692 WILLIAM HERBERT HOBBS 



level are the cause of the increased denudation and deposition 

 (doctrine of high rigidity) ; and, upon the other hand, it may be 

 assumed that these changes in level are the effect and not the cause 

 (doctrine of high plasticity — isostasy). 



While the name isostasy is of American origin 1 and the crystal- 

 lization of ideas always connected in time with the providing of a 

 pigeonhole for their assembling has been notably strong in this 

 country, the conception itself is much older and has long occupied 

 the attention of geologists in both Europe and America. 2 It 

 appears to have first found full expression a half-century earlier, 3 

 and it has ever since played an important role in the fields of geodesy 

 and geology. From the failure of astronomic and geodetic loca- 

 tions of position accurately to correspond, and from the "anom- 

 alies" of pendulum observations — -the so-called "anomalies" of 

 deflection of the vertical and of gravity (Ag.) — it has from the 

 beginning drawn most of its support. Woodward, writing in 1889, 

 says: "In general terms we may say that the difficulty in the way 

 of the use of pendulum observations still hinges on the treatment 

 of local anomalies and on the question of reduction to sea-level." 4 



Pratt's hypothesis. — -The remarkable anomalies in the deflec- 

 tion of the plumb line which were discovered in Northern India 

 along the base of the Himalayas led Archdeacon Pratt in 1859 s to 

 the rather astounding assumption that a mass of less density lies 

 beneath this great protuberance upon the lithosphere. Had the 

 venerable archdeacon conceived the earth to be rigid, as is generally 

 held today, it is not very likely that he would have arrived at this 



1 C. E. Dutton, "On Some of the Greater Problems of Physical Geology," Bull. 

 Phil. Soc. Wash., II (1889), 51-64. 



3 For an excellent summary of the evolution of thought along this line, see F. L. 

 Ransome, "The Great Valley of California: A Criticism of the Theory of Isostasy," 

 Bull. Dept. Geol. Univ. Cal., I, No. 14 (1896), 371-428. 



3 Sir John F. W. Herschel, "Letter to C. Lyell, Esq.," Phil. Mag., II (1837), 

 212-14. 



« R. S. Woodward, "The Mathematical Theories of the Earth," Am. Jour. Set. 

 (3), XXXVIII (1889), 341. 



s John Henry Pratt, "On the Deflection of the Plumb-Line in India Caused by the 

 Attraction of the Himalaya Mountains and of the Elevated Regions Beyond: and 

 Its Modification by the Compensating Effect of a Deficiency of Matter Below the 

 Mountain Mass," Phil. Trans., 1859, pp. 745-9 6 - 



