abstracts: geology 587 



GEOLOGY. — Manganese deposits of the Caddo Gap and De Queen 

 quadrangles, Arkansas. Hugh D. Miser. U. S. Geological 

 Survey Bulletin 660-C. Pp. 64. 1917. 



The manganese deposits of the Caddo Gap and De Queen quad- 

 rangles in west-central Arkansas comprise those that are at present 

 being most extensively exploited. They have been worked on a 

 small scale only and have yielded but a few hundred tons of marketed 

 ore. They occur in the Arkansas novaculite of Devonian age and are 

 mainly confined to two stratigraphic horizons, one being near the top 

 and the other near the middle of the formation. The novaculite, to- 

 gether with the other formations, has been closely folded and its out- 

 cropping edges stand up as parallel, nearly eastward-trending ridges 

 on which rock ledges or their debris abound. The manganese ores 

 consist of oxides, — psilomelane, pyrolusite, and manganite being most 

 abundant. Although these minerals may be found separately, as a 

 rule two or more are intimately mixed in the same deposit, and in 

 some places they are associated with iron oxides and manganiferous 

 iron ores. The manganese ores occur as nodules, pockets, and short 

 irregular veins from a fraction of an inch to 4 feet thick. Thicknesses 

 of 4 feet, however, are rare, and those of a foot are not common. The 

 ores occupy bedding planes or joint cracks or form a cement in nova- 

 culite breccia. 



Most of the manganese was probably deposited originally as finely 

 disseminated particles with the silica that formed the novaculite, a 

 rock that is regarded as belonging to the class of charts; the rest of 

 the manganese may have been deposited in disseminated form in the 

 overlying and underlying shales. Its concentration has been effected 

 by ground water, which has carried it down from the vast mass of 

 rock that has been eroded from the region; and its localized concen- 

 tration into deposits large enough to be of value appears to have been 

 dependent upon the amount of open space in the novaculite, which is 

 hard and compact and usually has no visible openings between the 

 layers or along the joints. Faults and the ends of plunging anticlines, 

 as is attested by a number of deposits occurring in such places, were 

 favorable places for the accumulation of ore. 



Most of the ores contain too much phosphorus for the manufacture 

 of ferromanganese. They usually contain too much iron for use in 

 chemical industries and electric batteries, and, where the quality is 

 suitable for these purposes, the quantity is generally too small for 

 profitable mining. The quantity of ore that can be mined at a profit 

 from any one deposit is small. H. D. M. 



