238 Bowie — Gravity Anomalies and Geologic Formations. 



is not held up by the rigidity of the earth's crust, and that no 

 area within the United States, as large as the State of Ohio, 

 for instance (41,000 square miles), is so supported. 



On the whole, the gravity anomalies in the United States are 

 very small, and there is, apparently, no connection between the 

 size and sign of the anomaly and the character of the topog- 

 raphy. On the other hand, there is, apparently, a relation be- 

 tween the surface geologic formation and the anomalies. 



Of the 124 stations in the United States, 10 are in areas 

 of Precambrian formation, 31 are in Paleozoic, 20 are in 

 Mesozoic, 30 are in Cenozoic, 11 are in Effusive or Intrusive, 

 and 22 are unclassified. The geological map of North Amer- 

 ica, by Gannett and Willis, of 1906, was used in making 

 decisions as to the geologic formation at the stations. 



Those stations were left unclassified which plotted on the 

 map near the dividing line between two formations, or when 

 in such areas as at Ely and Winnemucca, Nevada, where 

 there are rocks of several geologic ages within a few miles of 

 the station. 



The table shown below gives the means of the anomalies in 

 dynes (or centimeters) with and without regard to sign, for the 

 several groups of gravity stations. The average gravity in the 

 United States is about 980 dynes or centimeters. 



Geologic Number of 



Formation Stations 



Precambrian 10 



Paleozoic 31 



Mesozoic 20 



Cenozoic 29 



Intrusive and Effusive 11 



Unclassified 22 



All stations 123 



The mean with regard to sign for all anomalies is zero, and 

 without regard to sign it is -019 dyne. The anomalies in the 

 oldest formation have a comparatively large plus mean with 

 the sign considered. Eight of the ten Precambrian stations 

 have plus anomalies. The average size of the anomalies in the 

 oldest formation is much larger than the mean of all. The 

 most recent formation shows a decided minus anomaly with 

 regard to sign. The formations of the intermediate ages have 

 anomalies which are practically normal. The intrusive and 

 effusive anomalies have a mean of — *007 with regard to sign, 

 but the average size is much smaller than the mean of all. 

 The mean of the intrusive anomalies is — *003 and of the effu- 

 sive is — *011. The number of anomalies in each of these 



Mean with re- 



Mean without 



gard to sign 



regard to sign 



+ '016 



•026 



— •003 



•019 



+ •002 



•015 



— •008 



•021 



— -007 



•015 



+ •011 



•020 







•019 



