466 



ALBERT V. G. JAMES 



As the thin upper and the massive lower series of columns 

 approach one another their direction will be influenced by the plane 

 of the isotherms of each series. Both sets are perpendicular to the 

 planes of the isotherms (Fig. 13); Finally the two planes merge 

 into one another and if both sets of columns are vertical, there 

 will be a straight horizontal plane junction which simulates the 

 junction between two lava flow (Figs. 11 and 13), but the absence 



Fig. II. — Junction of the upper and lower columns of a lava flow. The sharp 

 division suggests the junction of two flows, but only one flow is represented. The 

 lower columns are much more massive than the upper. (Jackson's Creek, Sydenham, 

 Australia.) 



of vesicles and scoriaceous material, the exact similarity in density 

 and texture of the two series at their junction, and the actual 

 blending of the columns at times show that they belong to the 

 same flow. 



The resting of thinner columns on more massive basal ones is 

 very common. They are described by Scrope^ and Iddings^ and 

 are well shown at Sydenham. Dykes commonly show the edge of 

 the division plane between two sets of columns. 



' G. P. Scrope, Volcanoes (1862), p. 94. 



= J. P. Iddings, Amer. Jour. Sci., XXXI {li 



), p. 322. 



