CONDITIONS OF TOPOGRAPHIC DEVELOPMENT. 547 



and most important, as determined by structural peculiarities, in accordance 

 with which it is possible to unite many individuals under a single species or 

 class, as plains and plateaus ; mountains, embracing a great variety of sub- 

 species; volcanic structures; and so on with others of less- importance, all 

 these species having their own fashion of wasting from youth to old age. 

 Second, individuals differ even in the same species as to rate of development, 

 those of weak rocks in a moist climate being degraded faster than others. 

 Third, there is some variation in the rate of development of certain features 

 in individuals of different resistance ; thus, in a hard mass, deep, narrow val- 

 leys will be cut down close to baselevel, while the interstream mass is little 

 wasted ; in a soft mass, the valleys will be shallow and wide open, and the 

 stream channels will reach baselevel slowly, and not much sooner than the 

 interstream masses, because the rapid wasting of the surface keeps the streams 

 always overloaded and unable to cut channels of faint slope until the load 

 decreases. Fourth, there is a great contrast in the intensity of development 

 according to the altitude of the mass over baselevel : a low country cannot 

 have deep valleys cut into it, however long its rivers work ; a high country 

 will in a relatively short time take on forms of strong relief. Fifth, accord- 

 ing to the contrast of hard and soft members of the mass, there is a varia- 

 tion in the distinctness of the features of form during development ; in a 

 mass of horizontal beds, all of which are soft, there are no significant cliff 

 outcrops contouring around the slopes, such as appear when some of the beds 

 are harder than the others. Sixth, oscillations of level by which new cycles 

 are introduced are much more apparent near the seashore than in the dis- 

 tant interior of a large continent. 



It must be understood, also, that a single individual has many features; 

 the valleys of the large rivers being most unlike the main divides between 

 the basins. All these considerations, properly combined, go to explain the 

 topographic form of any geographic individual. 



"Age'' and "Date" of Topographic Forms. — When topographic forms are 

 thus described, age is not to be taken as a measure of time, but only as indi- 

 cating the degree of development of the region concerned : a mushroom may 

 grow old while an acorn has not advanced from its infancy ; a low weak 

 mass under plentiful rainfall may soon be reduced nearly to baselevel — that 

 is, to a nearly featureless peneplain — while in another part of the world a 

 very hard mass in a dry climate might scarcely lose its constructional form 

 in the same time. One would have become old in the same measure of abso- 

 lute time as that marking the youth of the other. The two might have the 

 same geological date of beginning, but one would become geographically old 

 while the other was still geographically young. As a corollary of this, it is 

 seen that, in a single individual composed of members of different hardness 

 and surviving several partial cycles of development, the weaker members 



