S14 “DYNAMICAL GEOLOGY. [Box III. 
not only have been forced to rearrange themselves as in cleavage 
a | 
& 
a 
structure, but to move upon each other to such a degree as to | 
acquire a “ fluxion-structure ” resembling that seen in rocks which 
have possessed true liquidity (p. 104). No large sheet of rock can be 
expected, however, to have undergone this internal change; the 
effects would probably be produced only here and there at places 
-where there was an escape from the pressure, as, for instance, 
along the sides of fissures," or in other cavities of rocks. As 
already remarked, this explanation ought not to be applied to the case 
of rocks like schists, which display foliation like a kind of fluxion- 
structure over areas many hundreds of square miles in extent.” 
A, Plication.—On the assumption of a more rapid contraction of 
the inner hot nucleus of the globe, and the consequent descent of the 
cool upper shell, a subsiding area requires to occupy less horizontal 
space, and must therefore suffer powerful lateral compression. The 
rocks will thus be crumpled, as, in the classic experiment of Sir 
James Hall (Fig. 81), layers of cloth are folded when a weight is 


Fic. 81.—HAt’s EXPERIMENT ILLUSTRATING CONTORTION. 
placed upon them and they are squeezed from either side? The 
mere subsidence of such a curved surface as that of our globe must 
thus necessarily produce much lateral compression with consequent 
contortion.* De la Beche long ago pointed out that if contorted and 
tilted beds were levelled out, they would require more space than 
can now be obtained for them without encroaching on other areas.° 
The magnificent example of the Alps brings before the mind the 
enormous extent to which the crust of the earth has in some places 
been compressed. According to the measurements and estimates of 
Professor Heim of Zurich, the diameter of the northern zone of the 
central Alps is only about one half of the original horizontal extent 
of the component strata which have been corrugated and thrown 
back upon each other in huge folds reaching from base to summit of 
lofty mountains, and spreading over many square miles of surface. — 
_ 
See the remarks made under “ Segregation Veins,” Book IV. Part VII. § i. 
See Daubrée, “ Géol. Expérim.” i. p. 392. 
Trans. Roy. Soc, Edin. vii. p. 86. 
Mr. J, M. Wilson has calculated that, if a tract of the earth’s surface, 345 miles 
in breadth, be depressed one mile, it will undergo compression to the extent of 121 
yards ; at two miles the compression will be 189 yards; at eight miles 598 yards (Geol. 
Mag. y. p. 206). The observed amount of compression in districts of contorted rocks, 
however, far exceeds these figures. 
® “Report, Devon and Cornwall,” p, 187. 
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