7 , Py 
922 PHYSIOGRAPHICAL GEOLOGY. [Book VII. _ 
slopes. It is important to observe that the depressions into which 
the first rain gathered on the surface of the newly upraised land 
would in most cases become the permanent lines of drainage. They 
would be continually deepened as the water coursed in them, so that 
unless where subterranean disturbance came into play, or where the 
channels were obstructed by landslips or otherwise, the streams would 
be unable to quit the channels they had once chosen. The per- 
manence of drainage-lines is one of the most remarkable features 
in the geological history of the continents. The main valleys of a 
country are usually among the oldest parts of its topography. - As 
they are widened and deepened the ground between them may be 
left projecting into high ridges and even prominent isolated hills, 
A chief element in the progress of land-sculpture is geological © 
structure—the character, arrangement, and composition of the rocks 
and the manner in which each variety yields to the attacks of the 
denuding agents. Besides the general relations of the so-called 
_ hard rocks to resulting prominences, and of soft rocks to depressions, 
the broader geotectonic characters have had a dominant influence 
upon the evolution of terrestrial contours. As illustrations of this 
influence, reference may be made to the marked difference between 
the scenery of districts composed of stratified sedimentary rocks, and 
that of areas of massive eruptive rocks such as granite. In the 
former case, bedding and joints furnish divisional lines, the guiding 
influence of which upon the external forms of the mountains is 
everywhere traceable. In the case of eruptive masses the rock is 
split open along joints only, which mainly determine the shapes of 
crest, cliff, and corry. 
Bedding produces a distinct type of scenery which can be traced 
from the sides of a mere brook up into tall sea-cliffs or into lofty 
mountain groups. Moreover, much of the ultimate character of the 
scenery depends upon whether the strata have been left undis- 
turbed ; for the position of the bedding, whether flat, inclined, vertical, 
or contorted, largely determines the nature of the surface. The 
most characteristic scenery formed by stratified rocks is undoubtedly 
where the bedding is horizontal, or nearly so, and the strata are 
massive. A mountain constructed of such materials appears as 
a colossal pyramid, the level lines of stratification looking like 
gigantic courses of masonry. Joints and faults traversing the 
bedding allow it to be cleft into blocks and deep chasms that 
heighten the resemblance to ruined architecture. Probably the 
most marvellous illustrations of these results are to be found in the 
Western Territories of the United States. The vast table-lands of 
_Colorado‘in particular offer a singularly impressive picture of the 
effects of mere subaerial erosion on undisturbed and nearly level 
strata (see frontispiece). Systems of stream-courses and valleys, 
river gorges, unexampled elsewhere in the world for depth and 
length, vast winding lines of escarpment, like ranges of sea-cliffs, 
terraced slopes rising from plateau to plateau, huge buttresses and 
