GEODETIC CONSIDERATIONS 73 



Whether we assume for the earth a nebular or a meteoric origin, the 

 tendency in the cooling globe to tetrahedral deformation of the outer 

 crust may be taken to be a vera causa, which, because of the constancy of 

 its action over long periods of time, may produce cumulative results at 

 times and places where its action is permitted by the balancing of other- 

 forces, as the rotation of the earth may cause the deflection of a stream 

 to the right under favoring conditions. 



The gravity determinations of Putnam, as interpreted by Gilbert, as- 

 sign to the earth a considerable effective rigidity, one capable of sustaining 

 the greater mountain chains and plateaus of the continents, while indi- 

 cating for the continental arch a condition of isostatic equilibrium. It 

 would seem that this might imply a rigidity not enough to hold up the 

 continents on the principle of the unsupported arch, but to give such 

 direction to the crumpling that must accompany the readjustment of the 

 crust that it should approach the tetrahedral form rather than the gen- 

 erally disseminated wrinkling of the baked-apple type. As the settling 

 of the depressed areas might cause a lateral transfer of the superficial 

 and thus lighter portions of the subcrust beneath the continental areas, 

 the tendency would be toward such a condition of isostacy that the crust 

 would be almost equally supported in all parts by the highly visGid sub- 

 crust. Continued shrinking of the nucleus would cause continued re- 

 newal of thrusts that, as epeirogenic forces, would tend to increase the 

 tetrahedral deformation, or, at other times localized as orogenic forces, 

 would add mountain chains like the Appalachian to the continent by an 

 impulse coming from the sea. When collapse of the earth's crust com- 

 menced at any point it would lose the advantage of the principle of the 

 arch, and this collapse would spread in all directions until limited by 

 the encroachment of other centers of depression. The tetrahedral form 

 need not be perfectly realized. Like a leaf stung on one edge, so that 

 the side grows unduly small, the one depression may be slightly hin- 

 dered in time of starting or rapidity of growth. Thus the Pacific may 

 have become over large in comparison with the other oceans. 



OTHER TETRAHEDRAL MAPS 



Map of Richard Owen. — As a document containing the first suggestion 

 of several of the points of the tetrahedral hypothesis, we present here 

 the quaint and interesting map of Richard Owen (plate 12), whose 

 peculiar book suggests the Naturphilosophie of Oken in its extension of 

 geology by strange and forced analogies into every department of human 

 knowledge. He has, however, accurately mapped the three protaxes 



