462 EMERSON— RECURRENT TETRAHEDRAL DEFORMATIONS. 



spiral nebulae comes as a welcome antecedent to either hypothesis, 

 and permits a great latitude in the amount of heat and volatilization 

 which may he assumed as the result of a given collision. 



At one extreme the conditions postulated by the usual plane- 

 tesimal hypothesis may prevail ; at the other with a maximum of 

 volatilization conditions approaching the older theory may be pres- 

 ent, a momentum derived from nebular contraction adding itself to 

 and modifying that caused by impact, so that in most favorable cases 

 even rings either temporary or permanent might be formed. We 

 can perhaps follow a satellite formed by the condensation of such 

 incandescent matter mixed with solid fragments in greater or less 

 quantity through to the present probable condition of the earth or 

 other planets, more easily than one made up of a cold and heteroge- 

 neous mass of discrete planetesimals ; and equally well or better 

 imagine it to assume in some degree the tetrahedral form. 



Chamberlin presents the calculation that shrinkage stresses of 

 the whole globe would support domed elevations on the earth only 

 eight miles high, but this is on the assumption that the earth material 

 is "firm crystalline rock."^^ But the crushing strength of the deep- 

 seated earth material should be taken as that of the steel dies of the 

 crushing machine rather than that of brittle rock (or indeed twice 

 that of steel as deduced from the rapidity of earthquake transmis- 

 sion), which would give a value for this elevation of the proper 

 order for even more than the continental protuberances. Indeed 

 Chamberlin in the same page seems almost to have contemplated 

 the very rhythmical mechanism we have assumed when he says : 

 " It is as if the shrinkage stresses accumulated to the full strength 

 of the stress-resisting power of the whole sphere and then col- 

 lapsed." 



There are good grounds to believe with Chamberlin^^ that the 

 greater earth movements affect all quarters of the globe together, 

 that they are periodic and that the " ocean basins become pro- 

 gressively deeper and more capacious, while the protuberances be- 

 come more protuberant," that " in the process of periodic adjustment 

 of the earth to its internal stresses, portions of the crust are thrust 



12 " Geology," I., 556. 



13 " Diastrophism as the Ultimate Law of Correlation," /oz^r. Geo., XVII., 

 685, 1909. 



