3IO TRANS. ST. LOUIS ACAD. SCIENCE. 



principal drainage (or indeed in any direction) would exhibit, as 

 its typical feature, horizontal surface-lines suddenly and sharply 

 giving place to slopes as steep as tenaceous clay long exposed to 

 the action of the elements will maintain ; and, moreover, such a 

 profile would shov/ that all the uplands have essentially the same 

 altitude, the slight southward inclination of the whole area and 

 the still less inclination east and west from the "grand divide" 

 being inconspicuous. This is imperfectly illustrated in the profile 

 of the accompanying generalized section, Fig. 6 (p. 331). It is 

 indeed manifest that the entire tract represents a single wide- 

 stretching plain in which the efiects of erosion are limited to the 

 excavation of the narrow ravines and broader valleys, leaving 

 the greater portion of the surface absolutely untouched. 



It is necessary to recognize a distinction among plains, easily 

 caught by the eye and essential in considering their origin, though 

 difficult to express in words. The type of surface commonly 

 comprehended under the term is more or less diversified by ele- 

 vations and depressions of greater or less vertical measure, regu- 

 larity, and symmetry ; the relative value of the relief (compared 

 to that of contiguous surfaces), as well as the absolute relief, is 

 consciously or unconsciously considered in defining the type, so 

 that what is a hilly tract in a plain country may become a plain 

 in a mountainous country ; the mean surface may be concave, 

 convex, or unequal, and the summits and depressions may be in- 

 dependent in their relations to the mean surface ; and, as the term 

 is commonly employed, it includes an endless variety of surfaces 

 between which the differences are differences in degree alone. 

 But there is another type of surface, in which there is more or 

 less close approximation to the" geometric plane, in which 

 (generally) the mean surface is not sensibly concave, convex, nor 

 unequal, and in which (generally) the summits and depressions 

 bear a constant relation to the mean surface ; and this type of 

 plain (exemplified by partially eroded terraces, certain mesas, 

 etc.) is discriminated at a glance, conveys a distinctive impres- 

 sion to the mind, and appears to differ in kind rather than in de- 

 gree from the other type. The standards or ideals to which these 

 types approximate are elusive, have never been clearly defined, 

 and may be indefinable; but it maybe suggested tbat the first 

 type is represented by any terrestrial surface in which the ele- 



