NO. 2.] SNAILS OF THE GENUS 10— ADAMS. 41 



Fortunately for our purpose the general physical history of the Southern Appalachian 

 province has been carefully worked out by McGee, Davis, Hayes and Campbell, Hayes, Camp- 

 bell, WiUis, Keith, and others. Their methods of study, as applied to this region, are relatively 

 new, but have become classical. In spite of the large amount of work already done, much 

 remains to be accomplished on the detailed ^history of the drainage, and there is no better field 

 in eastern North America for such studies than the area south of the Ohio and east of the Missis- 

 sippi rivers. 



a. Present geographic relations. — In tracing the evolution of the present geography and 

 topography, only the major features need be borne in mind, as the details are of such late origin 

 that they have little bearing on the early history. It should be understood that the general 

 Southern Appalachian province includes the area roughly bounded on the north by the New- 

 Kanawha and Ohio River divide, and elsewhere it is botmded by the Coastal Plain of the Atlantic 

 coast and the Mississippi embayment. Within these lowlands lie plateau belts, the Piedmont 

 Plateau on the east and the Cumberland Plateau on the west; while between these plateaus are 

 included two belts, extending northeast and southwest, the Southern Appalachian mountains 

 on the east, and on the west a lowland interrupted by low narrow axial ridges, the Appalachian 

 or Great Valley, plate 2. The highest parts of the Appalachian Mountains lie in east Tennessee 

 and western North Carolina. These mountains, on account of their relation to moisture- 

 bearing winds and their altitude, have an abundance of rainf aU and are the sources of many 

 large rivers. The lo shells occupy a part of the drainage of the Great VaUey above Chattanooga, 

 and to a very limited extent the Tennessee River below Chattanooga to the Muscle Shoals in 

 northern Alabama. 



h. Processes involved. — On account of the great complexity of the detailed physical history 

 of the Southern Appalachian region, it is desirable to introduce the problem by an outline of 

 the general principles involved, and by a brief statement of their methods of working. But 

 before considering these factors by groups, let us recall that in nature these influences do not 

 act separately but influence one another, and that generally several or all are acting at any given 

 time, although there are periods when certain factors are dominant. We may think of the 

 interaction of these forces as follows : The crustal movements of the earth, which relatively elevate 

 or depress it with reference to sea level, may be considered as producing a condition of unstable 

 equilibrium or a condition of tension. This is because, when once above the sea level, the sea, 

 the atmosphere, and running water, or various combinations of these, at once begia to influence 

 the land. Runnning water tends to seek' its equilibrium, which is ultimately at sea level, and 

 tends to carry with it the products of weathering and erosion of the land. Such a state of 

 tension wiU contiaue as long as the land surface is above sea level and possesses a slope down 

 which the water can flow, or, ia other words, as long as any land remaias above sea level. A 

 land surface cut down in this manner to near sea level, a peneplain, reduces to a corresponding 

 degree this state of tension and acquires to a correspondiag degree an equilibrium or adjustment 

 of the existing forces acting upon it. There are thus given to the environment definite and 

 determinate tendencies, whose activities show an orderly successional relation and resultant 

 topographic forms and drainage features. It is therefore possible to see that the inland physical 

 habitats of animals dependent upon the topography, although of great diversity in a humid 

 region, are determined by relatively few fundamental laws. 



Any interruption of such a condition of equilibrium will cause another condition of tension 

 and initiate a new cycle of changes, which in turn will tend to restore a new equflibrium. The 

 topographic forms and all other physical features of the region are thus the by-products of the 

 interaction of certain forces and agents acting under certain conditions. 



c. Crustal movements of the earth.— In the Southern Appalachian region the crustal move- 

 m.ents of the earth have been one of the dominant factors in the history of the environment. 

 While certain upward crustal movements appear to have been of exceedingly long duration, 

 yet in general the movements have been periodic. The records of certain of such periods 

 are well preserved, particularly at the close of the Carboniferous, during which period the 



