THE ORIGIN OF SOILS 39 



differ in chemical composition. Analysis of a portion 

 taken from about a foot below the surface showed that, 

 apart from water and ash, it contained 60*1 per cent, of 

 carbon, 32'4 of oxygen, 5*1 of hydrogen, and 1/9 of nitro- 

 gen. The lower layers contain a larger proportion of carbon 

 and less of the gaseous elements. The ash is very variable, 

 both in quantity and quality, because it is difficult to 

 secure samples .entirely free from adventitious sand and 

 other matters not properly constituents of the peat itself. 

 The proportion of true ash probably does not exceed 1 per 

 cent, of the dry matter, and is of much the same com- 

 position as the ash of other plants. 



The physical properties of peat are also peculiar. It 

 consists essentially of a mass of decaying vegetable matter, 

 and is soft and spongy in character. Its density varies 

 from about 1*4 to 1/6, and the weight of a cubic foot of the 

 dry matter, uncompressed, is only about 21 Ibs. In the 

 dry powdery condition it exhibits but little tenacity; it 

 is easily moved and blown about. It is extremely porous 

 and has a vast internal surface. Both on this account and 

 because it unites with water-forming colloidal hydrates, its 

 power of absorbing and retaining water far surpasses that 

 of any other soil-forming material. When the pores are 

 not blocked with water it can absorb large quantities of 

 air, but the oxygen is rapidly withdrawn by combination 

 with the substance of the peat itself. 



It has been shown that peat contains a considerable 

 proportion of nitrogen. It is the only rock, or soil-forming 

 material, which .naturally contains that element as an 

 essential constituent. Soils formed from peat are there- 

 fore naturally much richer in nitrogen than those of 

 mineral origin, but they are generally deficient in all the 

 other elements of plant food. 



Summary. It appears, then, that the crust of the ejarth 

 is composed of rocks. It undergoes erosion and denuda- 



