306 THE JOURNAL OF GEOLOGY. 



Many other similar cases of superficial enrichment in iron 

 deposits might be mentioned, but the above are enough to illus- 

 trate the point in question, and it will be seen that, of the regions 

 which are the active producers of iron ore in this country, almost 

 all, if not all, owe the existence, or at least the availability of 

 their large bodies of ore, to superficial concentration. 



Alteration in manganese deposits. — Manganese deposits are 

 affected by superficial influences in much the same way as iron 

 deposits. Many of the manganese deposits in the Cambrian and 

 Lower Silurian rocks of the Appalachian Valley were concentrated 

 in a manner somewhat similar, though not always so, to the iron 

 deposits in the same regions.^ 



In the Batesville manganese region of Arkansas, the ore 

 originally occurred in irregular masses in Silurian limestone, but 

 surface decay has leached the carbonate of lime out of the lime- 

 stone, leaving a red siliceous clay, which represents the less solu- 

 ble part of the original rock. This clay now lies in hollows on 

 the surface of the limestone and contains the masses of ore once 

 disseminated through that rock. The removal of the carbonate 

 of lime has concentrated the ore masses in the clay, and has also 

 rendered them more easily mined ; in fact, the only manganese 

 ore that can now be profitably mined in this region is that in the 

 residual clay.^ 



The frequent occurrence of deposits of bog manganese ore 

 in the areas of crystalline rocks, generally represents a concen- 

 tration of manganese resulting from the oxidation of dissemin- 

 ated carbonate and silicate of manganese in the country rock. 

 This oxidation product is taken into solution in surface waters, 

 and transported until subjected to such conditions that it is oxi- 

 dized and precipitated as a hydrous oxide. 



Alteratio?i in copper deposits. — In many copper deposits super- 

 ficial alteration has produced very remarkable chemical and 

 economic results, and this is especially well seen in the copper 



'R. A. F. Penrose, Jr., Journal of Geology, No. 4, Vol. I., 1893, pp. 356-370. 



^R. A. F. Penrose, Jr., Manganese : Its Uses, Ores, and Deposits; Arkansas Geol- 

 ogical Survey, 1890, Vol. I., pp. 166-209. 



