462 



ALBERT V. G. JAMES 



We can say that either the convex or the concave surface faces 

 the direction of cooHng. It is also evident that the soHd angles, a 

 and b, are likely to be broken off along YZ and YX (Fig. 3), since 

 these are planes of contraction and weakness. This seems to 



explain very simply a well- 

 recognized structure that has 

 given rise to much debate^ 

 (Fig. 3)- 



The above explanation 

 for the ball-and-socket struc- 

 ture also explains the prev- 

 alence of spheroids in the 

 columns. I cannot beheve 

 that they are due to weather- 

 ing^ or segregation, but 

 rather to the concentric 

 spheres of contraction. The 

 spheroids are not found 

 passing from one column to 

 another. In the diagram 

 (Fig. 3) each column is seen 

 to be divided actually or 

 potentially into blocks, each 

 composed of concentric 

 spheres separated by a con- 

 traction crack or line of 

 weakness along which 

 weathering will later take 

 place and finally give rise to ''onion structure." 



3. "Dutch cheeses.'^ — This structure is very common on slightly 

 weathered basalt columns (Fig. 4). The oblate spheroids are 

 seen piled one above the other. They appear to be oblate for this 

 reason: Contraction along ax (Fig. 5) is easier than along ab, 



^T. G. Bonney, Quart. Jour. Geol. Soc. (1876), p. 153; R. Mallet, Phil. Mag., L 

 (187s), p. 205; J. P. Iddings, Amer. Jour. Sci., XXXI (1886); R. B. Sosman, Jour. 

 Geol., XXIV (1916). 



2 J. Thomson, British Association Reports (1863), p. 89; R. Mallet, Phil. Mag., L 

 (1875), p. 220. 



Fig. 4. — "Dutch-cheese" structure 

 Each spheroid is about eight inches thick 

 ("Organ Pipes," Sydenham, Australia.) 



