196 



STRUCTURAL GEOLOGY. 



distorted form the direction of pressure is found to be 

 always at right angles to the cleavage-planes, i. e., the 



FIG. 112. A block of cleaved elate. (After Jukes.) 



fossils are shortened in that direction and elongated in 

 the direction of the planes (d). In many slates, especially 



Fig. 113. Cardium hillanum : A , natural form ; B and C, deformed by pressure. 



the purple Cumberland slates, much used in roofing, 

 oblong greenish spots are common. If they be closely 

 examined, they will be found to be very thin in the direc- 



