234 VEGETABLE SOIL DIAGEAM 19 



ceous 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 pre- 

 sent, 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. 



METEOBIC STONES. 



This name is given to stones which sometimes fall from the 

 sky, and they are also called aerolites. But it must not be sup- 

 posed that these stones are common, and this origin has been 

 wrongly ascribed to many fossils. True meteoric stones have al- 

 ways an irregular shape, and are generally of a black colour. 

 The shooting stars which are seen at night are meteoric stones 

 passing through the air, very few, indeed, of which, however, 

 reach the earth in the form of stones, being nearly always re- 

 duced to an impalpable dust in the highest regions of the atmo- 

 sphere. 



