JOINTED STRUCTURE AND CLEAVAGE. 
589 
containing peculiar organic remains.- Some of the contorted 
strata are of a coarse mechanical structure, alternating with 
fine-grained crystalline chloritic slates, in which case the 
same slaty cleavage extends through the coarser and finer 
beds, though it is brought out in greater perfection in j)ro- 
Fig. 624. 
Parallel planes of cleavage intersecting curved strata. (Sedgwick.) 
portion as the materials of the rock are fine and homogene¬ 
ous. It is only when these are very coarse that the cleavage 
planes entirely vanish. In the Welsh hills these planes are 
usually inclined at a very considerable angle to the planes 
of the strata, the average angle being as much as from 30° 
to 40°. Sometimes the cleavage planes dip towards the 
same point of the compass as those of stratification, but often 
to opposite points.* The cleavage, as represented in Fig. 
C24, is generally constant over the whole of any area affect¬ 
ed by one great set of disturbances, as if the same lateral 
pressure which caused the crumpling up of the rock along 
parallel, anticlinal, and synclinal axes caused also the cleav¬ 
age. 
Mr. T. McKenny Hughes remarks, that where a rough 
cleavage cuts flag-stones at a considerable angle to the planes 
of stratification, Fi^r. 625 . 
, Section in Lower Silurian slates of Cardiganshire, showing 
ot StratlhcatlOn, the cleavage planes bent along the junction of the beds. 
the rock is apt to 
split along the lines of bedding. He has also called my at¬ 
tention to the fact that subsequent movements in a cleaved 
rock sometimes drag and bend the cleavage planes along the 
junction of the beds in the manner indicated in the annexed 
figure. 
Jointed Structure. —In regard to joints, they are natural 
* Geol. Trans., 2d series, vol. iii., p. 461. 
the rock often 
splits into large 
slabs, across which 
the lines of bed¬ 
ding are frequent¬ 
ly seen, but when 
the cleavage 
planes approach 
within about 15° 
