4s. 
522 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boox ay: 
A region may have been subjected at successive intervals to the — 
compression that has produced cleavage. ‘The Silurian rocks of the 
south-west of Ireland were upturned and probably cleaved before 
a 
Fig. 248.—CLEAVED STRATA, WIVELISCOMBE, Wrst SomeERSET (B.). 
Showing the cleavage lines a a slightly undulating at the partings of the strata b b. 
the deposition of the Old Red Sandstone, which has in turn been 
well cleaved. Evidence of the relative date of cleavage may be 
obtained from unconformable junctions and from conglomerates. 
An uncleaved series of strata, lying upon the denuded edges of an 
older cleaved series, proves the date of cleavage to be intermediate 
between the periods of the two groups. Fragments of cleaved rocks 
3 SS 
= SS b 
> 
LSSSSS=a=ap 
SS: 
Fic. 249.—VEIN OF PorPHyRY (a) Crossinc DrvontaAn SLATES (0), PLymMouTH 
SOUND, BOTH BEING TRAVERSED BY CLEAVAGE (B.). 
in an uncleaved conglomerate show that the rocks whence they were 
derived had already suffered cleavage before the detritus forming 
the conglomerate was removed from them. An intrusive igneous 
rock, traversed with cleavage planes like its surrounding mass, points 
to cleavage subsequent to its intrusion (Fig. 249).? 
Part VI.—DISLocATION. 
The movements which the crust of the earth has undergone 
have not only folded and corrugated the rocks, but have fractured 
them in all directions. These dislocations may be either simple 
Fissures, that is, rents without any vertical displacement of the 
mass on either side, or Faults, that is, rents where one side has 
been pushed up or has sunk down. It is not always possible in a 
shattered rock to discriminate between joints and fissures which 
seem there to be both the simultaneous effects of the same cause, 
1 Dela Beche, “ Geol. Obs.” p. 620. ? De la Beche, op. cit. p. 621. 
