

404 STRUCTURAL AND FIELD GEOLOGY = 
modifying influence, it is evident that the process of degradation must 
be carried on most actively along lines of water-flow. As torrents, 
streams, and rivers cut their way down into the massif, larger and 
larger surfaces of rock become exposed to subaérial action. The 
shattered débris, detached from cliff and mountain slope, slowly or 
more rapidly enters the drainage system, gradually becomes reduced in 
size, and is eventually swept away in the form of gravel, sand, and mud, 
beyond the limits of the mountain area. In time, therefore, profound 
and broad valleys are ploughed out, and these continue to be deepened 
and widened, as the process of mountain-making goes on. Thus, in the 
great transverse valleys which radiate from the backbone of a growing 
mountain chain, the rate of erosion keeps pace with or even exceeds the 
rate of rock-folding and uplift. New or secondary mountains gradually 
come into existence along the flanks of the primary elevations—a 
mountain chain, in a word, grows by the successive addition of contigu- 
ous parallel ranges. But the large transverse rivers flowing out from 
the primary axis are not deflected by the younger ranges which thus 
slowly rise across their path. The rate of valley erosion exceeds the 
rate of crustal deformation, and thus mountain range after mountain 
range is successively sawn across by the primeval rivers descending 
from the axis of the chain. 
We may therefore conceive of the growth of a polygenetic mountain 
chain being continued through a long period of time—the gradually 
bulging and wrinkling crust being concurrently worn and furrowed by 
epigene action. The mountain-mass as a whole, however, continues to 
increase in elevation, notwithstanding the ravages of frost and glaciers, 
of rain and torrents, of streams and rivers. Only in the valleys does 
epigene action balance or exceed the elevating process. When at 
last all earth-movement ceases, the mountains are steadily reduced in 
height, while the valleys continue to be widened and deepened, until 
eventually the broad mountain-land may disappear and be replaced by 
a gently undulating plain—a flane of erosion. 
Many such plains are known. ‘That they occupy the site of vanished 
mountain chains is clearly indicated by their internal or geological 
structure. Some of these old plains of erosion, like that of the Belgian 
coal-fields, reach no great height above the level of the sea, while others 
attain considerable elevations, forming lofty plateaus. A study of such 
plateaus shows us that a chain of Original or Tectonic mountains, after 
it has experienced much denudation—after it has been reduced to its 
base-level and replaced by a plain of erosion—may again be uplifted. 
The crust may once more bulge up, and the plain be gradually carried 
to such a height that it then becomes a Alateau of erosion. Or, instead 
of being thus elevated, the plain may become submerged for a longer 
or shorter period of time. During gradual and long-continued sub- 
mergence, sediment may gather over the surface of the drowned land to 
such an extent that the site of the former mountain chain may eventually 
be buried under a thickness of many thousand feet of stratified materials 
—gravel, sand, mud, etc. Subsequently, the movement of depression 

