520 THE FORMS OF TISSUES [ch. 



further crystallisation will next set in. The hexagon will tend to 

 grow out into a six-rayed star; and later and more slowly the 

 material for further crystallisation will make its way between the 

 rays, and begin to build side-growths on them*. 



The basaltic column 



Hexagonal patterns are by no means confined to the organic 

 world. The basalt of Staffa and the Giant's Causeway shews a 

 wonderful array of prismatic columns of irregular size and form, 



Fig. 200. Basalt at Giant's Causeway. B}^ Mr R. Welch, Belfast. 



but mostly hexagonal; so also does the frozen soil of Spitzbergen; 

 starch sets on coohng into analogous prisms, but in a ruder fashion 

 as on a smaller scale; and all these are due to simple forces in a 

 simple field, namely to tension, or shrinkage, in a horizontal mass 

 or layer. Imagine a sheet or "sill" of intrusive basalt, thrust 

 in as a molten mass between older rocks. It is gradually chilled 

 by the cold air above or by the rocks on either side, and its inner 

 mass, cooling slower than the outer layer, contracts slowly. Nothing 

 hinders its vertical contraction, rather is this helped by its own weight 

 and by the load above; but no further lateral contraction can take 

 place without splitting the mass, once the basalt sets hard. Con- 



♦ Cf. Gerald Seligmann, Nature, 26 June, 1937, p. 1090. 



