
310 © DYNAMICAL GEOLOGY. —_— [Boor IIL. 
into the air.’ If this is the condition of rocks even at the surface 
we can realize that at great depths, where escape from strain is for 
long periods impossible, and the compression of the masses must be 
enormous, any sudden relief from this strain may well give rise 
to an earthquake-shock (p. 273). A continued condition of strain 
must also influence the solvent power of water permeating the rocks 
. 300). 
. (2. oleae Thien a mass of rock, owing to subsidence or 
any other cause, is subjected to powerful lateral compression, its Innate 
particles, which have almost invariably a longer and shorter axis, 
tend, under the intense strain, to rearrange themselves in the line of 
least resistance, that is, with their long axes perpendicular to the 
direction of the pressure, whereby a fissile structure is developed. 
Examined microscopically a section of a rock thus influenced shows 
(Fig. 72) a striking contrast to its original character (Fig. 78). 

Fic. 72.—Srction or CompressepD Ar- Fic, 73.—SEcCTION OF A SIMILAR Rock wHiIcu 
GILLACEOUS Rock IN WHICH CLEAV- HAS NOT UNDERGONE THIS MODIFICA- 
AGE STRUCTURE HAS BEEN DEVELOPED. TION, MAGNIFIED. 
MAGNIFIED. 
Rocks which have been thus acted on, and have acquired this 
superinduced fissility, are said to be cleaved, and the fissile — 
structure is termed cleavage. In Fig. 74, for example, where the 

Fic, 74.—Corved QUARTZ-ROOKS TRAVERSED BY VERTICAL AND HIGHLY-INCLINED 
1 n y “7 ™ {, NU 
Cieavacn, Sourn Srack Licurnovusn, Ancuusra (B.). 
original planes of stratification of the rocks are represented by wavy 
lines, and the new system of cleavage planes by fine upright lines, 
' Proc. Boston Soc. Nat, Hist, xviii. p, 272 (1876), 
