352 R. 8. Woodward — Mathematical Theories of the Earth. 



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 extent 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 there- 

 fore 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 seemingly divergent theories are really supple- 

 mentary. 



Closely related to the questions of secular contraction and 

 the mechanics 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 again ; and 

 whether our foundation be liquid or solid, or to speak more 

 precisely, whether the earth may not be at once highly plastic 

 under the action of long continued forces and highly rigid 

 under the action of periodic forces of short period, it is pretty 

 certain that some years must elapse before the arguments will 

 be convincing to all concerned. The difficulties appear to be 

 due principally to our profound ignorance of the properties of 

 matter subject to the joint action of great pressure and great 

 heat. The conditions which exist a few miles beneath the 

 surface of the earth are quite beyond the reach of laboratory 

 tests as hitherto developed, but it is not clear how our knowl- 

 edge is to be improved without resort to experiments of a 

 scale in some degree comparable with the facts to be explained. 

 In the meantime, therefore, we may expect to go on theorizing, 

 adding to the long list of dead theories which mark the progress 

 of" scientiiic thought, with the hope of attaining the truth not 

 so much by direct discovery as by the laborious process of 

 eliminating error. 



When we take a more comprehensive view of the problems 

 presented by the earth, and look for light on their solution in 

 theories of cosmogony, the difficulties which beset us are no 

 less numerous and formidable than those encountered along 

 special lines of attack. Much progress has recently been made, 



