INDUSTRIAL MINERALS DIAGRAM 20. 221 



quality of the land is bad, like clayey soils which contain too 

 much clay, or sandy soils which contain too much sand. When 

 there is too much lime in the soil, the surface, when moistened, 

 cakes, and forms a crust which prevents the air from penetrating 

 into the ground. Frost loosens the chalky soil, pulverises it, and in 

 this condition the earth is sometimes carried away by the wind, and 

 leaves the plants uncovered, and without proper support for the roots. 



But, in most cases, calcareous soils contain more or less silicious 

 sand or clay, and they then form excellent soils. When the 

 chalk or limestone is simply mixed with clay, the soil is very good 

 for the growth of corn and provender. If sand is also present, 

 the land which is composed of the three elements, sand, clay, and 

 chalk, also forms a soil which is favourable to the growth of most 

 cultivated plants, and especially to the growth of trees. Calcareous 

 soils generally yield more succulent and nourishing crops than 

 clayey and siliceous soils ; animals are generally stronger and 

 fatter there, and their milk is richer. 



4th. Peaty soils are those which contain a large proportion 

 of more or less decomposed organic substances, and these earths, 

 after being dried, lose a fourth of their weight when burned. 

 They are generally of a dark colour, on account of the large 

 proportion of organic matter which they contain. 



If peaty soil does not contain a sufficient proportion of mineral 

 substances, it is unproductive. The roots of plants cannot fix 

 themselves in it with sufficient firmness ; moreover, it dries up 

 too rapidly, and therefore the plants which grow upon it do not 

 always obtain sufficient moisture. 



INDUSTRIAL MINERALS. 

 [ DIAGRAM 20. ] 



After having briefly described the most remarkable phenomena 

 of the earth, we will notice the principal useful substances which 

 man has found in it. Those which he obtains from the Mineral 



