194 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1002. [bull.213. 
stones and the occurrence of the secondary cherts and spar in the 
dolomites are entirely in accordance with this theory. 
Processes of deposition of oxidized ores. — The oxidized ores of 
northern Arkansas are the carbonates and silicates, which have been 
derived from the sulphide ore bodies. They are, accordingly, rela- 
tively later, and have been produced since erosion has brought the 
sulphides into the zone of weathering. The descending waters carry- 
ing carbon dioxide have transformed the blende and galena. In some 
cases redeposition has taken place immediately, and not infrequently 
oxidized ores are found as incrustations on the sulphides. In other 
cases they are found along water channels or in the open spaces and 
on the surfaces of the country rock. In the exposed faces of ore- 
bearing beds and in the upper portions of workings secondary ores 
often predominate. When mining operations are carried into the 
rocks that are under cover or have been protected from the action of 
ground waters, the carbonates and silicates decrease, and galena and 
blende are found to be the predominating ores. 
Secondary deposition of sulphides. — The sulphide ores which were 
dissolved by descending waters have not all been redeposited within 
the belt of weathering. Such portions as were retained in solution 
upon reaching the belt of cementation were redeposited as sulphides, 
the processes in this case being the same as in the primary deposition 
and the ore bodies belonging to a second generation. In the lower 
horizons of the Ordovician dolomites considerable zinc ore is found 
which occurs as bright, clean crystals associated with drusy quartz 
or in openings formed by fracture. Such deposits are usually lean, 
and thus far no workable body of ore of this nature has been 
discovered. 
Observations have not shown that there is a criterion for clearly 
distinguishing the secondary sulphide ores, which may have origi- 
nated by migration from the primary deposits, since it is not improb- 
able that the solutions at the time of the first concentration may have 
deposited most of their ore in the upper horizons, in which case the 
deeper deposits would have the characteristics above described and 
assigned to the ores of the second generation. 
Sulphide deposits associated with secondary chert. — Where chert is 
the principal gangue of the blende and galena, deposition in the 
Ordovician has taken place by the replacement of the dolomites and 
the filling of fracture spaces and cementation of breccias. Secondary 
chert when freshly exposed usually has a bluish color. It may be 
distinguished from the other country rocks by means of its hardness, 
since it can not be scratched with a knife. It frequently has a banded 
or bedded appearance, which corresponds to the bedding of the orig- 
inal dolomite, and crystals of ore usually well formed and distinct 
occur within the mass. In case some of the sulphides have been 
leached out, molds of the blende are seen, which give the chert a 
