GEOLOGICAL PAPERS. Ill 



face. ... As compared with the Mississippi valley, the region is an elevated 

 one. To the north and west of it lie the Prairie plains; to the east and south- 

 east are the Gulf plains. . . . The structure of the Ozark region is simple. 

 The Boston mountains, on its southern border, are of a monoclinal type, while 

 the plateau portion, or northern part, has the form of a low dome, with local 

 faulting and minor undulations. To the south of the Ozark region is the Arkan- 

 sas Valley region, which has a folded structure, and beyond it the Ouachita 

 Mountain region, which is closely folded and thrust-faulted. • ... In the 

 southern portion of the Ozark region are the Boston mountains, which are a dis- 

 sected highland extending through northern Arkansas, from Batesville to near 

 Wagoner, in Indian Territory. The northern portion of the region is the Ozark 

 plateau, which consists of two divisions. The eastern one is the Salem upland 

 and the western the Springfield upland. They are separated by the Burlington 

 escarpment. (Adams, Physiography of the Ozark Region, 22d An. Rep. U. S. 

 Geol. Surv., pt. II, pp. 69-75.) 



In the extreme southeast corner of the state there is a small portion 

 of the Springfield upland. Its general surface is a structural plain 

 developed on the Mississippian limestone. The plain is limited on 

 the west by the valley of Spring river, which is located approximately 

 along the contact of the Coal Measure shales. The portion in Kansas 

 is the border of a westward-dipping structural plain, and, lying adja- 

 cent to the Cherokee lowland, is considerably reduced, and has but a 

 slightly greater altitude. The country rock of the Springfield upland 

 contains much flint. Its surface is accordingly in strong contrast with 

 the lowlands to the west, which are developed on soft shales and sand- 

 stones. The limestones, which constitute the upland, contain valua- 

 ble deposits of lead and zinc. The principal town of the area is 

 Galena. 



PRAIRIE PLAINS REGION. 



The Prairie Plains region embraces that portion of the Mississippi 

 valley which lies at a moderate elevation, and in which the processes 

 of reduction have brought the surface of the country to a generally 

 level plain. Its surface features have been developed by erosion, and 

 mountainous structures are nowhere present. The changes of level 

 which the Prairie plains have undergone were relatively slight as 

 compared with the mountainous regions which have been the centers 

 of the disturbances. To the northeast of the Prairie plains are the 

 Lake plains, which are related in their development to the Great 

 Lakes. To the south of the Prairie plains are the Gulf plains, which 

 have been developed as a result of the oscillations of the Gulf of 

 Mexico. To the east and west the Prairie plains pass by easy grada- 

 tions into higher altitudes and into higher plains or plateaux, which 

 are adjacent to the Appalachian region and the Rocky Mountain re- 

 gion. It will thus be seen that the Prairie plains occupy an interme- 

 diate position in the Mississippi valley. Their physiographic history 

 is related to that of the regions on the north, east, south, and west. 



