384 proceedings: geological society 



layers or channels, replacing the sand and clay of the rock; 5, this con- 

 centration was favored by periods of long denudation and deep weath- 

 .ering and was therefore most extensive on old peneplain surfaces; 6, 

 the purity of the ore is controlled largely by the purity of the rock re- 

 placed; 7, some of the richer minerals were apparently derived from 

 other minerals by dehydration after deposition, and some psilomelane 

 appears to have been deposited with quartz while it was in a gelatinous 

 state; 8, only one of the deposits, which passes downward into car- 

 bonate ore, was a replacement of the bedrock below the zone of sur- 

 face weathering. 



H. D. Miser: Manganese deposits of the Batesville district, Arkansas. 

 — The manganese ores consist of oxides and generally occur in irregular 

 shaped masses from less than one pound to 22 tons in weight, with 

 rough surfaces. Most of the masses are in clay; the others are in Ume- 

 stone, shale, chert, and sandstone, and there is much evidence that the 

 manganese oxides of which the masses are composed have replaced all 

 of these inclosing materials. The oxides have been derived from man- 

 ganese-bearing carbonates near the surface and do not extend below 

 the permanent water level of the district. The workable deposits 

 occur in the nearly horizontal Femvale limestone and Cason shale of 

 Ordovician age and in residual clays which were mainly derived from 

 these two formations. Most of the masses in the clays are residual, 

 having been freed from the above-named formations by their decomposi- 

 tion; the others have been formed by the replacement of the clays by 

 manganese oxides. 



The Cason shale was laid down in shallow marine waters on an old 

 land surface and was probably the source of all of the manganese. 

 The manganese was apparently deposited as a carbonate in the shale, 

 and since then there have been two principal periods of reconcentra- 

 tion, the first taking place during one or more of several stages of emerg- 

 ence and erosion that occurred between the Ordovician and Carboniferous 

 periods, and the other about the time of the completion of the Upper 

 Cretaceous or early Tertiary peneplain whose approximate elevation 

 is marked by many of the even-crested hills of the district. During 

 both periods the manganese was concentrated in the Cason shale 

 and much of it was carried in solution from the shale down into the 

 Femvale limestone. Synclines were present during the earlier period, 

 as well as the last one, and the)^ probably offered favorable conditions 

 for the concentration of the manganese ores, as is shown by the fact 

 that most of the ores occur in synchnes. 



E. L. Jones, Jr.: Manganese deposits of the Colorado River 

 Desert Region. — Deposits of manganese ore have been recently ex- 

 ploited in the desert region adjacent to Colorado River, that extends 

 from the Big Bend near Las Vegas, Nevada, to Yuma, Arizona. 



The deposits are fillings in veins and brecciated zones and replace- 

 ment deposits. The veins and brecciated zones cut all the rocks of 



