The Elements of economic geology

Bibliographic data

Monograph

Identifikator:: 1773832379

URN:: urn:nbn:de:zbw-retromon-172798

Document type:: Monograph

Author:: Gregory, John W. http://d-nb.info/gnd/11683014X

Title:: The Elements of economic geology

Place of publication:: London

Publisher:: Methuen

Year of publication:: 1928

Scope:: XIV, 312 S.; graph. Darst.

Digitisation:: 2021

Collection:: Economics Books

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Chapter

Document type:: Monograph

Structure type:: Chapter

Title:: Part III. Earthy minerals

Collection:: Economics Books

THE SOIL [QI fruits which are out of season. The essential value depends on composition, texture, porosity, and colour. Soi CownstiTuTiON—SAND, CLAY, AND Sitt—A bulk chemical analysis of a soil may be of little agricultural significance, as much of the material is not available as plant food. The constitution is often more instructive; it is determined by mechanical analysis, which after re- jecting the pebbles, divides the material that will pass through a sieve with holes ;-inch in diameter into sand, silt, and clay. Sand has often been regarded as composed of silica, and clay as composed of silicate of alumina; but some sand is com- posed of carbonate of lime, or of grains of felspar, and some clay consists of quartz ground as fine as flour. That the difference between clay and sand is physical and not due to composition is shown in mining. An ore is crushed to allow the extraction of its metal, and sorted into a coarse grade (above 04 mm.) ‘the sand,” and a finer, * the slime,” which is a clay. Their chemical composition is identical, and the difference is due wholly to the size of the particles. Sand is loose material in which the particles vary from J to gig of an inch (or I to '05 mm.) in diameter! Clay is material in which the particles are less than 554 of an inch (-005 mm.) in diameter. Material intermediate between sand and clay is known as silt. Clay of less than 002 mm. in diameter is a colloid and its effects on soils are independent of its chemical composition; colloids absorb from solutions materials that would otherwise be carried away by drainage ; they strengthen the imbibition (cf. p. 228) and control the reaction of the soils to lime. Lime may act differently on two soils of the same chemical composition, since it coagulates colloids into larger particles, and thus improves the texture, and confers no such benefit upon the coarser particles of silt. A sandy soil is said to be * light,” as it is friable and easily worked. A soil with more than 40 per cent. of fine sand cakes after rain, and has to be broken by rolling. A soil with 40 per cent. of sand and less than 5 per cent. of clay is rarely useful unless stable manure is abundant and the water conditions favourable. Many good loams hold less than L For a classification of the grades, and literature, see Tyrrell, Principles of Petrology, pp. 190-1. For methods see Boswell, British Refractory Sands, 1918, pp. 18-28.

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