adams] ZINC AND LEAD DEPOSITS OF NORTHERN ARKANSAS. 191 
The shales lying above the Mississippian limestones, on the contrary, 
have sufficient thickness to make them an important factor in deter- 
mining the movement of ground water. Formerly they extended from 
their present boundary, near the base of the Boston Mountains, north- 
ward into Missouri, and covered a considerable part of the Ozark 
Plateau. Before they were removed they acted as a confining or limit- 
ing horizon. Water entering the Ordovician dolomites and Mississip- 
pian limestones where they outcropped, and moving southward in the 
direction of the dip, was under a hydrostatic pressure beneath these 
shales. There may have been a first concentration of the ores, due to 
this circulation, and, if so, it could have taken place at some point 
between the upper portion of the Mississippian limestones which were 
below these shales and the bottom of the Ordovician. This reasoning- 
may be appealed to in accounting for the ore bodies now found in the 
Mississippian limestones near the base of the Boston Mountains, where 
these rocks have recently been uncovered by erosion. It is possible, 
however, that the concentration could have occurred through the 
agency of lateral circulation adjacent to the fractures in which the ore 
bodies are found, without appealing to causes which are of such wide 
influence. 
As erosion progressed the shales and other formations tying above 
the Mississippian limestones were speedily removed from the more 
central portion of the Ozark region, so that the conditions which at 
first existed, as above outlined, were not long maintained. The main 
streams of the region, such as White River and its tributaries, soon 
cut through these formations, so that water which may have formerly 
been under hydrostatic pressure found issuance in their valleys. At 
the present time there are no upper confining shales in the northern 
part of the field, and this condition has prevailed for a long period. 
The surface water has been free to descend into the Mississippian 
limestones and Ordovician rocks, or through the Mississippian lime- 
stones into the Ordovician, and the point of issuance of such portions 
as have reappeared in the surface flow has been in the valleys of the 
larger streams. There, no doubt, has been lateral movement along 
bedding planes and through the more permeable strata and open and 
brecciated beds and along the surfaces of local shale beds. The Sac- 
charoidal sandstone, which is a conspicuous formation and one which 
is relatively porous, has probably been a horizon of lateral movement, 
assisting in the transfer of the ground water to places where it could 
find its way into the adjacent beds. 
Relations of belt of weathering and belt of cementation. — Under the 
action of atmospheric agencies the rocks at and near the surface suf- 
fer loss of their materials and waste away. . This process may be 
described as weathering. Deeper in the earth the materials derived 
from the upper rocks are largely redeposited. This process is one of 
cementation. The belt of weathering and the belt of cementation are 
not separated by a sharp line, and, moreover, with the processes of 
