12 THE CLAYS OP ARKANSAS. 
The structure is Appalachian, and the massive sandstone and novac- 
ulite ridges, primarily arched by folding and compression, have 
become accentuated by the more rapid weathering of the upturned 
edges of interstratified shales until great contrasts of relief have 
resulted. The anticlinal ridges of the Ouachitas trend nearly east 
and west, are roughly parallel, and rise 500 to 1,000 feet above the 
intervening valleys, or to altitudes of 1,600 to 2,100 feet. 
Bordering the plateau region on the southeast and extending east- 
ward beyond the borders of the State across the Mississippi Valley 
and southward to the Gulf of Mexico is a lower belt, underlain by 
younger, softer strata of marls, chalks, clays, sands, and gravels. 
Low altitude and slight topographic relief characterize this portion 
of the Gulf Plains. Crowleys Ridge, in the northeastern part of the 
State, is a remnant of a once extensive plain that stood at a level 
slightly higher than the present ridge. The slope of the whole is 
gulfward, and the plain is trenched by many broad, shallow valleys 
bordered by terraces with steep escarpments. 
The older rocks are of Paleozoic age, ranging from early Ordo- 
vician sediments, through the Silurian and Devonian, to those of the 
later Pennsylvanian epoch of the Carboniferous period. The newer 
deposits represent interrupted sedimentation, beginning with the 
early Cretaceous and including Tertiary and Quaternary deposits. 
Besides the sedimentary deposits certain small areas of intrusive 
rocks occur at Fourche Mountain, south of Little Rock, and at Mag- 
net Cove, near Hot Springs. These rocks have been studied and de- 
scribed by the late J. Francis Williams, a and the areas near Little 
Rock which contain bauxite have been described by the writer, 6 but 
were afterward remapped by C. W. Hayes. c These rocks consist 
mainly of elseolite syenite, popularly termed granite, and were in- 
truded into the sedimentary rocks probably late in Cretaceous time. 
ORDOVICIAN SYSTEM. 
The Ordovician beds are the oldest sedimentary rocks certainly 
recognized in Arkansas. They consist of a thick series of magnesian 
limestones, sandstones, siliceous limestones, and cherts, and cover all 
or portions of the following counties in northern Arkansas: Ran- 
dolph, Lawrence (western half), Independence (northern part), 
Sharp, Fulton, Izard, Stone (northeastern part), Baxter, Marion, 
Boone, Searcy (northeastern part), Newton (northern part), Carroll 
(northwestern part), and small portions of Benton and Washington. 
a Williams, J. Francis, The igneous rocks of Arkansas: Ann. Kept. Geol. Survey Arkansas for 1890, 
vol. 2, 1891. 
*>Branner, J. C, The bauxite deposits of Arkansas: Jour. Geology, vol. 5, 1897, pp. 263-289. 
c Hayes, C. W., The Arkansas bauxite deposits: Twenty-second Ann. Kept. U. S. Geol. Survey, 
pt. 3, 1901, pp. 440, 440, and 454. 
