480 NOTES ON THE SOIL. 



the readiness and completeness with which plants growing in 

 it can withdraw the water for their use as required. The soil 

 is also a storehouse from which plants get the necessary ash 

 ingredients of their food, the lime, potash, phosphoric acid, 

 etc., which are formed by the breaking-down and solution of 

 the soil- grains. The soil is also a laboratory in which various 

 lowly plants (fungi and bacteria) are at work breaking down 

 dead organic matter, and converting it into nitric acid and 

 other forms in which it becomes available for the use of 

 higher plants. 



The Earth's Crust. By this term we understand the portion 

 of the earth's exterior which comes within our observation. Its thick- 

 ness is about 10 miles, so that it represents only a relatively thin layer. 

 Since the average density of the crust (2'8) is just about half that of 

 the earth as a whole, it is clear that the interior must have a very high 

 density, and cannot have either the same chemical composition or 

 physical characters as the outer layer. 



Rocks. A rock may be defined as any mass of naturally 

 occurring solid substance forming part of the earth's crust. Rocks may 

 be divided into two main groups : (1) those formed by cooling, at or near 

 the earth's surface, of masses of molten material, and termed igneous 

 rocks ; (2) those formed by the accumulation of material derived from 

 pre-existing rocks, and termed aqueous, stratified, or sedimentary rocks. 

 To the former group belong granites and basalts, and most of this group 

 are crystalline rocks. To the latter group belong sandstones, clays, 

 limestones ; most of these have been laid down under water, and they 

 are not usually crystalline. 



Elements of the Earth's Crust. Comparatively few ele- 

 ments enter into the composition of the common rock-forming minerals 

 (quartz, felspars, micas, hornblende, augite, olivine, calcite, gypsum, 

 kaolin, iron ores, etc.), and about eight out of the eighty elements now 

 known to chemists form about 97 per cent, of the earth's crust. The 

 percentages of these elements are : oxygen 50, silicon 25, aluminium 7, 

 iron 5, calcium 3, magnesium 2, sodium 2, potassium 2. Other ele- 

 ments which, though present in relatively small quantities, are widely 

 diffused and of considerable importance, are hydrogen, carbon, nitro- 

 gen, phosphorus, sulphur, chlorine. Only six of all the elements 

 mentioned occur in the free state namely, carbon, oxygen, nitrogen, 

 sulphur, hydrogen, and iron, the last two only in very small quantity. 



Origin of Soil. There are many agencies at work in 

 the formation of soils, and the processes of soil-growth are 

 in continuous operation. All soil-material is formed by the 

 breaking down of rocks, and soils may be divided into two 



