smith. 1 LEAD AND ZINC OF JOPLIN DISTRICT. 199 
Structure. — The rocks of the district have a low general dip to the 
northwest at an angle somewhat greater than the general inclination 
of the surface. Open folding' is common, both on a large and small 
scale. Noticeable faulting is not common over the quadrangle as 'a 
whole, though small slips occur here and there. In those parts of the 
district, however, where ore deposits occur, faulting and folding are 
both of more importance, but even here the amount of faulting, as 
a role, is not great. Both normal and reversed faults are found, 
both being the result of readjustment due to compression stresses. 
Where cherts predominate, brecciation has developed as a result of 
the faulting and folding. The individual faults are not known to 
continue for any considerable distance, but they occur (as do the 
folds also) mainty in zones. The ore deposits follow these zones of 
faulting and folding, and frequently come in and die out with the 
faults with which they are associated. The occur rence of cross folding 
and faulting, at an angle with the main lines, tends to make the zones 
of brecciated rocks more complex. As a result the deposits are, on 
the whole, extremely irregular. In at least one part of the district 
there is well-defined evidence of two periods of movement accom- 
panied by fracturing and faulting -of the rocks, both periods having 
been followed by ore deposition. 
In connection witli the phenomena of folding and faulting, shear 
zones and joints of compression and tension have been extensively 
developed, mainly in the cherts. Adjustments of small amount, also, 
have taken place along the bedding planes. The effects of these dif- 
ferent movements vary with the rocks. The shales have yielded to 
stresses by folding or by faulting; the limestones by folding and fault- 
ing with occasional complex fracturing. Stresses in the cherts, which 
are generally extremely brittle, are nearly always relieved by fractur- 
ing, more or less complex, and by brecciation, though occasionally by 
flexure. Many of the cherts are so brittle that only a slight amount 
of movement is necessary to cause complete brecciation, simple fold- 
ing being frequently sufficient. The brecciated cherts are often more 
or less firmly recemented, sometimes by a black secondary chert fre- 
quently containing disseminated sphalerite; sometimes by free sphal- 
erite or calcite, or by both. In many cases the cherts are thoroughly 
crushed, while still showing their original bedding planes. Examples 
have been noted of heavy-bedded cherts which have been cut by joint 
planes in several directions, the loosening of the mass by slight fault- 
ing or other cause giving rise to "bowldery ground" such as is char- 
acteristic of many of the mines. It is probable also that, during fold- 
ing, arching of the cherts occurred in some places, and that later, in 
many instances after the spaces had been filled with the material 
which now appears as the black secondary chert, the arches were 
broken down. It is mainly in the brecciated cherts that the ore 
bodies occur. 
