172 CONTINENTAL PROBLEMS OF GEOLOGY, 



ancient geograplij^ the two classes of evidence are equally iinportant. 

 But the strata, spread widely over the surface of the laud, are con- 

 spicuous phenomena, while unconformities are visible only here and 

 there and are usually difficult of determination. For this reason the 

 data derived from unconformity have never been assembled. Essays 

 toward ancient geography have dealt only with the minima of ancient 

 land, never with its maxima, and the question of continental growth 

 can not be adequately treated while half of the history is ignored. 



We may borrow a figure from the strand of a lake. As the waves 

 roll inward, each records its farthest limit by a line upon the sand and 

 each obliterates all previous wave lines which it overpasses. The ob- 

 server who studies thetransient record at any point may find a series of 

 lines, of which the highest is the oldest and th6 lowest is the newest, and 

 he may infer that the lake level was higher when the first wave left its 

 trace and that the water is receding from the land. But, if he continue 

 his observations through many days and fix monuments to record from 

 time to time the lowest land laid bare between the waves, he may dis- 

 cover that the highest wave line and the lowest record of ebb corre- 

 spond in time with the play of the largest waves, and that the lowest 

 wave lino and the highest record of ebb correspond to the play of smaller 

 waves, and thus reach the conclusion that the lake level has remained 

 unchanged. In the study of Time's great continental strand we are not 

 even able to observe directly the wave lines of rhythmic transgression, 

 but infer their positions from data often ambiguous; and of the lower 

 wave limits, the lines of maximum regression, Ave are absolutely 

 ignorant. 



It may be true that a priori considerations afford a presumption in 

 favor of continental growth, but such presumption should not be per- 

 mitted to give color to evidence otherwise neutral ; and moreover it is- 

 not impossible to discover an ffj)r/ori presumption in favor of continental 

 diminution. Assuming that hyj)ogene agencies cause continental areas 

 to rise above the ocean, the work of epigene agencies constantly tends 

 to remove the projecting eminences and deposit their material about 

 their margins, so as to extend the area of the continental plateau. Thus 

 we have a strong a priori presumption in favor of continental growth. 

 On the other hand, if we admittheprincipleof isostatic equilibrium, then 

 the continental eminences have low density ; and as they are worn away 

 by epigene processes the material which rises from below to restore them 

 has greater density and maintains a somewhat less altitude. The proc- 

 ess of isostatic restoration tends thus toward the permanent levelling 

 of continents, and if the hypogene initiative should cease the continents 

 would ultimately be reduced to ocean level, and finally, through proc- 

 esses of solution, to a level below the ocean; so, assuming the initiative 

 processes of the under earth to be of finite duration, tlie work of terres- 

 trial degradation, combined with isostatic restoration, should afford a 

 continental history characterized in an earlier stage by growth and in a 



