PROCEEDINGS OF THE ACADEMY AND AFFILIATED 



SOCIETIES 



THE PHILOSOPHICAL SOCIETY OF WASHINGTON 



The 786th meeting was held March 17, 1917, at the Cosmos Club, 

 President Buckingham in the chair; 51 persons present. The minutes 

 of the 785th meeting were read in abstract and approved. 



Mr. William Bowie read a paper on Our present knowledge of isos- 

 tasy, which was illustrated by a number of slides. After reviewing 

 briefly the conclusions reached by Prof. Hayford, Mr. Bowie discussed 

 the recent researches in gravity and isostasy. 



In these investigations the results obtained at 219 stations in the 

 United States, 42 stations in Canada, 73 stations in India, and 40 

 stations in other countries, were used. Isostasy seems to be about 

 as complete in each of the countries where extensive data were avail- 

 able as in the United States, although it was not possible to make 

 certain tests for the other countries, owing to lack of certain detailed 

 data. 



Isostasy in the United States, for the whole area, is practically 

 perfect. For local areas gravity is, on an average, 0.020 dyne from 

 normal. This anomaly corresponds to a mass of material of indefinite 

 horizontal extent and thickness of about 600 feet. Whether this 

 average deviation from normal in gravity is caused by a lack of 

 isostatic compensation or whether it is caused by the presence of 

 extra light or extra heavy material in the outer portion of the earth's 

 crust close to the station is a problem that cannot be entirely solved 

 by the use of present data. 



It was found, during the investigations of the Coast and Geodetic 

 Survey, that the gravity anomaly showed some relation to the geo- 

 logical formation on which a station is located. For instance, on the 

 pre-Cambrian formation, where the rocks are denser than normal, 

 gravity is in excess at nearly all stations. For the stations on the 

 Cenozoic formation, the density of the material of which is somewhat 

 less than normal the anomaly tends to be negative and all of the larger 

 anomalies of those stations are negative. There was not found any 

 relation between the gravity anomaly and the densities of the materials 

 of the Mesozoic and Paleozoic formations. 



Various tests were made for determining the most probable depth 

 of compensation. The result was a depth of 95 kilometers, but the 

 speaker emphasized the fact that this depth may be very indefinite 



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