Carroll and eastern Eenton counties. The average height of the Salem Plateau in Arkansas is about 1,250 feet above sea level. It is usually made up of less precipitous and less rough topography than that of either the Springfield Plateau or Boston Mountains.

The Arkansas Valley lying south of the Boston Mountains and north of the Ouachita Mountain region is, as a whole, fairly level. It contains, however, some isolated ridges and a few mountains which arise abruptly from the valley floor, such, for example, as Mount Nebo, Petit Jean Mountain and Magazine Mountain. Magazine Mountain is the highest in Arkansas, rising to 2,823 feet in height. Rich Mountain, in LeFlore County, Oklahoma, is, according to the U. S. Geological Survey, slightly higher, having an elevation of between 2,850 and 2,900 feet above sea level. The average elevation of the floor of the Arkansas Valley above sea level is from 300 to 600 feet.

The Ouachita Mountain region consists of a series of ridges, mainly east-west, which diminish in height from west to east. In the vicinity of Athens, Arkansas, the elevation of the highest siliceous ridges composing central portions of this system is 2,340 feet, while at their eastern intersection with the Coastal Plain their elevation is about 1,000 feet above sea level. The average slope of this system to the east is about fifteen feet to the mile. In the northern and southwestern portions of the Ouachita Mountain system, the ridges usually have an east-west direction, while in the southeastern portion near Hot Springs they are very much buckled and distorted, many of the ridges having a northeast-southwest direction.

The Athens Plateau lies between the Ouachita Mountains and the Coastal Plain and is a considerably dissected country which has been eroded by streams most of which flow south. The higher portions of this plateau have an elevation of from 400 to 1,100 feet above sea level.

TOPOGRAPHICAL MAPPING

The progress of the topographic survey in Arkansas by the U. S. Geological Survey is shown on the accompanying index map. Each of the rectangles outlined and numbered shows the area covered by a quadrangle which has been surveyed. The name of the resulting topographic map is indicated by the numbered list of sheets appearing below.

Each of the maps represented by the smaller quadrangles covers an area measuring 15 minutes of latitude and longitude (1/16 of a square degree), or from 246 to 250 square miles in area according to latitude. The scale on the smaller quadrangles is 1:62,500, or about one mile to one inch and the contour interval is 20 feet, with the exception of the Memphis sheet, which has a contour interval of five feet in Arkansas and ten feet in Tennessee. Each of the maps represented by the larger rectangles shows a quadrangle measuring

30 minutes of latitude and longitude (1/4 of a square degree), or from 963 to 992 square miles in area. The scale is 1:125,000, or about two miles to one inch, and the contour interval is 50 feet. There are twenty-eight published topographic maps covering quadrangles in Arkansas. The area covered by these surveys is 22,023 square miles, or 41.7 per cent of the area of the State.

These maps are obtainable from The Director, U. S. Geological Survey, Washington, D. C., for 10 cents each in orders amounting to less than $3.00. For orders of $3.00 or above the price is 6 cents. LIST OF TOPOGRAPHIC MAPS

GEOLOGY

The surface of Arkansas is almost entirely made up of sedimentary rocks; that is, rocks which were formed from sediments laid down ages ago in salt or fresh water seas and lakes. These sediments are found today more or less consolidated in the form of sandstones, shales, clays, chalks, limestones and do,lomites. There are also in the State about fourteen square miles of intrusive volcanic rocks which are scattered in relatively small patches. The blue or gray "granites" of Pulaski, Garland, and Saline Counties make up the principal body of these volcanic rocks. Other rocks of a vol

canic nature are found at Magnet Cove near Hot Springs, and in small patches in southern Pike, northeastern Scott, southern Logan and southern Conway Counties.

The sedimentary rocks of the Ozark, Ouachita, Arkansas Valley and Athens Plateau region were laid down during the Paleozoic era as the fossil remains in the rocks of this area were occupied by living organisms during the Paleozoic era, or what is known as the era of ancient life. The formations in the Coastal Plain area of the State were laid down during the Mesozoic and Cenozoic eras or the eras of middle and recent life. The above map shows the distribution of the Paleozoic, Mesozoic, and Cenozoic rocks.

The different rock formations of Arkansas, together with their vertical position and thickness in the Ozark region, the Arkansas Valley, the Ouachita Mountains and the Coastal Plain of Arkansas are shown on the accompanying chart.

The rocks of the Ozark region of Arkansas lie on the southern flank of the Ozark Uplift as it passes into the Arkansas Valley. The rocks have a gentle southward dip and, as a rule, the local folding and faulting is not great. The surface rocks of the Ozark region consist of dolomites, limestones, sandstones, and shales with the

dolomites and limestones predominating. The youngest rocks of this series are the Carboniferous shales and sandstones which cap the Boston Mountains. These grade into older rocks of Devonian, Silurian, and Ordovician age over the Springfield and Salem Plateaus. These formations have an aggregate maximum thickness estimated at 3,200 feet.

Practically all of the Salem Plateau area lying north of the White River in Arkansas is made up of dolomite of Ordovician age and has an estimated thickness along the northern border of Arkansas of between 2,300 and 2,600 feet.

The Arkansas Valley lies between the Ozark Uplift on the north. and the Ouachita Uplift on the south, and as such constitutes a