DO CONTINENTS GROW? 189 



unconformities are visible only here and there and are usually difficult 

 of determination. For this reason the data from unconformity have 

 never been assembled. Essays toward ancient geogra | >1 iv have dealt only 

 with the minima of ancient land, never with its maxima, and the ques- 

 tion of continental growth cannot 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 observer 

 who studies the transient record at any point may find a series of lines, 

 of which the highest is the oldest and the 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 correspond 

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

 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 Ave 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, we are absolutely ignorant. 



It may be true that it 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 a priori presumption in favor of continental 

 diminution. Assuming that hypogene 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 admit the principle of 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 pro- 

 cess of isostatic restoration tends thus toward the permanent leveling of 

 continents, and if the hypogene initiative should cease the continents 

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

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

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

 trial degradation, combined with isostatic restoration, should afford a 



