THEORY OF ISOSTACY 777 



THE THEORY OF ISOSTACY 

 BY W. M. DAVIS 



(Al)stract) 



1. Let it be assumed that the crust of the earth is too rigid to be affected 

 by the forces involved in isostacy, and that it has been deformed only by 

 other and stronger forces, acting in different epochs, over various areas, at 

 unequal rates, and with unlike intensities. Under this assumption the facts 

 that have been accounted for by isostacy may be explained by the accidental 

 agreement of depression and deposition, vi^hile certain other facts, such as the 

 formation of great trough valleys or the depression of land areas under deep 

 ocean water, for which isostacy gives no adequate explanation, may be ac- 

 counted for by disagreement between deformation and deposition. A merely 

 accidental agreement between depression and deposition is not improbable, for 

 small and slow movements excite small and slow transfer of materials, while 

 large and rapid movements excite great and active transfer of materials. 



2. The initiation of isostatic movement involves a non-isostatic movement. 

 The cessation of isostatic movement by the reversal of depression to elevation 

 or of elevation to depression has occurred under so many different conditions 

 as to structure and area, and especially as to load, that isostacy must have a 

 very subordinate value in the total movement. 



3. The peneplanation of mountains of various structures and at many differ- 

 ent dates has involved a long-maintained stability of extensive regions, while 

 they were suffering prolonged erosion and great loss of weight. After degra- 

 dation, some peneplains have been depressed and buried, others elevated and 

 dissected. All of this is inconsistent with isostacy. 



Discussion 



Prof. H. F. Reid: I am in entire accord with Professor Davis's ideas. 

 There are so many vertical movements of the crust that can not be due to the 

 simple restoration of isostatic equilibrium that it seems very doubtful if any 

 movements can be definitely assigned to that cause. Gravity observations 

 show that there is general isostatic equilibrium between the ocean beds and 

 the continents; but islands in the ocean are held up by the rigidity of the 

 crust, and the series of transcontinental gravity measurements made some 

 years ago and discussed by Mr. Putnam and Mr. Gilbert show that areas like 

 the Appalachian Mountains and the Rocky Mountains do not owe their alti- 

 tudes to isostatic equilibrium. 



GEOLOGICAL PROGRESS MAP OF OKLAHOMA 

 BY CHARLES N. GOULD 



(Abstract) 



With the possible exception of Triasslc and Jurassic formations, Oklahoma 

 contains rocks of every geological age above the Lower Cambrian. Pre- 

 Cambrian granites and Lower Paleozoic sediments are exposed in the four 

 mountain regions, namely, the Wichita, Arbuckle, Ouachita, and Ozark. 



