192 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1902. [bull. 218. 
erosion the lower limit of the activity of the atmospheric agencies has 
constantly migrated downward. These belts are related to the topog- 
raphy of the country, the plane separating them being higher in the 
hills than in the valleys. Consequently, during the long period which 
has been required for the removal of tin' stratified rocks to their pres- 
ent limits, there has been a shifting downward and southward, as the 
streams have cut their valleys deeper and the escarpments have 
retreated southward. In the northern part of the field the belt of 
weathering which was formerly in the Mississippian limestones has, 
since the removal of these beds, reached the Ordovician rocks. To 
the south, as a result of the rugged topography, it lies partly in the 
Ordovician and partly in the Mississippian. At the base of the Boston 
Mountains, where the the shales a in 1 sand si ones have been but recently 
removed, it has descended but a short distance into the upper port ion 
of the Mississippian Limestones. The rocks in the northern portion of 
the zinc and lead district of northern Arkansas may accordingly be con- 
sidered as exhibiting the more advanced stages of the process of 
weathering and erosion. 
ORE DEPOSITS. 
Source of the <>n.s. — It is generally accepted that- the zinc and Lead 
deposits of this region have been accumulated by the action of circu- 
lating waters which have dissolved the ores which were first broadly 
disseminated in the limestones of the region. Water has dissolved 
and carried them in solution to certain places where the conditions 
were favorable for their redeposil ion. Stating it differently, they 
have been derived from the belt of weathering and the belt of cemen- 
tation, and largely deposited in the belt of cementation. A study of 
the nature of the ores and their gangue materials and of the geologic 
history of the region makes it apparent that at least the latest concen- 
tration of the ores in the Ordovician has been largely the result of 
downward and lateral movements. The metallic sulphides may have 
been mainly derived from the Mississippian Limestones, which for- 
merly had a wider distribution, from the Ordovician, or from both 
formations. An examination of the Mississippian rocks shows that 
they have been leached by surface waters. Where they are exposed 
in railway cuts they exhibit decay to considerable depths, and within 
the area of their outcrop there are numerous sink holes in which the 
water disappears into underground channels. The surface cherts 
which have been derived from the weatliering of these rocks are fre- 
quently porous and spongy, thus indicating the loss of silica. In the 
Ordovician secondary silica is not infrequently a gangue of the ores. 
The Mississippian limestones contain notable deposits of zinc and 
lead at many localities in the Ozark region, and where there is ore in 
the Ordovician the Mississippian limestones have formerly overlain 
the area. The mines of southwestern Missouri around Joplin are in 
