32 JoLY — An Estimate of the Geological Age of the Earth. 



dense vapour, or sinking into hotter layers beneath immediately resumed its 

 vapourous state. Its condition was, in fact, highly unstable as regards upward or 

 downward motion ;* finally the temperature sank till water established itself upon 

 the surface : here and there over hotter areas doubtless flashing into vapour, but 

 gradually gaining a resting-place upon the surface.f For a long period the fall in 

 pressiu'e attending its own condensation must have maintained it in a state of 

 ebullition. 



Effects were produced at this stage which may well claim here a moment's 

 consideration. 



Sensible shrinkage due to secular cooling, and the great earth-folding which 

 has since wrinkled the Earth's surface had not yet taken place. Let us suppose a 

 depression anywhere upon the comj^aratively uniform surface receiving the 

 precipitated water. Over this area the pressure is increased, elsewhere it is 

 reduced. The effect of this is to cause, on the one hand, a further depression of 

 the early sea bottom, and to establish a drainage into it, and on the other to 

 facilitate the expansion and extrusion of any heated volatile matter held in solution 

 in the lavas beneath the dry land ; a diminution of density of the land masses and 

 corresponding upheaval. Further precipitation of water would widen and deepen 

 the early oceans. Finally the uniform pressure of about 300 atmospheres becomes 

 concentrated as a pressure of some 400 atmospheres over perhaps -j-\- of the Earth's 

 area, if we assume some such concentration of water as at present exists. The 

 several conditions attending the gradual precipitation of the gaseous envelope 

 upon the surface render it improbable that a uniform ocean covering the entire 

 globe ever existed, even if it could have remained in equilibrium on a thinly crusted 

 Earth possessing an energetic substratum. 



The effects of this new distribution of pressure must have been to flood the 

 land areas with lavas extruded from beneath. A change of pressure of from 300 

 atmospheres to one comparatively nil might be represented by an unloading of our 

 present continental areas to the extent of 3600 feet of rock of a specific gravity of 

 2-5. J And this unloading must have been effected in a comparatively short 

 period — ^"instantaneously," if contrasted with the slow unloading effected by 

 denudation. § Such a redistribution of pressures must have inaugurated remarkable 



* " A Theory of Sunspots." By J. Joly. Eoy. Dub. Soc. Proc, N.S. Vol. viii., 1898, pp. 697-700. 



f Professor SoUas, F.ll.S., in his lectures in dwelling on the facts of the inception of ocean basins, 

 has frequently pointed out that these must have dated from the rainfall attendant on the fall of tempera- 

 ture to the critical temperature of water. 



{ One effect of this would be that over the land surfaces the melting point of a rock such as Diabase 

 would be raised about 8° C. This would tend to confer some greater rigidity on the exposed crust of the earth. 



§ It is not to be supposed that tidal disturbances permitted this allocation of the surface to take place 

 (juietly, and without swaying at each vibration of our satellite, then possibly much closer to the terrestrial 

 surface. 



