University of California. 



[Vol. 2. 



In the same way, in the divides formed in both hard and soft 

 rocks, the cutting in neither can be of indefinite extent. For, as 

 the stream can cut only as the slopes and summits are lowered, so 

 the summits are lowered only as the stream cuts. As in the case of 

 the stream bed, then, there will be differential erosion, and this will 

 be greater in extent than that found along the stream itself, as 

 would naturally follow from the principles already laid down. 

 Monadnocks will be developed in the rocks of unusual resistance, 

 and the areas of least resistant rocks may sink considerably below 

 the general level. But with all this the differentiation will be 

 limited — the maximum, which can not be exceeded, being deter- 

 mined mainly by the differences in the angles assumed by the 

 various rocks — and will be apparent mainly in detail. The effect 

 of the whole, as in the case of the stream profile, will be one of 

 more or less uniformity, giving an .evenness of the upland sky-line. 

 The differentiation due to erosion will increase progressively to a 

 maximum at maturity, beyond which it will decrease until (theoret- 

 ically) it disappears entirely in extreme old age. 



As a corollary to the above it may be stated that, at the begin- 

 ning of a cycle of erosion, and throughout its earliest stages, the 

 uplands -will be formed by surfaces belonging to the time just prior 

 to the opening of the cycle. Thus, flat-topped hills or ridges, 

 showing pronounced truncation (provided they do not form a part 

 of the graded slopes) may be unhesitatingly classed as not belong- 

 ing to the existent cycle of erosion, but as remnants of older forms. 

 This applies especially in cases where both hard and soft rocks are 

 truncated indifferently. In such cases the more resistant rocks will 

 preserve the truncated character for a longer time than the softer 

 ones. 



If at the beginning of the cycle of erosion the upland sky-line 

 is markedly irregular — a condition of things which is the excep- 

 tion rather than the rule — it will depend on circumstances whether 

 or not the uplands will tend to approach uniformity of altitude after 

 graded slopes have been attained. Among the main factors deter- 

 mining the result are the character of the development of the 

 matured drainage lines, the difference in original elevation, and the 

 amount of variation in the resistance of the rocks. With great 

 original diversity there may be no tendency to uniformity till old 



