570 J. W. Spencer — Terrestrial Gravity and observed 



for a long period, over large areas (less than continental) of 

 both excess and deficiency of gravity, and over those glaciated 

 and not.* The variations of load given are quite equal to the 

 weight of former glaciers, yet without a subsequent readjust- 

 ment to isostatic conditions, since the disappearance of the ice 

 caps. 



Before studying the effects of gravity, I had commenced 

 investigations, in order to ascertain the amount of recent 

 deformation of the mountain zone, south of the glaciated 

 areas, for comparison with that of the lake district. These 

 are now less important, in this connection, since the discovery 

 of the remarkable anomalies of gravity, as found by Hayford 

 and Bowie, in the investigations of the U. S. Coast and Geo- 

 detic Survey, under the direction of Superintendent O. H. 

 Tittmann. 



These studies confirm the physiographic evidence of defor- 

 mation, based upon the drainage features both pre-Glacial and 

 post-Glacial. 



On the Causes of the Changes of Level. 



Hitherto, I have suggested no hypothesis as to the cause of 

 the late continental elevation and subsequent depression, which 

 first enabled the rains, streams and rivers to excavate the deep 

 valleys and canons along the border of the continental mass, 

 and then submerge the frontal plateaus, slopes and valleys. 

 It has lately been urged on me by one of the most learned 

 investigators of Europe, Sir John Murray, that my task does 

 not end with the description of the phenomena themselves 

 without trying to find their relationship to the final causes. 

 This problem is so stupendous that I should not have dared to 

 announce a working hypothesis had not the anomalies of 

 gravity been found coinciding with my own investigations in 

 the physiographic deformities, since extended and confirmed 

 by Taylor, Leverett, Goldthwait, Fairchild, Coleman. 



My hypothesis of the primary causes of the great changes 

 of level on land and sea is that they are due in some way to 

 differential contraction under the ocean and land areas, although 

 very little is known of the physics of the earth. This is not a 

 new idea, and has been expressed in one form or another. For 

 geological purposes it seems to answer best. Under it the 



* The excess and deficiency of materials, even at St. Paul and Seattle, are 

 small compared with the mountain and plateau masses Bowie states that 

 the gravity determinations indicate that the average excess or deficiency at 

 the stations of observation is about 630 ft., while the average elevation of 

 the United States is about 2,500 feet ; so that the crust is in approxi- 

 mate equilibrium for the country as a whole, at the present time. But the 

 ocean is near its highest level, so far as the coasts are concerned, while 

 formerly it was much lower. (Spencer.) 



