924 PHYSIOGRAPHICAL GEOLOGY. — [Boox VII. 
solitary stacks standing like islands out of the plains, great mountain 
_ masses towering into picturesque peaks and_ pinnacles, cleft by 
innumerable gullies, yet everywhere marked by the parallel bars 
of the horizontal strata out of which they have been carved—these 
are the orderly symmetrical characteristics of a country where the 
scenery is due entirely to the action of subaerial agents and the 
varying resistance of level or little disturbed stratified rocks. 
On the other hand where stratified rocks have been subjected to 
plications and fractures, their characteristic features may be gradu- 
ally almost lost among those of the crystalline masses which under 
these circumstances are so often found to have been forced through 
them. The Alps may be cited as a well-known example of this 
kind of scenery. The whole geological aspect of these mountains is 
suggestive of former intense commotion. Yet on every side are to 
be seen proofs of the most enormous denudation. ‘I'wisted and 
crumpled, the solid sheets of limestone may be seen as it were to 
writhe from the base to the summit of a mountain, yet they present 
everywhere their truncated ends to the air, and from these ends it is 
easy to see that a vast amount of material has been worn away. 
Apart altogether from what may have been the shape of the ground 
immediately after the upheaval of the chain, there is evidence on 
every side of gigantic denudation, The subaerial forces that have 
been at work upon the Alpine surface ever since it first appeared 
have dug out the. valleys, sometimes acting in original depressions, 
sometimes eroding hollows down the slopes. Moreover they have 
planed down the flexures, excavated lake-basins, scarped the moun- 
tain sides into cliff and cirque, notched and furrowed the ridges, 
splintered the crests into chasm and aiguille, until no part of the 
original surface now remains in sight. And thus the Alps remain a 
marvellous monument of stupendous earth-throes followed by a pro- 
longed and gigantic denudation. 
‘Jn massive rocks the structure-lines are those of joints alone, 
and according to the direction of the intersecting joints the trend 
and shape of the ridges are determined. The importance of rock- 
joints, not only in details of scenery, but even in some of the 
main features of the mountain outlines of massive rocks, is hardly 
at first credible. Yet it is along these divisional lines that the rain 
has filtered, and the springs have risen, and the frost wedges have 
been driven. On the bare scarps of a high mountain where the 
inner structure of the mass is laid open, the system of joints is seen 
to have determined the lines of crest, the vertical walls of cliff and 
precipice, the forms of buttress and recess, the position of cleft and 
chasm, the outline of spire and pinnacle. On the lower slopes, even 
under the tapestry of verdure which nature delights to hang where 
she can over her naked rocks, we may detect the same pervading 
influence of the joints upon the forms assumed by ravines and crags. 
Each kind of eruptive rock has its system of joints, and these in large 
measure determine its own characteristic form of scenery, 
