196 THE MATHEMATICAL THEORIES OF THE EARTH. 



jected, and one which some* consider crucial against it, is the vohinietric 

 amount of crumpling shown by the Earth at the present time. This is 

 a difficult quantity to estimate, but it appears to be much greater than 

 the theory can account for. 



The opponents of the contractional theory of the Earth, believing it 

 quantitatively insufficient, have recently revived and elaborated an 

 idea first suggested by Babbage t and Herschel in explanation of the 

 greater folds and movements of the crust. This idea figures the crust 

 as being in a state bordering on hydrostatic equilibrium, which can not 

 be greatly disturbed without a readjustment and consequent movement 

 of the masses involved. According to this view the transfer of any 

 considerable load i'rom one area to another is followed sooner or later 

 by a depression over the loaded area and a corresponding elevation 

 over the unloaded one, and in a general way it is inferred that the ele- 

 vation of continental areas tends to keep pace with erosion. The proc- 

 ess by which this balance is maintained has been called isostasy,! and 

 the crust is said to be in an isostatic state. The dynatnics of the super- 

 ficial strata with the attendant phenomena of folding and faulting are 

 thus referred to gravitation alone, or to gravitation and whatever op- 

 posing force the rigidity of the strata may offer. In a mathematical 

 sense, however, the theory of isostasy is in a less satisfactory state than 

 the theory of contraction. 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 ex- 

 tent it is adequate remain to be determined. Moreover, isostasy does 

 not seem to meet the requirements of geological continuity, for it tends 

 rapidly towards stable equilibrium, and the crust ought therefore to 

 reach a state of repose early in geologic time. But there is no evidence 

 that such a state has been attained, and but little if any evidence of 

 diminished activity in crustal movements during recent geologic time. 

 Hence we infer that isostasy is competent only on the supposition that 

 it is kept in action by some other cause tending constantly to disturb 

 the equilibrium which would otherwise result. Such a cause is found 

 in secular contraction, and it is not improbable that these two seem- 

 ingly divergent theories are really supplementary. 



Closely related to the questions of secular contraction and the me- 

 chanics of crust movements are those vexed questions of earthquakes, 

 volcanism, the liquidity or solidity of the interior, and the rigidity of 

 the earth's mass as a whole; — all questions of the greatest interest, but 

 still lingering on the battle-fields of scientific opinion. Many of the 

 "thrice slain" combatants in these contests would fain risk being slain 

 agaiu; and whether our foundation be liquid or solid, or, to speak more 



* Notably, Rev. Osmond Fislier. See his Physics of the Earth's Crust, chapter viii. 



t Appendix to the Ninth BridgewaterTreatise (by C. Babbage), second edition, Lon- 

 don, 18:<8. 



t Button, Capt. C. E. On some of the Greater Problems of Physical Geology, 

 BuUeiin Philosojyhical Society of Washington, vol. xi, pp. 51-G4. 



