564 J. W. Spencer — Terrestrial Gravity and observed 



cal evidence that within the interval covered by the geologic 

 record, many thousands of feet of material have been eroded 

 . . . and deposited in other parts, that changes of elevation of 

 the surface, amounting to thousands of feet, have been pro- 

 duced in this and other ways, and that these changes have con- 

 tinued to take place in recent times."* Let us examine some 

 of the anomalies of gravit}^ in their relationship to the theory 

 of changes of land in late geological days, of which we have 

 found so much evidence in the submarine valleys indenting the 

 border of the continental mass. 



Relationship of the observed Earth-movements and Intensity of 

 Gravity in Eastern America. 



Preliminary studies of the recent terrestrial deformation in 

 the Appalachian belt indicate that there is an increasing rise 

 from zero at Washington to three feet per mile, or possibly 

 five feet or more, in proceeding up the Potomac valley. 

 Southeast of the city there is depression to the mouth of the 

 Chesapeake Bay, where the original valley is deeply buried by 

 the sands of the bottom of the bay. On following up the 

 Susquehanna valley, I have not so far found a greater deforma- 

 tion indicated than three feet per mile. In Georgia there is 

 the same evidence of tilting in recent times, with a rise towards 

 the mountain zone and a sinking towards the sea. The 

 intensity of gravity at Washington, according to Bowie,f 

 shows an excess of weight equivalent to about 1255 feet of 

 rock, while at Virginia Beach, 150 miles distant on the coast, 

 a deficiency of weight occurs equal to 1600 feet of rock 

 Thus, if the whole region had been equally heavy, the differ- 

 ence of weight would represent a subsidence of 2855 feet. 



The physiography of the region presents unusual features, 

 in that the coast at Cape Hatteras is situated only a few miles 

 from the border of the continental shelf, an entirely different 

 condition from that of any other point along the coast. Also, 

 beneath the sea, a very large and deep embayment in the con- 

 tinental slope occurs in front of Chesapeake Bay (with the 

 valley of which it was doubtless connected), similar to the fea- 

 tures about the canon of the Hudson. It has appeared to me 

 that the coastal plain of the region about Cape Hatteras has 

 been built out upon the continental shelf by the materials 

 brought from the mountain zone behind, and in support of this 



* Science, vol. xxxiii, p. 3, 1911. 



f In this paper only the New Method (Hayforcl) anomalies of gravity are 

 considered. They are shown on Chart No. 2, Spec. Pub. No. 12, Coast and 

 Geodetic Survey. According to Bowie, '003 dyne (unit of force) increase 

 in gravity may be assumed, as a working hypothesis, to be caused by a disk 

 of rock of average density (of 2*67), of indefinite horizontal extent and 100 

 feet in thickness, lb., p. 22. 



