Geology and Natural History. 185 



SCIENTIFIC INTELLIGENCE. 



I. Geology and Natueal ' History. 



1. The Geodetic Evidence of Isostacy, with a Consideration 

 of the Depth and Completeness of the Isostatic Compensation 

 and of the Bearing of the Evidence upon some of the Greater 

 Problems of Geology ; by John F. Hayford, C.E. Proc. of 

 the Wash. Acad, of Sciences, vol. viii, pp. 25-40, May 18, 1906. — 

 The nature of this short paper is well indicated by its title, and 

 instead of merely stating the more conspicuous conclusions, it may 

 be desirable, in order to indicate its great importance, to give a 

 brief historical review of the place of this subject in geological 

 literature and of the problems upon which it bears, the historical 

 side not being developed in the paper itself. 



Since the time of Babbage and Herschel the idea has been enter- 

 tained by certain geologists, that the greater features of the 

 earth's surface were sustained, not by virtue of the internal 

 rigidity, but in large part rested in equilibrium because of a 

 lessened specific gravity of the crust beneath the more elevated 

 masses ; the facts leading to this hypothesis having been pointed 

 out by Petit, who in 1849 discussed the deficiency of gravity 

 beneath the Pyrenees,* and by Archdeacon Pratt of Calcutta, who 

 a few years later called attention to the striking deficiency of 

 mass found to exist by the Indian Trigonometrical Survey 

 beneath the Himalayas and the Plateau of Thibet. f 



This conception involves far-reaching consequences in regard 

 to the nature of the earth's interior, implying a superficial hetero- 

 geneity of density and a capacity of viscous nowage toward that 

 figure of equilibrium which should tend to bring equal pressures 

 upon all parts of the earth's interior at a certain depth. For this 

 condition of equilibrium Dutton proposed the term of isostacy,! 

 now fully incorporated into geological literature. The acceptance 

 of the principle of isostacy, however, immediately raises the ques- 

 tions as to how deep is the zone within which the heterogeneity 

 of density producing the larger and broader surface features is 

 confined ? What is the percentage variation in the density of 

 various parts of this zone ? What is the departure from true 

 isostatic adjustment exhibited by the surface features? How 

 quickly will response take place to destroyal of the isostatic 

 adjustment through erosion and sedimentation ? What is the 

 initial cause of the internal differences in density which lead to 

 the larger features of the earth's surface and which having once 

 originated, isostacy tends to preserve ? 



It is seen that these questions involve fundamentally the prob- 



* Comptes rendus de l'Acad. des Sc, xxix, p. 730. 

 f Phil. Trans. Roy. Soc, vol. 145, p. 53. 



X " On Some of the Greater Problems of Physical Geology," Bull. Phil. 

 Soc. of Washington, vol. xi, p. 53, 1889. 



