CHEMICAL WORK OF THE ATMOSPHERE 39 



CO2 ill 10,000 parts 

 by weight. 



Air from sandy subsoil in forest 38 



Air from loamy subsoil in forest 124 



Air from surface soil of forest 130 



Air from surface soil of vineyard 146 



Air from pasture soil 270 



Air from soil rich in humus 543 



The union which goes on continually of the COo with the min- 

 erals of the Belt of Weathering in the earth's crust is rather an 

 accompaniment of the processes set going by the ground water, than 

 a direct activity of the atmosphere. The use of CO, in solution 

 of limestones likewise belongs there. On liberation in the process 

 of decarbonation, the CO2 commonly returns to the atmosphere. 



Laterization. Laterite (Meigen-64; Clarke-17) is the prod- 

 uct of rock decay in pluvial tropical regions, and is especially 

 characterized by the high content of iron oxide. Its color is mostly 

 a deep red, the iron being often so abundant as to form concretions 

 of brown or red iron ore. The depth to which the laterite extends 

 may be very great, being over 300 feet in Brazil. Its areal extent 

 is likewise great : thus, according to the estimates of von Tillo, it 

 covers 49% of the surface of Africa, 16% of Asia and 43% of 

 South America. 



Typical laterite is characterized by the formation, during the 

 process of decomposition, of a considerable amount of hydrous oxide 

 of aluminum, chiefly in the form of Hydrargillite (AI0O3.3H2O), 

 whereas the ordinary products of rock decay in moist temperate 

 climates are the hydrous silicates of aluminum, kaolinite (H^Ab- 

 SioOg), or its ferric equivalent, nontronite (H^FcoSioOg). Often 

 the laterite still shows the form or structure of the gneisses or other 

 crystalline rocks from which it is derived. 



The diminution or total disappearance of the silicic acid in com- 

 bination with bases is one of the characteristic features of lateriza- 

 tion. Thus the laterization of a dolerite near Bom.bay, India, in- 

 volves a loss of SiOo from 50.4% in the fresh to 0.7% in the 

 weathered, while at the same time there is a proportional increase 

 in the ALO3 from 22.2% in the fresh to 50.5% in the weathered 

 rock, and an increase in FcoO.. from 9.9% to 23.4%. In the 

 kaolinization of an English dolerite in South Staffordshire, the 

 change from the fresh to the altered rock includes a change in SiOo 

 from 49.3% to 47.0%, in AbO^ from 17.4% to 18.5%, of Fc.Oa 

 from 2.y% (+8.3% FeO) to 14.6%, and so forth. Laterization 



