141 



COLUMNAR ARRANGEMENT. 



present markings of cleavage, which have some resemblance 

 to stratification, and might be mistaken for such. The 

 student will, however, easily distinguish between the two, 

 not only by the mineral composition of the rocks, but by 

 observing that the lines resembling stratification run into 

 one another in various directions, or are only continued for 

 short distances, as in fig. 71. 



FIG. 71. 



COLUMNAR ARRANGEMENT. Several of the unstratified 

 rocks, more especially basalt, together with porphyry and 

 greenstone, occasionally assume a columnar form. These 

 columns occur of three, four, five, six, or eight sides, but 

 those with four and six form the prevailing structure. They 

 are either continuous from top to bottom, as in fig. 72, 



FIG. 72. 



or they are jointed, and consist of a number of short prisms, 

 piled on each other, as in fig. 73. 



