MATERIALS OF THE EARTH 



17 



This term means not simply that the rock occurs in large bodies, 

 but that the rock has no distinct cleavage. It is not in beds, and 

 it is not schistose. Sills and extrusive flows of lava often take on 

 the form of sheets. When one extrusive sheet of lava overlies an- 

 other, the succession of sheets has some resemblance to stratified 

 rock; but the materials of the several sheets show little indication 

 of arrangement in layers. The material of the sheets, however, 

 sometimes has a peculiar structure developed by the flow of the 



Fig. 7. Columnar structure in igneous rock. Giants' Causeway. 



lava after it had become stiff from cooling. This structure is known 

 as flow structure (Fig. 6). In some places, the lava, on cooling, 

 develops a columnar structure (Fig. 7) which appears to be the 

 result of contraction as the lava cools. The columns are generally 

 perpendicular to the surface of cooling. 



The explanation of the columns is probably somewhat as 

 follows: The surface of the lava contracts about equally in all 

 directions on cooling. The contractile force may be thought of 

 as centering about equidistant points. About a given point, the 

 least number of cracks which will relieve the tension in all direc- 

 tions is three (A, Fig. 8). If these radiate symmetrically from 

 a point, the angle between any two is 120, the angle of the hex- 



