190 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1902. [bull. 213. 
The fracturing of the Ordovician rocks was produced by compress- 
ive forces, and in certain zones has a considerable vertical extent. A 
second and equally important result was brecciation, which was pro- 
duced by the differential movement of the strata. The variation in 
the structure of the dolomite series, which is in places massively 
bedded and in other places thin bedded, laminated, and even shaly, 
resulted in the lateral movement being taken up in varying degree 
by the individual beds, so that the motion was such as is produced 
by forces acting in couples. The brecciation is due to the tendency 
of the pieces resulting from the breaking of certain brittle strata to 
shear past each other, or to rotate with the horizontal movements of 
the adjacent beds, so that the fragments are relatively displaced. 
In the Mississippian limestone the compression produced princi- 
pally fracturing and Assuring. The walls of the fissures not infre- 
quently exhibit slickensiding, which has been produced by the rocks 
moving past; each other horizontally. The Mississippian limestones 
do not exhibit brecciation, excepting in fracture zones or where they 
have been crushed by the dragging of the beds along normal faults, 
which in most cases are due to a later adjustment of the rocks of the 
area. 
The fracturing and brecciation above mentioned are probably due 
to stresses induced at the time of the folding in the Ouachita Moun- 
tain and Arkansas Valley regions. Al the close of the Carboniferous 
period the thick sediments which had accumulated in what is nowcen- 
tral Arkansas and western Indian Territory were folded in a manner 
which, suggests that they were thrust to the north. In the Ouachita 
Mountains there are close folding and thrust faulting; in the Arkan- 
sas Valley region open folds. Tn the southern border of the Ozark 
region, and particularly in the area here under discussion, the gen- 
erally horizontal position of the rocks was retained, but there was 
considerable movement of individual beds. This movement was one 
of accommodation, and resulted in fracturing without marked dis- 
placement. It took place largely along the bedding planes and 
resulted in brecciation of the beds. The normal faulting in this area 
is of later date, and is probably due to the readjustment following 
the crushing, or to subsequent oscillations of level. 
Geologic conditions influencing circulation of ground water. — The 
rocks which constitute the Ordovician system and the Mississippian 
limestones of the northern Arkansas district may be considered as 
relatively quite permeable. There are local beds of shale in the 
Ordovician through which water would not readily pass, and the 
Devonian, which has a very limited extent in the southern part of 
the field, is of about the same character and importance in controlling 
the path of the ground water. The shales do not have a wide influence, 
since they do not form persistent horizons. Where they occur they 
probably diverted the solutions laterally, but no localization of ore 
deposits seems to be directly due to them. 
