
_ = 
~ 
Pant I, Sxcr, iv.§3.] PLICATION AND FRACTURE. 315 
He computes the horizontal compression of the whole chain at 
120,000 métres, that is to say, that two points on the opposite 
sides of the chain have, by the folding of the crust that produced 
the Alps, been brought 120,000 métres, or 74 miles, nearer each 
other than they were before the movement.’ 
Though the sight of such colossal foldings of solid sheets of rock 
impresses us with the magnitude of the compression to which the 
crust of the earth has been subjected, it perhaps does not convey a 
more vivid picture of the extent of this compression than is afforded 
by the fact that even in the minuter and _ microscopic structure of 
the rocks intricate puckerings are visible (Fig. 19). So intense has 
been the pressure, that even the tiny flakes of mica and other 
minerals have been forced to arrange themselves in complex foldings. 
On an inferior scale, local compression and contortion may be 
caused by the protrusion of eruptive rocks. The characters of 
plicated rocks as part of the framework of the terrestrial crust are 
given in Book IV. Part IV. 
5, Jointing and Dislocation.—Almost all rocks are traversed 
by vertical or highly inclined divisional planes termed joints 
(Book IV. Part II.). These have been regarded as due in some way 
to contraction during consolidation. But their regularity and fre- 
quent persistence across materials of very varying texture suggest 
rather the effects of internal pressure and movement within the 
erust. In an ingenious series of experiments Daubrée has imitated 
joints and fractures by subjecting different substances to undu- 
latory movement by torsion and by simple pressure, and he infers 
that they have been produced by analogous movements in the 
terrestrial crust.” 
But in many cases the rupture of continuity has been attended 
with relative displacement of the sides, producing what is termed a 
fault. Daubrée also shows experimentally how faults may arise 
from the same movements as have caused joints and from bending 
of the rocks. Faults must be regarded as connected rather with 
the elevation than with the subsidence of ground. Instead of 
haying to occupy a diminished diameter, rocks get more room by 
being pushed up, and as they cannot occupy the additional space by 
any elastic expansion of their mass, they can only accommodate 
themselves to the new position by a series of dislocations.2 Some 
portions will be pushed up farther than others, and this will happen 
more particularly to those which have a broad base. These will rise 
- more than those with narrow bottoms, or the latter will seem to sink 
relatively to the former. Each broad-bottomed segment will thus 
be bounded by two sides sloping towards the upper part of the block. 
This is found to be almost invariably the case in nature. A fault or 
dislocation is nearly always inclined from the vertical, and the side 
1 “ Mechanismus der Gebirgsbildung,” 1878, vol. ii. p, 213. 
2 Géol. Expérim. Part I. sect. ii. chap. ii. 
5 See J. M. Wilson, Geol. Mag. v. p. 206. 
