18 We PASENNY 
ARKANSAS! 
Arkansas has been divided into four main geological sections: (r) 
the Gulf Coastal Plain, south and east of = line passing through 
DeQueen, Arkadelphia, Little Rock, and Pocahontas; (2) the Arkan- 
sas Valley, between a line from Van Buren to Heber Springs on the 
north and a line from Waldron to Little Rock on the south; (3) the 
Ouachita Mountain region, between the Arkansas Valley and the 
Gulf Coastal Plain; (4) and the Ozark region, north of the Arkansas 
Valley. 
Gulf Coastal Plain —The southern part of the Gulf Coastal Plain 
from DeQueen to Lake Village is indicated as a large negative anom- 
aly, interrupted by a north-south trend of magnetic “‘highs’” from 
Hamburg to Star City, which seems to be the northward continuation 
of the magnetic high trend from Winnsboro to Bastrop in Louisiana. 
The well known local anomaly at Rison is due to peridotite in- 
trusions with as much as 20 per cent magnetite content. 
The positive vectors from Stuttgart to Forest City seem to be- 
long to a large northwest-southeast magnetic high trend, toward 
which are also directed the negative vectors at Wynne and Helena. 
The large irregular positive and negative vectors farther north sug- 
gest very localized and relatively shallow sources of magnetism. 
Ouachita Mountain region.—In the Ouachita Mountain region it 
is interesting to note that the two positive vectors at Mena and 
Mount Ida are almost horizontal and directed toward the Arkansas 
Valley, instead of south toward the axis of the Ouachita Mountains, 
as would be expected if the Ouachita Mountains were the result of 
uplift in the Basement complex. 
If it be assumed that the Basement complex is the cause of the 
magnetic anomalies in this region, the magnetic conditions suggest 
that this complex lies deeper below the Ouachita Mountains than 
below the Arkansas Valley, because the magnetic lines of force seem 
to come out of the ground at DeQueen and Murfreesboro, to assume 
a horizontal direction over the mountain range, and to penetrate into 
the ground in the Arkansas Valley. 
Thus the magnetic conditions seem to be in favor of the theory 
that the Ouachita Mountains are the result of a huge overthrust, as 
explained by W. A. J. M. van Watershoot van der Gracht.? Similar 
1 The local magnetic vectors are based on the information given in ‘United States 
Magnetic Tables and Magnetic Charts for 1925,” op. cit. 
2 W. A. J. M. van Watershoot van der Gracht, ‘‘Permo-Carboniferous Orogeny in 
South-Central United States,” Bull. Amer. Assoc. Petrol. Geol., Vol. 15, No. 9 (Sep- 
tember, 1931), pp. 991-1057. 
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