1(5 THE CLAYS OF ARKANSAS. 
exposed. Rocks of this formation also occur on the tops of the out- 
liers immediately north of the Boston Mountains. 
The formation consists of beds of sandstone and shale, with a few 
thin local layers of limestone. The sandstone beds range in thick- 
ness from 3 feet to more than 50 feet. One of these beds, and in 
places two, near the base of the formation, are conglomeratic, con- 
taining waterworn quartz pebbles of small size. These gritty beds 
at and near the base of the Winslow formation were described by 
the Arkansas Geological Survey in the report on Washington County 
as the "Millstone grit." The shales, which constitute probably 75 
per cent of the formation, are as a rule black and carbonaceous, 
though less so than the shales of the Morrow formation. Coal occurs 
within this formation but only in beds too thin to be profitably worked. 
The Winslow formation in the Boston Mountain region extends up 
to the base of the series of rocks that contain the workable coal beds 
in the Arkansas coal field, and its total thickness is estimated to be 
more than 1,500 feet. 
South of the Boston Mountains the Winslow formation is repre- 
sented in a part of the thick series of sandstones and shales that 
underlie the coal-bearing rocks in the Arkansas coal field. This 
series of rocks was referred to in the publications of the Arkansas 
Survey as the "Lower or Barren Coal Measures." The uppermost 
formation in this series has been described in the report of the Arkan- 
sas coal field a and in publications on the Indian Territory coal field 
as the Atoka formation. This formation in Oklahoma and Arkansas 
ranges in thickness from 3,000 to probably 5,000 feet. Beneath the 
Atoka formation there are other formations of sandstone and shale 
of probable Pennsylvanian age, aggregating 12,000 to 15,000 feet in 
thickness. 6 These rocks make the rugged country of the Ouachita 
Mountain region south of the Arkansas coal field, and the Arkansas 
River Valley east of the coal field and south of the Boston Mountains. 
In the study of the fire clays and clay shales of the State the shale 
beds of the "Lower or Barren Coal Measures," as well as of similar 
beds in the productive coal-bearing rocks, are of much importance. 
In the region of the Boston Mountains and in the lower region of 
similar formation on the east the rocks of the "Lower Coal Measures" 
lie flat, or approximately so, and their outcrops and distribution are 
thus very simple and are easily traced. In the region south of this 
area of horizontal beds, however, the same rocks are thrown into 
folds that become more and more abrupt toward the south. The 
upper portions of these folds are worn away by erosion, and the sand- 
stones are left as ridges while the soft beds of shale are degraded. 
a Bull. U. S. Geol. Survey No. 326, 1907. 
bBranner, J. C, Thickness of the Paleozoic sediments in Arkansas: Am. Jour. Sci., 4th ser., vol. 2, 
1896, pp. 229-236. 
