VOLCANOES AND IGNEOUS INTRUSIONS 



333 



328 



Basaltic columns in a lava flow near 

 the city of Mexico. 



Columnar Struc- 

 ture of Lava. — A 



striking feature of 

 many ancient lava 

 flows whose lower 

 portions have been 

 exposed to observa- 

 tion by erosion is 

 their columnar struc- 

 ture, the lava being 

 broken up into an- 

 gular columns which 

 are often six-sided. 

 If the lava sheet is 

 horizontal, the col- 

 umns are vertical 

 (Fig. 328) ; if it has been intruded into a fissure (dikes), the columns 



are horizontal (Fig. 

 329). One may ob- 

 serve similar joints 

 in dried mud and 

 starch, but in these 

 substances the sides 

 are much less regular. 

 The explanation of 

 columnar jointing is 

 to be found in the 

 contraction of the 

 lava, resulting from 

 cooling and loss of 

 gas, and the conse- 

 quent cracking of the 

 rock. Since the least 

 expenditure of energy 

 is required to relieve 

 the strain when three 

 cracks radiate from 



equidistant points at 



Fig. 329. — A lava dike (depressed) showing the basal- _ , 



tic jointing at right angles to the walls. Maine, angles of 120 , the 

 (Photo. F. Bascom.) formation of six-sided 



