. 
- Boox VITI.] : AGE OF MOUNTAINS, 917 
required to discriminate the enclosure from the rocks of which it 
appears to form an integral and original part. Some of the 
recorded examples of fossils of an older zone occurring by them- 
selves in a much younger group of plicated rocks may be thus 
accounted for. : 
The inward dip and consequent inversion traceable towards the 
centre of a mountain chain lead up to the fan-shaped structure 
(p. 019), where the oldest rocks of a series occupy the centre and 
overlie younger masses which plunge steeply under them. Classical 
examples of this structure occur in the Alps (Mont Blanc, St. 
Gothard), where crystalline rocks such as granite, gneiss, and schist, 
the oldest masses of the chain, have been ridged up into the central 
and highest peaks. Along these tracts denudation has been of 
course enormous, for the appearance of the granitic rocks at the 
surface has been brought about, not by actual extrusion into the 
air, but by prolonged erosion, which in these higher regions, where 
many forms of subaerial waste reach their most vigorous phase, has 
removed the vast overarching cover of younger rocks under which 
the crystalline nucleus lay buried. 
With the crumpling and fracture of rocks in mountain-making, 
the hot springs must be connected, which so frequently rise along 
the flanks of a mountain chain. A further relation is to be traced 
between these movements and the opening of volcanic vents either 
along the chain or parallel to it, as in the Andes and other prominent 
ridges of the crust. Elevation, by diminishing the pressure on the 
parts beneath the upraised tracts, may permit them to assume a 
liquid condition and to rise within reach of the surface, when, driven 
upwards by the expansion of superheated vapours, they are ejected 
in the form of lava or ashes. Mr. Fisher supposes that the lower 
half of the double bulge of the crust ina mountain, by being de- 
pressed into a lower region, may be melted off, giving rise to 
siliceous lavas which rise before the deeper basaltic magma begins 
to be erupted, 
VELaA RE 
is 42D A) a6 } | x NY. A 
b a b 
Fig. 434.—Sercrion or 4 Mountain CHArn sHOWING TWO PERIODS OF UPHEAVAL. 

é 
A mountain chain may be the result of one movement, but pro- 
bably in most cases is due to a long succession of such movements. 
Formed on a line of weakness in the crust, it has again and again 
given relief from the strain of compression by undergoing fresh 
erumpling and upheaval. ‘The successive stages of uplift are usually 
not difficult to trace. The chief guide is supplied by unconform- 
ability (p. 599). Let us suppose, for example, that a mountain range 
(Fig. 434) consists of upraised Lower Silurian rocks (a), upon the 
upturned and denuded edges of which the Carboniferous Lime- 
stove (b b) lies transgressively. The original upheaval of that range 
