FACTORS IN COLUMNAR STRUCTURE IN LAVAS 463 



because no tearing apart of the crystals is necessary along ax. 

 In order that tension in all directions may be approximately 

 equal, am must be shorter than ax. Therefore, the shape of the 

 spheroids is dependent on the ratio of the tension between ax and 

 am. The shape of the spheroids is also dependent on the down- 

 ward pressure of the columns. This pressure tends 

 to resist horizontal cracking and therefore the 

 greater the downward pressure the more prolate 

 the spheroid will be. The actual shape of the 

 spheroid is determined by the downward pressure 

 of the column tending to make it prolate, and 

 the difference in tensions along am and ax which 

 tends to make the spheroid oblate. 



4. Chisel structure. — At the "Sydenham Organ 

 Pipes" horizontal markings f " to 2" apart, are seen 





,>--v---< 



Fig. 5. — Dutch- 

 cheese structure. 

 The shape of the 

 spheroids de- 

 pends on the bal- 

 ance between 

 the difference in 

 tension between 

 ak and am, which 

 tends to make 

 the spheroid ob- 

 late and the 

 downward pres- 

 sure of the col- 

 umn tending to 

 make the sphe- 

 roid prolate. 



Fig. 6 

 Figs. 6 and 7.- 



FiG. 7 



-Diagrammatic section showing horizontal 

 bandings abah, cdcd round the columns. Rough chiselings are 

 shown on each band. 



circumscribing most of the columns (Figs. 6, 7, and 9). The hori- 

 zontal Hues are found along the whole length of the columns and 

 between them there are peculiar irregularly curved lines similar 

 to those made by roughly chiseling wood (Figs. 7 and 9). The 

 horizontal Hues appear to represent successive stages in which the 

 columns were formed, i.e., as cooling proceeded, the vertical plane 

 ahcd extended to ef, then to gh, and so on. The "chiselings" 

 curve sometimes to the left, sometimes to the right, and are 

 probably due to the heterogeneity of the rock as it parted. 



5. Columns formed hy cooling from two opposing surfaces.— 

 Cooling takes place not only from the upper surface but also from 



