100 STUDIES FOR STUDENTS 
arched divides to the streams. The surface soil, greatly refined 
in texture by long exposure to the weather in its deliberate jour- 
ney slowly creeps and washes to the streams, and the relief is 
reduced to smaller and smaller measures. The condition of 
grade, at first developed in the lower course of the larger 
rivers, next in their branches and headwaters, then on the 
valley sides and over the hills, has thus been extended all over 
the region. The organization that at maturity characterized the 
water streams has come in old age to characterize the streams 
and sheets of waste all over the land surface. From the begin- 
ning to the end of this process, there is steady progress without 
break or interruption through the normal cycle. There is an 
essential unity of development through the whole of it. It is 
very desirable that this unity should be expressed in the terms 
employed in the description of land sculpture and land form; 
and that the balanced condition of water streams and waste 
streams alike should be expressed by such a term as grade, 
rather than that an artificial distinction between them should be 
introduced by speaking of the balanced rivers as defining a 
‘‘baselevel,’’ while balanced waste streams are given some other 
name by which their close affinity to graded water stream is con- 
cealed. 
An old land surface, sheeted over with a graded soil cover, 
is a peneplain of erosion or of gradation; it passes slowly into 
a plain of gradation. It is almost the realization of that 
imaginary baselevel surface described by Powell as ‘inclining 
slightly in all its parts toward the lower end of the principal 
streams draining the area through which the level is supposed to 
extend, or having the inclination of its parts varied in direction 
as determined by tributary streams ;’’ but this imaginary surface 
is elusive and intangible, because of the impossibility of defining 
the stage of stream development when it should be introduced, 
and the length of graded stream course that it should follow; 
while on the other hand the graded land surface is a reality 
whose gradual development and slow change is one of its essen- 
tial characteristics. 
