256 AN ESTIMATE OF THE GEOLOGICAL AGE OF THE EARTH. 



atmosphere as to be attended by pressure conditions exceeding the 

 critical pressure an instant change of state occurred. The water result 

 ing— almost still a vapor, but possessing a surface, although a highly 

 energetic one— probabl}- floated in the equall}' dense vapor, or sinking 

 into hotter la3^ers beneath immediately resumed its vaporous state. 

 Its condition was, in fact, highly unstable as regards upward or down- 

 ward motion;' finally the temperature sank till water established itself 

 upon the surface, here and there over hotter areas doubtless flashing 

 into vapor, but gradually gaining a resting place upon the surface.^ 

 For a long period the fall in pressure attending its own condensation 

 must have maintained it in a state of ebullition. 



Efl'ects were produced at this stage which nvdy 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 an}' where upon the comparatively 

 uniform surface receiving the precipitated water. Over this area the 

 pressure is increased, elsewhere it is reduced. The efl'ect 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 tby land; a diminution of density of the land 

 masses and corresponding upheaval. Further precipitation of water 

 would widen and deepen the carl}' oceans. Finalh' the uniform pres- 

 sure of al)out 300 atmospheres becomes concentrated as a pressure of 

 some 400 atmospheres over perhaps eight-elevenths of the earth's area, 

 if we assume some such concentratioii 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 ener- 

 getic substratum. 



The eflects 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 3,600 feet of rock of a specific gravity of 2.5.=' And this 



^A Theory of Sunspotn. By J. Joly. Rov. Dub. Soc. Proc, n. s., Vol. VIII., 

 1898, pp. 697-700. 



2 Professor SoUas, F. R. S., in his lectures in dwelhng on the facts of the inception 

 of ocean basins, has frequently pointed out that these nuist have dated from the rain- 

 fall attendant on the fall of temperature to the critical temperature of water. 



^One effect of this would be that over the land surfaces the melting ]ioint of a rock 

 such as Diabase would he raised about 8° C. This would tend to confer some greater 

 rigidity on the exposed crust of the earth. 



