352 



SCIENCE 



[N. S. Vol. XLV. No. 1163 



1912 map, wliicli was based on 124 stations 

 only. 



Mr. Bowie's general conclusion in regard 

 to isostasy in the United States is as follows : 



The group of publications of the Coast and Geo- 

 detic Survey dealing with deflections and gravity 

 values shows that isostasy exists in a form nearly 

 perfect in the United States as a whole, also that 

 there is nearly perfect isostasy in areas which 

 form comparatively small percentages of the area 

 of the entire country. 



The abundant evidence presented supports 

 this conclusion fully. It is important to note 

 that a conclusion of this general character 

 was originally based, not much more than ten 

 years ago, on a small fraction of the evidence 

 now available and that although the amount 

 of evidence has increased greatly, and the 

 evidence has been studied much more in- 

 tensely, the general conclusion has simply been 

 confirmed abundantly and repeatedly — being 

 modified only in details. 



The largest anomaly noted in the United 

 states to date, at Seattle, Washington, — .093 

 dyne, was recognized as a very large anomaly 

 as early as 1910, when only 89 gravity stations 

 were available in the United States. Since 

 that time with an increase in the number of 

 stations to 219 it is interesting to note that 

 no other anomaly larger than .059 dyne has 

 been found in the United States. So also 

 when the study was extended to Canada (42 

 stations) the largest anomaly there discovered 

 was only — .045, and in India (73 stations), 

 the largest discovered was — .078 dyne. In 

 other parts of the world only four anomalies 

 have been found which are larger than that at 

 Seattle. Each of these four is on an oceanic 

 island or near a steep continental shore. The 

 anomalies referred to in this paragraph are 

 the residuals after topography and isostatic 

 compensation have been taken into account in 

 the computation. The extension of the com- 

 putation of isostasy, in a form based upon a 

 few stations in a restricted area, to other sta- 

 tions and to other areas, shows an agreement 

 with the facts which is substantially as close 

 at the new stations and in the extended area 

 as at the original stations in the original area. 



Two additional gravity stations having now 



been occupied near Seattle, it is evident that 

 this largest anomaly in the United States, 

 — .093 dyne, is a comparatively local matter, 

 not a characteristic of a large area. The 

 writer imderstands that additional unpub- 

 lished observations emphasize this statement 

 still further. 



It is evidently desirable to supplement 

 studies of gravity made by means of stations 

 scattered widely, and somewhat uniformly, 

 over a large area such as the United States, 

 by intensive studies of selected small areas in 

 which a large munber of stations are con- 

 centrated. The beginning of one such study 

 has been made in the vicinity of Washington, 

 D. 0., where observations have been made at 

 seven stations within twenty miles of the 

 capitol. 



No discernible general relation is found be- 

 tween areas of recent erosion or recent deposi- 

 tion, on the one hand, and areas of negative 

 anomaly or of positive anomaly, respectively, 

 on the other hand. 



The strength of the proof of the validity of 

 the method of computation used in the Coast 

 and Geodetic Sm-vey for taking into account 

 the effects of topography and isostatic com- 

 pensation upon the intensity of gravity de- 

 pends in part upon the extent to which the 

 remaining anomalies seem to be independent 

 of the topography around the stations. Hence 

 special intensive studies upon this point have 

 been made repeatedly at the survey. The 

 latest evidence, as set forth in this publica- 

 tion, shows the remaining anomalies to be 

 almost, if not quite, independent of the topog- 

 raphy. There is a slight apparent tendency 

 for coast stations to have negative anomalies. 

 But the indications are that this tendency is 

 due to the prevalence of Oenozoic formations 

 along the Atlantic and Gulf Coasts of the 

 United States, not to the nearness of the sta- 

 tions to the coast. 



No other method of computation which has 

 been published gives anomalies which are thus 

 independent of the topography. 



There is found to be a decided tendency for 

 gravity stations located on Cenozoic forma- 

 tions in the United States and in India to 



