Ransome. ] 



The Great Valley 



397 



figure to which gravitation tends to reduce a planetary body, irre- 

 spective of whether it be homogeneous or not." But within this 

 broad statement there are really two problems involved, — one per- 

 taining to the department of mathematical physics, and one to that 

 of observational geology, although the two, as is usual in such 

 cases, show a certain amount of overlapping. Few geologists will 

 be disposed to deny that great changes in relative density, or the 

 transference of relatively large masses from one portion of the 

 spheroid to another, would disturb the existing isostasy and lead to 

 some modification of the form of the spheroid, whatever strong 

 views they may hold as to the earth's rigidity under ordinary cir- 

 cumstances. The question which immediately concerns the geolo- 

 gist, however, is whether there is anything, in the past history or 

 present condition of the earth's surface, that can lead us to suppose 

 that any such isostatic adjustment need be taken cognizance of in 

 dealing with purely geological phenomena. Among the latter 

 there are no processes through which more stupendous shiftings of 

 material from one portion of the earth's surface to another are 

 effected, than those which are embraced in the terms denudation, 

 transportation, and deposit or sedimentation, and the isostatic prob- 

 lem, for the geologist, may be considered as practically narrowed 

 down to the single question of whether this transfer of material has 

 any applicable effect upon the solid crust * of the earth. Can we 

 regard it as sinking under and because of the added load, and rising 

 because it is lightened by erosion, or must we look upon it as a 

 floor whose movements take place independently of such superficial 

 transferences of material. 



Woodward.f in the same year, sums up the case for and against 

 isostasy as follows: "As yet we can see only that isostasy is an 

 efficient cause if once set in action; but how it is started and to 

 what extent it is adequate remains to be determined. Moreover, 



*In using the term "crust" for the solid external portion of the spheroid, 

 the writer nowhere means to imply that there is any sharply defined shell, 

 definitely separated from an interior portion of different character. 



t Mathematical Theories of the Earth, Am. Jour. Sci., 3 ser., Vol. 

 XXXVIII, p. 351; alsoScience, N. S., Vol. I, p. 194; Trans. N. Y. Acad. Sci., 

 Vol. XIV, p. 73. 



