 13. Where do we find soils formed from chalk? From limestone? REFERENCES ^ Clark. F. W., Bulletin (UO U. S. Geological Survey. The Data of Geo- chemi
. Soil physics and management. abundant in rooks?0. What is the chief value of zeolites? 7. What is the importance of non-silicates as soil formers? S. Which are the principal non-silicate minerals in soils of luuuid regions? 9. How are igneous rocks formed : 10. Give distinctions between granites, svenites. diorites, and diabases. 11. Distinguish between chemical precipitates and seilimentary How are soils formed from calcareous rocks? 13. Where do we find soils formed from chalk? From limestone? REFERENCES ^ Clark. F. W., Bulletin (UO U. S. Geological Survey. The Data of Geo- chemistrv, iSUii. p. 34.* Merrill, G. Kocks. Eock-Weathering and Soils. 1906, p. 15. CHAPTEE II WEATHERING EocKS are broken down into soil material throngh the processesof weathering (Figs. 3, 4, and 5). These may be divided into (1)physical agencies that break the rock into smaller pieces withoutaffecting it chemically, and (2) chemical agencies that change thecomposition of the minerals forming the rock and in so doing exert. Fig. 3.—Irregular weathering of rock due to Joints and stratification. Note talus at base.(Chamberlain and Salisbury, Courtesy Henry Holt & Co.) a marked influence upon its physical character. The work of thephysical agencies is disintegration, while that of the chemical agen-cies is decomposition. Each is accompanied and aided by the otherin its work and the changes tend to produce more stable forms underexisting conditions. As an illustration, feldspars are not very stableminerals under ordinary conditions, and hence break down intosubstances that are more stable. The chemical changes producehy^drous aluminum silicate, carbonates and free silica which aremuch more stable than the feldspar from which they are , the clay is much more stal^le than the original mineral 11 12 SOIL PHYSICS AND MANAGEMENT or even the coarse soil coustitueiits because it has approached moreclosely the limit of mechanical division. The others may be brokendow
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