SOIL. — ITS FORMATION AND COMPOSITION. 8 1 



shops some carbonate of soda (44); a white insoluble 

 gelatinous looking deposit is produced. When this sub- 

 stance, which is alumina^ is collected on a cloth and dried, 

 It becomes a fine white powder, which when mixed with 

 water, forms a tenacious mass which can be moulded into 

 shapes, and it is upon its presence that porcelain, tiles, 

 and brick clays depend for their useful properties. It is 

 never found in a separate state in the soil. The substance 

 which is termed clai/ by the farmers is composed of alumina 

 chemically combined with silica (50) and a small amount of 

 oxide of iron ; pure porcelain claij, such as is produced by 

 the disintegration or decay of the granite of Mourne, usually 

 consists of about 42 parts of alumina, and 58 of silica. 



117. Though alumina is an invariable ingredient of soils, 

 yet it is not regarded as directly contributing to the nourish- 

 ment of plants. The incombustible part of the soil is therefore 

 distinguished from the ash of the plant by the presence of 

 this substance. Its presence in the soil communicates to it 

 tenacity and the property of retaining moisture; and Liebig 

 is of opinion that by possessing the power of absorbing 

 ammonia from the atmosphere it contributes to fertility. 



118. It has already been stated that the differences which 

 exist in the qualities of the soils of a country depend in a 

 great degree upon the nature of the rocks from which they 

 are derived- It will, therefore, be useful to make you ac- 

 quainted with the composition of the rocks from which the 

 arable soil of this country is derived. 



By examining a geological map of Ireland, that of Griffith 

 for example, it will be found that different districts are dis- 

 tinguished by certain marks or colours according to the kind 

 (jf rock of which they are composed. Looking at the shading 

 {blue) which denotes the limestone formation, it will be per- 

 ceived what an immense extent it occupies, extending east 

 and west, from Dublin to Galway, a distance of 120 miles; 

 and north and south measuring 150 miles, and seldom in its 

 course ri&ing more than 300 feet above the level of the sea. 

 Next in importance, will be observed the clay-slate formation, 

 which in the north occupies so much of the counties of 

 Down, Louth, Monaghan, Longford, and Armagh, and in the 

 ijuth constitutes the prevailing rock of Wexford, Waterford, 

 ork, and Kerry. Marked by shades of a different kind, 

 ,red) and iuterposed between the limestone and the coast, 



