FACTORS IN COLUMNAR STRUCTURE IN LAVAS 465 



the liquid rock seems to be comparatively unimportant, the con- 

 ductivity of the rock beneath is very low, and the loss of heat by 

 rising gases is unimportant in lowering the temperature of the 

 bottom layers. The isothermal planes will therefore be widely 

 separated in the upper layers and closely packed together in the 

 lower. Two sets of columns will be formed. The upper are long, 

 thin, and often irregular, while the lower set are short, thick, 



Fig. 10. — Weathered chisel structure. The chisel markings have been weathered 

 o£E but the horizontal bandings have developed into platy structure. (Jackson's 

 Creek, Digger's Rest, Australia.) 



and much more perfect, owing to the homogeneity of the lower 

 lava (Figs. 11 and 12). 



It must be remembered that straight hexagonal columns are 

 produced only in regularly cooHng, quiet, homogeneous lavas. The 

 upper part of the flow is more heterogeneous than the lower because 

 (a) gases from the lower layers are irregularly distributed near the 

 top; (b) the upper surface is churned to some extent by the move- 

 ment; (c) volcanic rain percolates the vesicular lava; (d) water 

 may collect on the partially cooled lava. These four factors tend 

 to produce a heterogeneity in the upper part of the flow and there- 

 fore only very imperfect columns, if any, are formed. 



