462 Action of the Atmosphere upon newly-deepened Soil. 



Silica 



Alumina 



Peroxide of iron 



Lime 



Potash 



Magnesia 



Water 



Insoluble matter and talc 



Miessen. 

 Forchammer. 



46-46 

 36-37 



1-47 



13-61 



altogether dissolved out. Forchammer considers that the yellow 

 clay of Denmark consists of granite, the felspar of which has 

 been altered, whilst its mica remains unchanged, its quartz 

 forming the sand of the clay ; while the blue clay results from 

 syenite and greenstone, which have no mica. The clay derived 

 from the potash felspar is wanting in lime, and not so favourable 

 to vegetation as that from minerals which contain both lime and 

 potash or soda. The red colour of the clay is owing to the 

 presence of peroxide of iron. In the blue clays the colour would 

 appear to be occasionally owing to carbonaceous matter acting 

 on this peroxide and converting it back into the dark protoxide. 

 Thus, below beds of peat, even in districts where the clay is 

 otherwise red, a blue-coloured clay is usually found ; and beneath 

 vegetation red marls and sandstones are sometimes seen converted 

 into a green or bluish-green colour, the carbonaceous matter ab- 

 stracting part of the oxygen. (Z)e la Beche.) An interesting 

 instance of this occurred in the parish of Culsamond, in this 

 county. In cutting a ditch about 8 feet below the surface, large 

 quantities of bog-iron ore (hydrated oxide of iron) were found 

 mixed with decayed oak wood. The mixture was of a beautiful 

 light-blue colour in consequence of the action of the iron and 

 vegetable matter on each other. [Stat. Account of Aberdeenshire, 

 p. 730.) Another instance was observed by the writer in his 

 own house ; during wet weather a little rain-water had come in 

 at a skylight window, and trickling over a piece of rusty iron 

 dropped upon a wooden floor. This water, impregnated with 

 the iron rust (peroxide), gave the wood at first a brownish-red 

 colour, but after a time the carbonaceous matter of the wood 

 acting upon the iron abstracted part of the oxygen, and converted 

 the spot into a dark-bluish green stain. 



The shales and slate-clays of the sedimentary rocks may be 

 viewed as arising in the same way as the other clays spoken of, 

 viz. by the decomposition of felspar and other silicates. They 

 will vary in fertility according to the amount of alkalies .that 

 remain in them ; they are also often impregnated with foreign 

 matters, bitumen, organic remains, &c. 



