THE PAST AND FUTURE OF GEOLOGY. 189 



be e(]nally well effected by time. It is not, liowever, a question raised 

 as to the nature of the force, but as to its enerj^y : it is not a question of 

 necessity one way or the other, but of interpretation ; it is a (jutstiou 

 of dynamics and not of time, and we cannot aci^ept the introduction of 

 time in exphuiatiou of problems the real difliculties of which are thereby 

 more often passed over than solved. Time may and must be used as 

 without limits ; there is no reason why any attempt should be made either 

 toexteiulor to curtail it; but while there is no need for I'rngality, there is no 

 wisdom in prodi<^ality. After all, it will be Ibnnd that, whichever theory 

 is ado[)ted, the need will not be very different ; the mountain range, for 

 the gradual elevation of which the one will ask 100,000 years, the other 

 may require for its more sudden elevation a force taking the same num- 

 ber of years to accumulate its energies. 



We must, however, judge of the past by the features it has stamped 

 on the land,* and these we must interpret not entirely by our own ex])e- 

 rience, not alone by our estinuite of force, but by our knowledge of what 

 amount of force the energy due to the thermal condition of the ^^lobe 

 can develop on known dynamical principles, and by our observation of 

 what those forces have eli'ected in past times. 



However we may differ in our interpretation of the present theiuial 

 state of the globe, most geologists agree in accepting the hypothesis of 

 central heat as the one best in accordance with known facts relating to 

 subterranean temperature, the eruption of igneous rocks, the action of 

 metamorphism, and the crushing and contortions of rock-masses. The 

 radiation of heat into space has been accompanied by a gradual con- 

 traction of the central mass, and a shrinking of the crust, to which the 

 trough of oceans, the elevation of continents, the protrusion of mount- 

 ain-chains, and the faulting of strata are to be attributed. The ques- 

 tion is whether that contraction was accompanied by a like gradual 

 yielding and adaptation of the solid crust to the lessening circumference 

 of the globe ; or whether the resistance of the rigid crust was only over- 

 come at intervals by paroxysmal efforts. This latter Avas the view 

 held by most of our early geologists, and is still the prevailing one 

 abroad. 



It is not necessary here to deal with the first steps of the problem. 

 Let us take it after, for exam[)le, the re-adjustment of the crust (when it 

 must have been many miles thick) which resulted in the elevation of 

 such a mountain-chain as that of the Alps; and here I must assume a 

 point in advance. The resisting strata having given way to the ten- 

 sion to which they had been subjected, a state of equilibrium and re- 

 l)Ose would for a time ensue. As the secular refrigeration subsequently 

 proceeded, the tangential force due to contraction resumed action ; and 

 while certain larger areas were depressed, chiefly b^' the action of gravity, 



* The evidence of facts witli respect to the ghiciul ])eiio(l has led to the adniissiuu of 

 a greater iutensity of cold ; so we contend that the evidence of the pas*^, at times, 

 respecting the greater effects of heat, is equallj' definite. 



