EVIDENCE OF HISTORY 257 



been attributed to the formation of colloidal 

 aluminum silicate on the surface of the mineral 

 particles." 



Referring to this possibility, the authors say 

 (page 23, Bulletin 30) : 



"Silica is a normal product of the decom- 

 position of the minerals and has accumulated 

 in relatively large proportions in the soil. It 

 may be present as quartz, amygdaloidal silica, 

 or perhaps as other forms. But undoubtedly 

 there is a distinct tendency through solution 

 and redisposition for it to be gradually 

 changed from other forms into quartz. Under 

 arid conditions, as is well known, it is common 

 to find the quartz crystals and crystal frag- 

 ments with sharp clean edges, while they are 

 usually more or less rounded in soils from 

 humid areas. But it is astonishing to find how 

 often even in humid areas the quartz does show 

 clean sharp edges and smooth surfaces, with 

 well-marked terminal planes, apparently due 

 to redeposition from solutions. These facts are 

 important because they tend to negative some- 

 times expressed opinions, that the silica as it is 

 formed from the minerals either as a colloid 

 or in some other form, coats the minerals and 

 prevents further solvent action on them. Evi- 

 dence is w r anting for the existence of such silica 

 coatings on the minerals of the soil." 



