Boox VII.| INFLUENCE OF DENUDATION. 925 
A few of the more important features of the land may be briefly 
noticed here in their relation to this branch of geology. In the 
physiography of any region, mountains are the dominant features 
- (p. 87). A true mountain chain consists of rocks that have been crum- 
pled and pushed up in the manner already described. But ranges 
of hills almost mountainous in their bulk may be formed by the 
gradual erosion of valleys out of a mass of original high ground. In 
this way some ancient table-lands have been so channelled that they 
now consist of massive rugged hills, either isolated or connected 
along the flanks. minences detached by erosion from the masses 
of rock whereof they once formed a part, have been termed hills of 
circumdenudation. Their isolation may either be due to the action 
of streams working round them, apart altogether from geological 
structure, or to their more resisting constitution, which has enabled 
them to remain prominent during the general degradation of the 
whole surface. 
Table-lands (p. 40) may sometimes arise from the abrasion of 
hard rocks and the production of a level plain by the action of the 
sea, or rather of that action combined with the previous degradation 
of the land by subaerial waste (p. 451). Such a form of surface may 
be termed a Table-land of Denudation. Notable examples are to be 
seen in the extensive “fjelds” or elevated plateaux of Scandinavia, 
‘many of which, rising above the snow-line, form the gathering 
eround for the glaciers that descend almost to the sea-level. 
Fragments of a similar table-land may be recognized among the 
Grampian Mountains of Scotland. But most of the great table-lands 
of the globe seem to be platforms of little-disturbed strata, either 
sedimentary or volcanic, which have been upraised bodily to a con- 
siderable elevation. These may be termed Table-lands of Deposit. 
But whatsoever its mode of origin, the plateau undergoes a gradual 
transformation under continued denudation. No sooner are the 
_ rocks raised above the sea than they are attacked by running water, 
and begin to be hollowed out into systems of valleys. As the valleys 
sink, the platforms between them grow into narrower and more 
definite ridges, until eventually the level table-land is converted 
into a complicated network of hills and valleys, wherein, neverthe- 
less, the key to the whole arrangement is furnished by a knowledge 
of the disposition and effects of the flow of water. The examples 
of this process brought to light in Colorado, Wyoming, Nevada, 
and the other western Territories, by Newberry, King, Hayden, 
Powell, Gilbert, Dutton, and other explorers, are among the most 
striking monuments of geological operations in the world. ‘The 
erosion of the ancient table-lands of Scandinavia and Scotland, and 
their conversion into systems of hilly ridges and valleys, convey less 
impressive but still instructive complete evidence of the efficacy of 
subaerial waste. 
Watersheds are of course at first determined by the form of 
the earliest terrestrial surface. But they are less permanent than the 
