402 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1910, 



the highlands and mountain ranges of the continent. In the direct 

 studies for gravity the scope of these computations has been extended 

 to the features of the entire earth. 



This topograpliic correction in Hayford's investigations occupies 

 the place Avhich the calculation of the " mean plain " takes in those 

 of Putnam and Gilbert. But the plain of reference for the topo- 

 graphic correction under the assumption of complete isostatic com- 

 pensation is the " theoretic plain " at the altitude of the station indefi- 

 nitely extended in all directions. The mean plain and the theoretic 

 plain will rarely if ever coincide, and the corrections therefore have 

 different values. It is much to be desired that the "mean plain" 

 correction and Faye's method, as used by Putnam and Gilbert, should 

 be applied to all available data with the scope and detail employed 

 by Hayford in order that we may have a comparison of the two 

 methods on equally reliable results. The reason for this statement 

 will appear presently in considering certain geological data that bear 

 on the choice of method. 



Hayford found that at eacli station there remained residual de- 

 flections of the plumb line after all the corrections had been made, and 

 he regarded these residuals as evidence of departures from complete 

 isostatic compensation. He says on this point: 



For the United States and adjacent ai-eas it is safe to conclude from the 

 evidence just summarized that the isostatic compensation is so nearly complete 

 on an average that the deflections of the vertical are thereby reduced to less 

 than one-tenth of the mean value which they would have if no isostatic com- 

 pensation existed. One may properly characterize the isostatic compensation 

 as departing on an average less than one-tenth from completeness or perfection. 

 This statement should not be Interpreted as meaning that there is everywhere 

 a slight deficiency in compensation. It is probable that under some areas there 

 is overcompensation as well as undercompensation in others. 



Interpreting the preceding estimate in terms of altitude, Hayford 

 places the average departure for the Continent of North America 

 from that altitude which would correspond to perfect compensation 

 at 250 feet. He further states that the maximum horizontal extent 

 which a feature, such as a mountain, can have and escape compensa- 

 tion is between a square mile and a square degree. 



It is evident that Hayford's studies on isostasy exceed all previous 

 ones in exhaustive detail and in precision. Nevertheless there are 

 geological considerations which suggest that the assumption of com- 

 plete compensation is less satisfactory as a basis of reasoning than 

 that of partial compensation and partial rigidity. 



To present these considerations we must proceed from the fact that 

 the features of continents are not permanent. They are the transient 

 effects of two processes, uplift and erosion, which are opposed to each 

 other, and which act intermittently. During certain epochs, of 

 which the present is one, uplift has been dominant. Then continents 



