adams] ZINC AND LEAD DEPOSITS OF NORTHERN ARKANSAS. 189 
workings has made it difficult to produce clean concentrates. With 
the continuance of deeper workings and the following of the ore bodies 
into the hillsides the sulphides are found to predominate, and this 
difficulty largely disappears. While it is impossible to predict with 
certainty the future of the field, there are mines now opened which 
are capable of large output, and many of the prospects are promising 
and well warrant fuller exploitation. With the completion of the 
railroads the northern Arkansas district promises to assume its true 
commercial importance. 
GEOLOGY. 
The ores are found in two formations, the lower being the Ordo- 
vieian dolomites, and the upper the Mississippian limestones. The 
Ordovician rocks occur extensively in Baxter County and in the 
northeastern part of Marion county, and in the other portions of 
the field, along the valleys of the streams, where erosion has cut down 
to them. The Mississippian limestones lie to the south and south- 
west, forming an irregularly fringed and dissected belt, lying some- 
what higher and extending to the base of the Boston escarpment. 
The Mississippian limestones formerly extended farther to the north 
and northeast and overlaid the Ordovician, but they have been 
removed by the wearing away of the land surface through the action 
of atmospheric agencies. In addition to these formations, which are 
the principal ore-bearing rocks, there are some thin formations found 
between the two in certain parts of the field, but for the immediate 
discussion of the problems connected with the ore deposits they need 
not be described. The Saccharoidal sandstone, however, which lies 
above the Ordovician dolomites, and accordingly separates the lower 
from the upper ore-bearing rocks, should be mentioned, since it is a 
convenient datum in this field. It is known locally as the "sand 
ledge," and is referred to in determining the horizons of the mines 
and prospects which occur below it in the Ordovician. To the south, 
lying upon the Mississippian limestones, and accordingly higher in the 
geologic column, are the shales and sandstones, which are extensively 
developed in the Boston Mountains. 
STRUCTURE. 
An examination of the mines of the district shows that the ore 
bodies are related to two classes of structure, viz, simple fractures 
and breccias. The rocks, considered broadly, are found to be nearly 
horizontal. Locally, however, they are undulating, and occasionally 
have well-marked dips. There are well-defined normal faults, which 
are later than the fracturing and brecciation above mentioned, but, 
except in certain instances where fault breccias have been developed, 
there is only a minor amount of mineralization along the fault planes 
or in the material filling the normal fault fissures. 
