1 86 Bulletin of Laboratories of Denison University [Voi. xii 



principal agent involved in the chemical decay of the rocks. 

 Accompanying such changes, the metallic bases of the silicates 

 combine with the various acids and are removed in solution as 

 salts of these acids, the insoluble parts of the minerals remain- 

 ing where formed, to make up the residual mantle. Manganese, 

 with other of the base-forming elements, is thus removed in 

 solution by the streams, and under favorable conditions of 

 oxidation finally precipitated with the sediments on the floor of 

 the water-bodies into which the streams drain. 



Definite evidence from field-study in this area, by the 

 writer, is lacking to indicate the exact form in which the man- 

 ganese was laid down in the rocks, whether as carbonate or 

 oxide or both. 



After examination of the chemical behavior of manganese, 

 Dunnington ^ calls attention to the probability of manganese- 

 sulphate having taken an important part in the formation of de- 

 posits of manganese-ores. In view of the results from this ex- 

 amination he says : ^ 



"It appears possible that many deposits of manganese in calcifer- 

 ous rocks owe their formation to the action of solutions of sulphates, 

 and possibly an illustration of such action is presented in the man- 

 ganese-deposits of Crimora, Augusta county, Virginia. . . ." 



Professor Dunnington then outlines the conditions under 

 which he conceives the Virginia deposits to have been formed. 



3. Local Accumulation of the Manganese. — If, as indicated, 

 the manganese was regularly or irregularly disseminated in a 

 finely divided state through the limestones and quartzite in 

 a greater or less quantity, then some secondary action or pro- 

 cess must explain their present local accumulation. Segregation 

 to any appreciable extent, if at all, of the finely disseminated 

 particles of manganese does not appear to have taken place in 

 the original unweathered rock. The agencies which promoted 

 the decay of the rocks inclosing the manganese particles were 



' Dunnington, F. P., "On the Formation of Deposits of Oxides of Man- 

 ganese," Amer. Journ. Set., 1888 (3 s.), vol. xxxvi., pp. 175-17S. 

 * Idtd., p. 177. 



