STRATIFIED ROCKS. 195 



show the strata strongly folded, while the light lines 

 show the cleavage nearly vertical, cutting through these 





FIG. 111. Strata, cleavage-planes, and joints. 



in parallel planes. In Fig. Ill, three kinds of structure, 

 which should be kept distinct in the mind, are shown. 

 The rectangular block-faces are joints ; the strong lines, 

 s s, slightly inclined to the right, are strata ; while the 

 highly inclined lighter lines are cleavage-planes cutting 

 through both. 



Cause of Slaty Cleavage. Slaty cleavage is undoubt- 

 edly caused by a mashing together of the whole rock-mass 

 in a direction at right angles to the cleavage-planes, and 

 an extension in the direction of these planes ; and, since 

 cleavage -planes are usually nearly vertical, it is the result 

 of a mashing together horizontally, and an up-swelling or 

 extension vertically of the whole cleaved mass. 



Proof. This is proved (a) in field-observation by the 

 folding of the strata (Fig. 110), and (b) in hand-speci- 

 mens by the crumpling of the finest laminae in the direc- 

 tion indicated above. Fig. 112 represents a block of 

 slate eighteen inches long, in which the lamination-lines 

 are shown crumpled by the pressure. In the position of 

 the block it is evident that the crushing was horizontal. 

 The cleavage-planes, represented by the light lines, are 

 vertical. One cleavage-face, c p, is shown. The same is 

 proved, also (c), by distorted fossils often found in cleaved 

 slates (Fig. 113). By comparing the natural with the 



