ACADEMY OF SCONCES] ^g LAST EIGHT YEARS 283 



THE GEOLOGICAL ASPECTS OF ISOSTASY 



It may be pointed out in continuation of the last sentence, that there is a still more strictly 

 geological aspect of the problem of isostasy to which Gilbert referred only incidentally, but to 

 which his references are nevertheless illuminating. If the earth's crust is to-day in a state of 

 almost perfect isostatic equilibrium, it has probably been similarly isostatic during the greater 

 part of past geological time; for it would be unreasonable to suppose that an earth crust, 

 which has had so enormously long an opportunity to bring itself into equilibrium, had not 

 attained that state before to-day. The purely geological question then arises: How has the 

 isostatic condition of the crust been maintained through the many deformations and erosions 

 and depositions that it has experienced in past geological time? Gilbert's essays fortunately 

 contain a few passages that bear on this matter. 



The first, already quoted, is found in his second essay of 1895, in which the great interior 

 plain is referred to as a region that has been so long exempt from orogenic corrugation that all 

 the topographic effects of the corrugation have been obliterated by erosion; and as therefore 

 being a region in which isostatic equilibrium ought to be well established, if established any- 

 where. It is furthermore said in the same essay in connection with the gravitative excess of 

 the Rocky Mountain mass: 



The upland may be conceived to have originated from the horizontal compression of some crustal tract 

 and the consequent upthrust of superificial portions — a process which would result in local excess of matter 

 approximately to the full extent of the uplift. Excess would continue until the protuberance was removed by 

 erosion. 



The third essay of 1895 rephrases the first of the above statements but with the significant 

 addition of "viscous flow" to other agencies of adjustment: The great central area of the 

 United States "has been exempt for a succession of geological periods from orogenic disturb- 

 ances, and during that time has had exceptional opportunity for the gradual rehef, through 

 viscous flow, degradation and sedimentation, of the strains engendered by gravity in connection 

 with anomalies of density. It seems, therefore, a priori probable that this plateau is in approxi- 

 mate equilibrium," as it was found to be. 



Gilbert's idea therefore was that mountains produced by crustal corrugation or by subsur- 

 face compression and surface upthrust would thereby lose the isostatic equilibrium that they 

 might previously have had, but that they would gradually regain the equilibrium when their 

 surface excess of matter was removed by erosion, or when the surface excess was counterbalanced 

 by a deep-seated viscous outflow. In other words, although isostasy would be temporarily lost 

 in regions and at times of great crustal disturbance, it would spontaneously come to prevail again 

 in regions and at times of crustal calm ; and it may be added that as great crustal disturbance is a 

 relatively rare event in any one region, isostatic equilibrium may be supposed to have been 

 generally prevalent over the earth through geological time. 



It is instructive to note that, as to the relative efficiency of surface erosion and of deep- 

 seated underflow in bringing about and in maintaining crustal equilibrium, some pointed argu- 

 ments impugning the value of the latter process are introduced in the essay of 1913. The most 

 significant comments there made on this phase of the problem concern an old hypothesis regard- 

 ing underflow, at a depth of mobility, from a region that has been weighted down by deposition 

 of sediments toward an adjacent region that has been simultaneously lightened by the erosion 

 of the surface rocks. It is first shown that, to whatever extent the deep-seated readjustment by 

 underflows lags behind the disturbance of equilibrium by surface erosion and deposition, there 

 should be to that extent an excess of mass in the region of deposition and a defect in the region 

 of erosion; and hence "under the hypothesis of interpretation which correlates excess of mass 

 with plus anomalies and defect of mass with minus anomalies we should expect to find as a 

 general fact of the anomaly map, an anomaly gradient from ocean to land in coastal regions," 

 such as the eastern United States. It is then pointed out that, while such a gradient exists 

 on certain northern and southern stretches of our Atlantic coast, " the gradient is unequivocally 

 oceanward between Delaware and Florida"; but he did not here take account of the expectable 

 oceanward gradient which, as shown by Helmert (1909), should, even on a perfectly isostatic 

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