EMERSON— RECURRENT TETRAHEDRAL DEFORMATIONS. 467 



lowed by an exfoliating spherical mass as large as the cross-section 

 of the column. There are samples of all these shapes in the col- 

 lection at Amherst. 



Thus no support can be drawn from analogy of the ball and 

 socket structure of trap for the explanation of the large earth fea- 

 tures as a result of variations in rotation. ^^ 



It is further difficult to see how this oscillation on unknown but 

 very long period and of unknown but very slight amplitude can 

 " attract " the other deforming agencies and form bands of fissuring 

 and elevation radiating at 120° and culminating where the move- 

 ments pass through the zero point. The amplitude and period and 

 total duration of these oscillations are left wholly indeterminate and 

 as we exaggerate the nebular character of the original knot and 

 minimize the mass and period of falling and variation of falling of 

 the planetesimals, which is the cause of variation in rate, we may 

 have conditions where the whole effect would be small or even neg- 

 ligible. It is further interesting to note that when a line of tension 

 is drawn from the south pole to the fulcrum line at the south point 

 of Australia, it is then continued northwest with the full width of 

 Australia across the East Indies, bending north in Asia with the full 

 width from Afghanistan to eastern China, and there is no corre- 

 sponding northeast line to divide the Pacific. In the same way the 

 line drawn from the south pole to South America goes northwest 

 with the full width of South America and bends north in North 

 America with a width from southern California to Georgia and 

 there is no northeast line to divide the Atlantic. In the case of 

 Africa the treatment is different, and the line from the south pole is 

 made to branch, although at much too small an angle at the south 

 point of Africa, and the branches to run up the two coasts to Af- 

 ghanistan and the Atlas mountains and then converges to the north 

 pole and a hypothetical ocean is made to occupy the area from 

 Arabia to Scandinavia. It is more consistent and consonant with 

 the other arrangements to have made Africa a "yield tract" exactly 

 analogous to South America and Australia. The line along the east 

 border of the continent, closely parallel to the corresponding line 



IS Polished cross-sections of trap columns show a wholly homogeneous 

 texture. R. B. Sosman, " Types of Prismatic Structure in Igneous Rocks," 

 Jour. Geol., XXIV., 228, 1916. 



