Ch. 30] DISPERSION PATTERNS 539 



subject to date. Thyssen (1942) has published a useful summary of 

 the basic principles to be considered in vegetation sampling as a pros- 

 pecting method. 



DISPERSION PATTERNS 



Sampling campaigns in geochemical prospecting work should be 

 planned with due regard for the geometry of the dispersion pattern. 

 The characteristic dispersion pattern of an element under given con- 

 ditions depends on both the mechanics of the dispersion process and 

 the chemical equilibrium of that element with its environment. Many 

 of these factors are only very poorly understood, and much more 

 research will be necessary before it is possible to evaluate them 

 properly. 



Dispersion patterns in the zone of weathering may be conveniently 

 grouped according to their characteristic form, following a modified 

 form of the system of classification set up by Sergeev (1941) : 



Residual and vestigial weathering products. A variable proportion 

 of the weathering products of an ore deposit may remain in the gossan 

 or residual soil directly over or very near their source in the bed- 

 rock. "Residual" elements remain behind in relatively large propor- 

 tions compared with their concentration in the parent material, whereas 

 "vestigial" elements are present only in very minor though still de- 

 tectable quantities, the remainder having been removed in solution. 



Dispersion halos. The diagnostic ore elements have migrated later- 

 ally from the ore body to form a more or less symmetrical distribution 

 pattern or "halo" (Russian "oreol") in soil or vegetation. 



Dispersion fans. Glacial action scatters the erosion products of an 

 ore deposit in the form of a fan opening out in the direction of ice 

 movement and converging toward the source. It is probable that 

 soluble material moving in the ground water also assumes a similar 

 form, where the fan opens out in a direction down the slope of the 

 water table. 



Dispersion trains. Where the weathering products of an ore deposit 

 enter the surface drainage as dissolved salts or water-borne sediment, 

 their distribution takes the form of a "train" (Russian "potok") con- 

 trolled by the linear drainage pattern of the stream. 



The three principal agents of dispersion are air or other gas, glacial 

 ice, and water. Gaseous dispersion is important in geochemical pros- 

 pecting for petroleum, but at the present time it is of little more than 

 academic interest in ore prospecting. Glacial and aqueous dispersion, 



