academy of sconces] DISCUSSIONS OF ISOSTASY 217 



for this theory postulates such a distribution of crustal densities as to make the correction 

 zero. Next the mean value of gravity, reduced to sea level and to latitude 40°, for the 11 sta- 

 tions between the Alleghenies and the Rocky Mountains is adopted as the standard with respect 

 to which the similarly reduced values for other stations to the east and west are to be compared; 

 these 11 stations being selected to furnish the standard because they stand on "a vast plain, 

 1200 miles in its smallest dimensions," which "has been exempt from orogenic corrugation 

 . . . for at least five of the great periods of geologic chronology. . . . The topographic evi- 

 dences of earlier corrugation have been practically obliterated. Here, if anywhere on the 

 continent, isostatic equilibrium should be established." The chief results of geological interest 

 are reached when the corrected values of gravity for other stations are compared with this 

 standard gravity. 



In the expression of these results a device is introduced for which geologists may well be 

 grateful, in that it changes their phrasing from a geodetic to a geologic form. Geodesists usually 

 say that gravity at a certain station has a positive or a negative anomaly of, for example, 0.078, 

 without delaying to state that corrections for altitude and latitude are of course understood to 

 have been made, and without thinking it necessary to add that the numbers used represent 

 the departures of reduced gravity-acceleration values from a standard value in thousandths of 

 a centimeter per second of time; and particularly without explaining that a positive anomaly 

 means that the reduced value of the locally observed gravity is stronger than standard gravity, 

 or that a negative anomaly means that it is weaker. The geodetic form of statement is excellent 

 for geodesists, but when a geologist hears that gravity at Pikes Peak has a positive anomaly 

 of 0.078, he generally has to think twice — or oftener — before clearly conceiving just what is 

 meant. Gilbert was good enough to rephrase these mystic figures in what he called "rock- 

 feet," or the thickness of a sheet of rock, the attraction of which causes local gravity, corrected 

 for altitude and latitude, to differ from standard gravity; or, in other words, the number of 

 feet by which the earth crust is in excess of isostatic requirements at a station where gravity is 

 too strong, or in defect where it is too weak. It is convenient to remember that the excess or 

 defect of gravity expressed in thousandths of a centimeter will if multiplied by 30 give the 

 excess or defect in rock-feet. The excess or deficiency may mean either that the land surface 

 is too high or too low, or that the rock column below the station is too dense or not dense enough. 



Thus stated, it appears that for the 11 stations in the great interior plain the average excess 

 or defect of the crust is only 240 rock-feet. As to other stations, Gilbert says regarding Pikes 

 Peak, a mountain 14,084 feet in altitude, and Gunnison, a deep valley at an altitude of 7,677 

 feet: "Gravity at the two stations exceeds the isostatic requirement by 2,300 and 2,200 rock- 

 feet," and that is exactly what geologists wish to know when isostasy enters their discussions. 

 As to the two high-level stations just mentioned, he goes on to say: "The evident suggestion 

 is that the whole Rocky Mountain plateau, regarded as a prominence on a broader plateau, is 

 sustained by the rigidity of the lithosphere." Similarly three stations in Yellowstone Park 

 "indicate gravitational excesses of 1,500, 1,800, and 2,300 rock-feet." In a closing summary 

 the Rocky Mountain region from Pikes Peak to Salt Lake City is said to have an excess of 

 1,345 rock-feet and this is taken to indicate a "greater sustaining power than is ordinarily 

 ascribed to the lithosphere by the advocates of isostasy." Washington and Philadelphia, 

 standing on the "fall line " at the inner border of the Atlantic coastal plain, were similarly found 

 to have an excess of mass of 1,600- and 1,300 rock-feet, respectively; hence "a study of the 

 anomaly at the fall-line promises to throw light on the general question of isostatic adjustment 

 between regions of progressive degradation and deposition." 



This instructive discussion was shortly followed by a concise summary of the same conclu- 

 sions for geological readers under the title of " New light on isostasy": 5 



The whole [Rocky] mountain mass above the level of its base [where it adjoins the Great Plains] is in excess 

 of the requirement for isostatic adjustment; or, in other words, is sustained by the rigidity of the earth . . . 

 These results tend to show that the earth is able to bear on its surface greater loads than American geologists, 

 myself included, have been disposed to admit. They indicate that unloading and loading through degradation 

 and deposition cannot be the cause of the continued rise of mountain ridges with reference to adjacent valleys, 

 but that, on the contrary, the rising of mountain ridges, or orogenic corrugation, is directly opposed by gravity 

 and is accomplished by independent forces in spite of gravitational resistance. 



« Journ. Geo!., iii, 1895, 331-334. 



