226 JOSEPH BARRELL 



were local and close compensation, then as erosion removed the 

 softer surrounding rocks there should be isostatic upwarping of such 

 areas of denudation and relative downwarping of the uneroded 

 crystalline areas. Such warping of the Mohawk, St. Lawrence, 

 and Champlain valleys with respect to the Adirondacks has not 

 been noted, though the problem from the standpoint of field 

 evidence has not been fully studied. The physiographic evidence 

 that residual mountain masses known as monadnocks or unakas 

 have not been shown, however, to be marked by local downwarping 

 and, on the contrary, certainly stand in relief due to circumdenuda- 

 tion, combines with the geodetic evidence of the average excess 

 of gravity for the resistant areas of pre-Cambrian formations, to 

 suggest effective rigidity against the stresses produced by erosion. 

 The evidence, however, as developed thus far from the geodetic 

 standpoint shows that there are more important factors than that 

 of the surface geologic formation, since the larger anomalies are 

 much greater than these figures which have been discussed and 

 hold but little relation to either relief or surface geology. In fact 

 Hayford and Bowie do not find any discoverable relation between 

 the anomalies in general and the topography. 



It is thought by the writer, however, that if stations were 

 located especially to test the intensity of gravity over various 

 broad plateaus remaining by circumdenudation and the intensity 

 compared with that over adjacent broad areas of lower level, the 

 mean differential anomalies due to the surface excess of mass in 

 the plateau over the lowlands would rise to a larger figure than 

 the 0.008 to 0.012 dyne which has remained to be explained in 

 the present discussion. These figures are low because certain 

 pre-Cambrian areas, like those in the vicinity of Baltimore and 

 Washington, have been lowered by prolonged denudation and do 

 not stand markedly above the level of younger formations. Further- 

 more, the tendency of broad pre-Cambrian areas to stand above' 

 sea-level is very probably of an isostatic nature. This implies 

 under such areas a slightly lower mean density to the whole zone 

 of compensation which would diminish the anomaly due to the 

 surface elevation. In individual areas of 100 to 200 km. radius, 

 however, such a relation of positive anomaly to pre-Cambrian 



