90 FORMATION OF SOILS. 



in carbonic acid, water containing that gas continues to exercise 

 an action upon it, and effects a progressive disintegration of its 

 constituents. 



Forchammer considers that the yellow clay, which occurs so 

 frequently in Denmark, consists of granite, the felspar of which 

 has been altered, whilst its mica remains unchanged, and its 

 quartz forms the sand of the clay. 



The magnetic and titanic oxides of iron existing in granite are 

 still found in the clay as peroxide of iron and titanic acid. 



The blue clays arise from syenite and greenstone ; for in these 

 mica is absent (Forchammer). 



The great strata of clay at Halle have had their origin in the 

 disintegration of porphyry.* 



The white basis of the clay is easily distinguished by moisten- 

 ing it : while the felspar may be recognised by its yellow color 

 (Mitscherlich). The silica, dissolved by the potash, or soda, is 

 sometimes found deposited in a crystalline form on the crystals 

 of felspar ; this is often observed in the trachyte of the Seven 

 Mountains near Bonn (Mitscherlich). Most sand-stones contain, 

 mixed with them, silicates with alkaline bases. In the sandstone 

 of the Holy Mountain near Heidelberg, many unchanged frag- 

 ments of felspar are observed, which are partly changed into 

 clay and form white points in the sandstone. 



The analysis of the porcelain clays proves that the felspars 

 from which they were formed have not reached their utmost 

 limit of disintegration, for they still contain potash. The porce- 

 lain clays are those which are refractory in the fire, and do not 

 melt when exposed to the strongest heat of our furnaces. The 

 difficult fusibility of the porcelain clays depends upon their 

 small proportion of the alkaline bases, potash, soda, lime, mag- 



* The decomposed felspar, porcelain clay of Mori, near Halle, con- 

 iists of — 



Silica 71-42 



Alumina - - - - 26*07 



Peroxide of iron - - 1'93 



Lime 013 



Potash 0-45 



