70 



G. F. BECKER FINITE STRAIN IN ROCKS. 



other cracks must form. Either, then, rupture will take place at new 

 centers or the original cracks will branch at their ends. A greater 

 amount of relief for a given length of crack will be afforded by the latter 

 method. The branches will be thrown off at angles of 120° for the same 

 reason that the first cracks formed this angle. As the process continues 

 the branching will be repeated, and it is evident that the surface will be 

 divided into regular hexagons. The more slowly the cooling progresses 

 the smaller will be the tension in the exposed surface and the larger will 

 the hexagons be. 



FiciuiiK 1?,. —Primary tension Cracks. 



In some cases tension may accumulate in one of the hexagons after 

 division to such an extent that a secondary rupturing takes place. This 

 too will begin by three radiating cracks at the center, and these must be 

 perpendicular to the greatest tensions or to the lines joining opposite 

 angles of the hexagon. They will thus divide the hexagon into inequi- 

 lateral pentagons, and the secondary fissures will be at right angles to 

 the sides of the hexagon. 



Figube 14.— Secondary tension Cracks. 



The tensions are due to stresses acting at the isothermal surfaces and 

 tangential to them. Hence, if the mass cools faster at one side than at 

 the other, the resulting columns will be curved and will everywhere be 



