UN STRATIFIED OR IGNEOUS ROCKS. 



221 



monly five or six. The columns are not usually continu- 

 ous, but short- jointed, like a vertebral column (Fig. 130). 

 The position of the columns is usually perpendicular to 

 the cooling surface. Thus, in vertical sheets, like dikes, 

 they are horizontal, and an outcropping dike often pre- 

 sents the appearance of a pile of corded wood (Fig. 131). 



FIG. 131. Columnar dike, Lake Superior. (After Owen.) 



In overflow-sheets the columns are vertical (Fig. 129), 

 and at the base of a cliff of such rocks are found piles of 

 separated and disjointed columns. 



The cause of this structure is shrinkage by cooling. 

 Many substances shrink by drying., and break into pris- 

 matic columns. Mud thus forms polygonal prisms by 

 sun-cracks. Wet starch, poured into boxes and drying, 

 breaks into prismatic pencils. In the case of lava, the 

 shrinkage is by cooling, instead of drying, and the prisms 

 are far more regular. 



Examples of this structure are found in every country, 

 and give rise to many remarkable scenes. In Europe, the 



