adams.3 ZINC AND LEAD DEPOSITS OF NORTHERN ARKANSAS. 193 
1 he Mississippian limestones, and in northern Arkansas, as has already 
been stated, prospecting has shown that in the portion of the district 
where they have been but recently exposed to the action of surface 
waters, probably as a result of a first concentration, they carry consid- 
erable lead and zinc. 
Classification of ore deposits. — The most important deposits of the 
district are the sulphide ores of lead and zinc, or, as they are com- 
monly called, galena and blende. In the Ordovician dolomites there 
are two principal classes of these deposits, which are characterized by 
the gangue material. One class is distinguished by the presence of 
secondary chert, which occurs as a siliceous replacement of the dolo- 
mites, or filling fractures in these rocks; in the other there is asso- 
ciated with the ore a large amount of dolomite spar, which forms a 
cementing material in the breccias. In certain of the mines there is, 
in addition to these main ore bodies, accessory ore which replaces the 
country rock to some extent adjacent to the main ore body without 
the development of secondary chert or spar. 
In the Mississippian limestones the primary ore deposits are accom- 
panied by secondary chert and calcite. They are related to fractures, 
and in some instances to fault planes. In the latter case they usually 
occupy breccias. Accessory ore replacing the country rock is some- 
times present with these deposits. 
The northern Arkansas field contains important deposits of oxi- 
dized ores. These are the carbonates and silicates. They are derived 
from the primary sulphides, and are due to the alteration of the sul- 
phides by the action of surface waters. In discussing the genesis of 
the ores the important problem is the origin of the sulphide deposits, 
the relation of the oxidized deposits to the sulphide deposits being 
evident. 
Processes of primary deposition of sulphide ores. — The action of 
ground waters in the belt of weathering, and to a considerable extent 
in the belt of cementation, resulted in the solution and transportation 
of the ores. As the water percolated downward and moved laterally, 
and perhaps later upward, it reached a place where deposition took 
place. In the early part of the journey of the waters, through the 
action of the carbon dioxide and the humic acids, silica was taken 
into solution, and the waters accordingly contained it in notable 
quantities, along with the ores in solution. In the later part of the 
journey these waters caused the solution of lime and magnesium 
carbonate and the deposition of silica and sulphides, the resulting 
ores supposedly having been transported as sulphates. The reduction 
of the metals to sulphides was probably accomplished through the 
agency of organic matter and pyrite in the rocks, directly or indi- 
rectly, and deposition of the sulphides occurred along with the 
formation of the secondary chert and spar. 
The superposition of the original cherts in the Mississippian lime- 
Bull. 213—03 13 
