294 B. WILLIS DISCOIDAL STRUCTURE OF THE LITHOSPHERE 



SuBOCEAi^ic Structure 



PROBLEM AND PREMISES 



In the preceding discussion the fact that igneous rocks are extruded 

 along lines parallel to the outlines of oceanic basins is cited as evidence 

 of th^ir relation to oceanic underbodies and is a basis for the argument 

 that those underbodies have a discoidal structure. It is desirable to 

 approach this relation and conclusion independently from a different 

 angle. If the eruptives do originate chiefly beneath the oceans, there is 

 a reason for their predominance in the suboceanic masses, and that reason 

 must be known, if the hypothesis is to advance beyond the purely specu- 

 lative stage. We are thus confronted with the problem, What is the con- 

 stitution of the oceanic underbodies? Do they consist in large part of 

 those basic rocks which are the deep-seated equivalents of the basalts and 

 andesites that are so copiously erupted about their margins? 



HYPOTHETICAL ALTERNATIVES 



Isostatic theory finds it convenient to assume the affirmative in re- 

 sponse to the preceding question, because basic rocks are heavy and so 

 also are, by hypothesis, the low-lying underbodies; but this does not 

 explain why. 



That subtheory of the planetesimal hypothesis which assumes that the 

 inf ailing matter was dust, and that in its passage through the atmosphere 

 it was so sorted as to establish the existing differences of density in 

 oceanic and continental underbodies*^ assigns an original cause for the 

 distribution and leaves only the subsequent processes of segregation to 

 be explained. But that subtheory is a special postulate of a particular 

 hypothesis and, brilliant and logical as it is, does not occupy the field to 

 the exclusion of alternatives. 



Among alternative hypotheses to account for the character of the 

 oceanic underbodies is the postulate that the planetesimals gathered into 

 knots and formed masses of notable size whose differences of density are 

 perpetuated in the existing features of the globe; but the suggestion is 

 undemonstrable, and there are dynamic consequences possible as a result 

 of impact of bodies of suboceanic size which suggest prudence. That the 

 heat of impact may have produced local melting is not, however, an ex- 

 treme postulate. There is a tendency in certain current speculation to 

 return to a hypothesis of a more or less molten globe, to appeal to gravi- 

 tational sorting in the molten magma as a process which resulted in the 



*8 T. C. Chamberlin : The origin of the earth, 1916, pp. 193-200. 



