EMERSON— RECURRENT TETRAHEDRAL DEFORMATIONS. 469 



as they are assumed to have been located by the rotation process, 

 and would have gained the advantage of any sorting action of the air 

 and water currents in concentrating the heavier matter over the 

 sea bottoms and the lighter over the land. This would tend to in- 

 crease the tetrahedral depressions and promote the breaking down 

 of the elevations and the spherical recovery. 



The chapter is introduced by a diagram from Darwin showing 

 that the tidal stresses are eight times as great in the central as in the 

 equatorial regions. This dynamical basis for the theory is largely 

 non-existent, since as shown by BarrelF^ the citation is from an 

 earlier and erroneous calculation later corrected by Darwin, who 

 shows that the central stresses are only two and two thirds greater 

 than the equatorial. 



Barrell says further concerning the theory : 



" It is not clear that earth strains due to the causes invoked could initiate 

 such a primary segmentation, in fact calculations on the stresses which the 

 reviewer has made to test this sub-hj'pothesis pointed to quite a different 

 method of yielding. The distribution of continents and oceans does not 

 accord very closely with it, and the evidence of isostacy does not indicate 

 that the density differences between continents and ocean basins reach below 

 the outer fiftieth of the earth's radius. This hypothesis of juvenile shaping 

 should therefore be accepted with much reserve and does not appear to be 

 as well supported as are the conclusions of the previous chapters." 



The remarkable paper by Professor Lane^" fits all the crevices 

 of the tetrahedral theory. There is a surface layer for orogenic 

 purposes, a deeper plastic (asthenospheric) layer to facilitate flow- 

 age, a deeper layer for epeirogenic purposes, indeed, for tetrahedral 

 purposes and provision for periodic collapses. A nut with its acute 

 distal point and its obtuse proximal end is a suggestive model of the 

 tetrahedral earth ; a triangular beechnut would have been simply 

 perfect. 



Two tables have been published giving the periods of elevation 

 and depression of the North American continent. The table of 

 Shimer is based largely on the geological maps of Chamberlin and 



I'' Science, XLIV., p. 244, 1916. 



-0 A. C. Lane, " On Certain Resemblances between the Earth and a 

 Butternut," Scientific Monthly, 1915, p. 132. 



PROC. AMER. PHIL. SOC, VOL. LVI. EE, AUGUST 3, I917. 



