PRISMATIC STRUCTURE IN IGNEOUS ROCKS 225 



columns, on the other hand, are usually about 0.2 m. or less in 

 diameter; their length is often 20 m. without a joint, and their 

 total length may be over 40 m. It should be noted, however, that 

 the composition of the rock may have a considerable effect on the 

 size of columns under given conditions of cooling, the more salic 

 rocks forming larger columns than the more femic rocks. 



3. Shape of 'cross-section. — Convection columns, if perfect, 

 should all be hexagonal. The more uniform the conditions have 

 been, the greater the proportion of hexagons; in any case, the 

 hexagonal sections will be in the majority. Seven-sided figures 

 will be common, produced by the trunkation of one angle of a 

 hexagon; pentagons will also occur frequently, by the elimination 

 of one side of a hexagon. But three- and four-sided figures will 

 be very rare. 



In contraction columns, on the other hand, pentagons are likely 

 to be the prevailing type, and four-sided figures are fairly numerous, 

 while hexagons become less important. This distribution of poly- 

 gons arises from the fact that a mass cracking under the stresses of 

 its own thermal contraction, although theoretically it should break 

 into perfect hexagons of equal area, actually tends to yield by the 

 formation of master-cracks which are then joined up by the forma- 

 tion of shorter cracks. 1 An example of thermal contraction prisms 

 on a large scale is seen in the soil polygons of Arctic regions; a map 

 of a set of these polygons, in a recent article by Lefhngwell, 2 shows 

 clearly the contraction- type fissures described above. 



The relative frequency of polygons in some of Benard's artificial 

 convection cells, 3 in the Giant's Causeway, 4 and in a columnar dike 5 

 is shown in Table I. 



1 "The rock may rather be said to be divided into numerous perpendicular fissures; 

 than to be prismatic, although the same picturesque effect is produced." — Lyell, 

 description of Torre del Greco. 



2 E. De K. Lefhngwell, Jour. Geol., XXIII (1915), 653. 



3 The photograph used for this computation was one taken while the liquid was 

 cooling and the polygons were undergoing gradual changes, leading to the formation 

 of 5- and 7-sided figures. Under steady conditions of heat flow the cells were hexagons 

 almost without exception. 



4 J. P. O'Reilly, Roy. Irish Acad. Trans., XXVI (1879), 641-734- 



3 A. Geikie, Ancient Volcanoes of Great Britain, illustration, p. 459. 



