EMERSON— RECURRENT TETRAHEDRAL DEFORMATIONS. 465 



at the three ends of the corresponding set of fissures from the south 

 pole, dividing the equatorial band into six about equilateral triangles, 

 set saw-tooth-wise. Three alternating ones would be placed base to 

 base with the three north polar triangles above defined. The three 

 intervening ones would be placed base to base with the three tri- 

 angles formed around the other pole by three lines similar to those 

 first mentioned and drawn to the south pole from where the zigzag 

 line touched the southern fulcrum line. The six quadrilaterals made 

 each of two triangles base to base on the fulcrum lines; three touch- 

 ing the north and three the south pole, and interlocking saw-tooth- 

 wise across the equator would by their see-saw motion on the ful- 

 crum lines relieve the stresses rising from the variations in the rota- 

 tion. It is further assumed that all other stresses, as shrinkage, 

 tides, erosion effects, would be localized as elevations along these 

 lines and reach a maximum with special protuberances at their inter- 

 section. These lines become then of great width and are the nuclei 

 of the continents and are called yield tracts rather than fissure lines. 

 The formation of basaltic columns and especially the ball and 

 socket structure, with protuberances rising at the points where three 

 cracks meet, and connected by lower ridges along the cracks, is taken 

 as an instructive illustration of how the rising in ridges along these 

 fissure tracts would occur and the especially marked protuberances 

 at their junction would be formed, and is considered almost a proof 

 that the process has really taken place. There seems, however, only 

 partial resemblance between the two cases. The tensile strains are 

 here alternating; in the basalt coincident and continuous. The trap 

 column furnishes an analogy only for the action at the poles and 

 only for the first half of the cycle, and it is not exact there. As ex- 

 pansion proceeds tension is relieved by three fissures radiating from 

 the pole but this tension and fissuring are not equal along the three 

 lines to the next angles as in the trap but decrease outwardly to zero. 

 When the second half of the cycle begins it may first close up the 

 fissures and then bring the polar regions into a state of compression 

 with maximum at the pole, a state of things not occurring in the 

 trap, where there is no compression and so no elevation. This com- 

 pression might relieve itself by folding or mashing along lines of 

 weakness with little regard to the 120° law or to the former fissure 



