GEOLOGY: N. M. FENNEMAN 
19 
lands on the other. So also, forms which are not classified together 
may be superficially similar; for example, a young coastal plain and a 
peneplain. Hence this map, which distinguishes physiographic types 
based on a genetic classification, does not in all cases make the distinc- 
tions which are most obvious to the casual observer. On the whole 
however, this discrepancy is not great. A very large proportion of 
all the boundaries shown on this map are familiar features. To have 
based the divisions purely on superficial features in proportion to their 
magnitude, would not have resulted in the making of units suited to 
scientific treatment. 
Important physiographic differences between adjacent areas are, 
in a large proportion of cases, due to differences in the nature or structure 
of the underlying rocks. Where this is the case the two areas are dis- 
tinguished on the geologic as well as the physiographic map. Distinc- 
tions based on geologic age also correspond to physiographic distinctions 
where the forms are so recent as to be in their first erosion cycle, as is 
generally the case with sheets of glacial drift. When these facts are 
remembered, it is not surprising that a large proportion of the boundary 
lines shown on the accompanying map are also geologic lines. 
The segments here presented are of three orders, called respectively 
major divisions, provinces and sections. The basis of distinction 
among coordinate units is very much the same in all the orders. On 
the whole it may be said that contrasts in structure are stronger and 
more general between adjacent major divisions than between adjacent 
divisions of lower orders. Naturally also, the degree of topographic 
homogeneity is greatest in the units of the lowest order, but the reasons 
for calling one area a major division and another a province or a sec- 
tion are not clearly defined. 
The degree of homogeneity in the several divisions of the same order 
is not in all cases the same. Homogeneity is strong in the Dissected 
Till Plains (12-e) which are practically ever3rwhere submaturely dis- 
sected plains of moderate relief; also in the Snake River Plain (19-d) 
which is everywhere a young lava plain. On the other hand, the East 
Gulf Coastal Plain (3-d) is a heterogeneous area, for it grades from a 
young marine plain with undeveloped drainage near the coast to a 
maturely dissected, belted coastal plain farther inland; this case illus- 
trates the inclusion of several types in one section by gradation, where 
no good dividing line is known, and where practical convenience requires 
that the types be considered in their mutual relations. Again, the 
Nevada Basin (21-b) comprises isolated mountain ranges (probably 
dissected block mountains) separated by aggraded desert plains; here 
