EROSION AND ITS SIGNIFICANCE. 133 



Estimates of the earth's age from a consideration of the 

 thickness of the sedimentary deposits are also valueless for 

 a similar reason. 



Estimates also as to the earth's age based on the salinity 

 of the ocean apparently need modification, because all the 

 factors do not appear to have been considered. Thus dur- 

 ing an ephemeral mountain period, such as the present one, 

 the volume of rock accessible to wandering waters which 

 yield up their harvest of salts to the rivers is much greater 

 than that during the incomparably longer life of the low 

 lying plain of erosion. 



(3) Peculiarities of certain topographies. — Controversy 

 has raged round the origin of certain topographic features 

 for many years. Thus the Yosemite Valley of California 

 has been explained variously as the result of stream, of 

 fault, or of glacial action. The fiord and lake basins of 

 New Zealand have also been explained in the same manner, 

 so also the forms of the Scottish Highlands, of the Nor- 

 wegian fiords, and of the Alpine valleys and lakes of 

 Switzerland. While all these processes have doubtless 

 been active in all the regions mentioned during recent 

 times, it is necessary to decide as to what share each has 

 had in moulding the landscape. In this connection brief 

 mention only is made of the Yosemite and several New 

 Zealand forms. 



Yosemite. — It is certain that glaciers have occupied the 

 lower 2,000 feet of the valley in recent time and it is highly 

 probable that they have produced the peculiar "steps " and 

 "treads" at the Nevada and Vernal Falls. On the other 

 hand there are no moraines in the valley worth serious 

 consideration, and the upper valley slopes are certainly 

 those due to sapping action. The most difficult thing to 

 explain on the assumption of the origin of the Yosemite 

 profiles by ice corrasion alone is the position of the Bridal 



