M'"GEE NOTES ON TilE OEOLOGY OF MACON CO., MO. 3II 



ments and individual inequalities do not fall into a system of 

 horizontal lines or planes, and which is intuitively referred to an 

 independent mean surface : while the second type appears to be 

 represented by any surface of which a considerable portion natu- 

 rally falls into one or more horizontal or slightly inclined planes 

 to which the whole surface is instantly and intuitively referred, 

 the inequalities being simultaneously excluded as fortuitous and 

 non-essential. Certainly the distinction between these types of 

 surface, elusive and intangible as it appears on paper, is con- 

 stantly recognized on the ground by students of terrestrial confi- 

 guration, and, indeed, lies at the very basis of that branch of 

 geologic science which interprets in terms of geologic history the 

 significant language of topographic forms. Contrasted profile 

 sketches representing them, which are partly ideal though largely 

 actual, are exhibited in Fig. 3. 



The interfluvial plateaus of Macon county belong to the latter 

 type of plain ; sensibly, they are not simply plains — they are 

 planes. 



This strongly individualized topography is not confined to the 

 southeastern quarter of Macon county ; it extends over nearly the 

 whole of northeastern Missouri and from 50 to 100 miles into 

 Iowa ; and, indeed, essentially the same topographic type is main- 

 tained (except in the immediate vicinity of the large rivers) 

 throughout southern Illinois and southwestern Indiana, as well as 

 throughout a considerable area in southern Missouri and northern 

 Kentucky — though its southern limit has not been accurately de- 

 termined. Over the greater part of this area of probably at least 

 50,000 square miles the topographic type exemplified in the 

 tract under review, though locally masked by other types, re- 

 mains characteristic and easily recognizable ; the entire area is one 

 of level plains, or rather a single plain, dissected by deeply and 

 sharply incised waterways. This configuration is intimately asso- 

 ciated with the superficial deposits of the region ; and, although 

 the barely adolescent drainage systems have not yet fully invaded 

 the plains, the drainage, together with the topographic forms 

 developed thereby, represent on a grand scale what is elsewhere* 



* Eightli Annual Report of the U. S. Geol. Survey. (In press.) 



