Smith.] Some AspCCtS of El'dsioH. I ] I 



age. Any marked uniformity in the final result in such a case 

 would, of course, be due to peneplanation. 



Orogenic Movements. — The formation of mountain ranges, the 

 geographical units with which we have been dealing, takes place 

 commonly by folding or faulting, or both, along lines of weakness, 

 which are oftenest parallel to the continental margin. The general 

 movements which form mountain ranges, since they are likely to 

 be more or less uniform throughout the geographical unit, tend to 

 uniformity of upland outlines. So, generally speaking, these move- 

 ments would result in diversity of altitudes only when such diver- 

 sity already existed in the region undergoing disturbance. For 

 example, the uplift through faulting of the Sierra Nevada 

 Mountains, which constitute such a unit, has not disturbed the 

 uniformity which existed before, and which is still seen on the 

 western slopes of the range. 



On the other hand, local orogenic movements, since they tend 

 to deform the parts of a unit, are likely to result in diversity of 

 altitudes. Some so-called monadnocks may be due to this cause. 

 Since, in general, according to the views already expressed, erosion 

 does not tend, on the whole, to marked irregularity of upland fea- 

 tures over large areas, it would seem that where such irregularity 

 exists, it must be attributed mainly to local disturbances. To a 

 minor extent, diversity of upland altitudes, or serration of crest 

 lines, is produced by alpine glaciation. This, however, like ordi- 

 nary stream erosion, affects rather the detail than the general 

 result. 



"Tilting." — As we go down stream the volume of the stream 

 increases, and the valley usually widens, owing to lateral corrasion, 

 while the grade of the channel decreases. The neighboring graded 

 slopes, however, still have an angle approximating those found at 

 greater altitudes along the stream course (provided, of course, that 

 they are in rocks under similar conditions as regards resistance, 

 attitude etc.). The reason for this is that, as we go down stream, 

 vertical corrasion is more or less replaced by lateral corrasion, 

 which, by cutting away the base of the slopes, increases their grade 

 near the bottom, and thus ultimately over the entire surface. Lat- 

 eral corrasion is thus fully as effective as vertical corrasion for 



