CHAPTER VI. 



THE SOIL — TTS FORMATION AND COMPOSITION. 



108. We have for so far been occupied with the con- 

 sideration of the nature of the raw materials which exist in 

 the air and the earth for the development of the vegetable 

 tribes, and of the peculiar forms so important to man which 

 they are made to assume. It will be interesting and useful, 

 now that we have finished this inquiry, to direct our attention 

 to the soil, that great storehouse, from which plants procure 

 the earths, the alkahes, and the metallic compounds which are 

 invariably found to enter into their structure, forming their 

 skeleton and giving them strength and firmness. 



1 09. By the term soil is understood that layer of earth 

 which the farmer ploughs and cultivates, and which gives 

 support to his crops. Its colour and appearance vary very 

 much. It occurs as the rich black mould of the garden, the 

 turfy soil of the bog, the heavy red clay, or light-coloured 

 calcareous covering. Its depth also varies from a few inches, 

 as where it covers the clay-slate rocks of Down and Mona- 

 ghan, to many feet, as in the deep sandy and alluvial beds 

 of earth which constitute the arable land in many parts of 

 Ireland. 



110. When we examine the soil of a cultivated field 

 minutely, it is found to consist of various proportions of sand, 

 gravel, and dust, mixed with decaying remains derived from 

 the roots and leaves of plants that have grown upon it, and 

 the bodies of worms and insects that have lived in it. When 

 we penetrate below these we find a layer of earthy materials, 

 broken pieces of rock, and hard clay, containing fewer traces 

 of animal or vegetable matter. This is what is termed the sub- 

 soil, and beneath this everywhere at varying depths is to be 

 found a solid floor, composed of that kind of rock which pre- 

 vails in the district. In one place we Avill find it consisting 

 of a blue or white limestone, in another, of the black basalt, 

 and in other localities, of the red sandstone or grey slate. 

 There was a time when no soil covered the hard rock, and 



