PRISMATIC STRUCTURE IN IGNEOUS ROCKS 231 



is set up in the surface; the cracks then produced are soon widened 

 by solution. A photograph of an excellent example of this type of 

 structure in diabase is given in Fig. 5. Internal expansion may also 

 account for the prismatic surface structure of " bread-crust bombs," 

 although this remains to be proved. 



Fig. 4. — A polygonal structure in a cement briquette, caused by internal 

 expansion. 



SUMMARY 



From the physical standpoint, several types of prismatic struc- 

 ture in igneous rocks can be distinguished. The first and most 

 common is due purely to thermal contraction in the crystallized 

 rock; examples are numerous and familiar. A subordinate type 

 of contraction structure is produced when the contraction and 

 separation occur while the magma is still partly crystalline and 

 partly liquid ; this type is illustrated by an occurrence in a diabase 

 sill in eastern Pennsylvania. 



