100 G. D. Hubbard — Colloids in Geologic Problems. 



in caves, as at Mount Tabor or Beanie's Gave, Ohio, many 

 of which are precipitated by organic agencies, are truly 

 colloids which may in the course of time be dehydrated 

 and pass over into the crystalline goethite, or even into 

 hematite. 



Some bauxite was formed by passing through a col- 

 loidal state. Colloidal alumina is soluble in water and 

 has been carried some distance before coagulation has 

 taken place. The coagulation is a result of the meeting 

 with electrolytes in solution, whose charge is different 

 from that of the colloid, and therefore the alumina loses 

 its negative charge ; the solution first becomes turbid, 

 then the hydroxide actually flocculates and collects into 

 larger and larger lumps, and finally falls to the bottom. 

 Apparently the continued growth of these lumps by the 

 successive addition of layers of aluminum hydroxide 

 builds up the oolitic and pisolitic bodies of typical bauxite. 

 Some bauxite is certainly not of this origin. 



These gel ores of iron and aluminum start by decom- 

 position of silicate rocks in the weather zone. Descend- 

 ing waters carry them down into other rocks, or surface 

 waters carry them away to marshes and bogs where the 

 the precipitating conditions are found, and the ores are 

 dropped. In the cooler, higher latitudes the iron ores 

 are formed, while the alumina and silica remain 

 together 10 ; and in the warmer or subtropical climates 

 the laterites or bodies of bauxite are formed. Thus the 

 ore deposit may witness to the character of the climate in 

 force when the deposit was made. The precipitation in 

 other rocks would probably take place above the water 

 table; hence in high and dry regions there would be a 

 long vertical range for deposition, which could take place 

 anywhere between the surface and the water table. 



It has been pointed out by Krusch 11 that the manganese 

 oxides may also be laid down as gel ores, and subse- 

 quently become crystalline. He makes a point also of the 

 fact that this colloidal ground mass of manganese oxides 

 and hydroxides is capable of adsorbing other substances 

 which may permeate it, e.g., barium and potassium com- 

 pounds. In this way, he accounts for psilomelane, a 

 manganese mineral containing barium and potassium. 



10 Schlosingj T., Compt. Bed., 132, p. 401, 1901. 



11 Krusch, P., Mining and Scientific Press, 170, 418, 1913. 



