NEW DATA FROM TESTS ON GLACIAL BRICK (LAYS 367 



Weight, dry, in tons 

 Moore's description. per cubic meter. 



1. Black River limestone, residual clay 1.766 .021 10 students' tests 



2. Upper Cambrian, Galesbury residual clay . . 1.72 2 

 o ^ 1.82 2 



7. \ 15 ° 2 



5 f Various stream clays in a limestone country, number 5 not tested 

 e'.J 1-78 7 



7. Stony wash from the Readville shale 1.97 6 



s. Juniata clay 1 .93 5 



1.82 34 tests 



Applications to gkavitattve Anomalies 



The diagram (figure 1 ) shows the connection between the weight per 

 cubic volume wet and the weight per cubic volume dry, or with the pores 

 filled with gas, assuming no shrinkage and assuming the weight of the 

 rock substance 2.8. It is easy to see how it can be modified to suit other 

 specific gravities of rock substances, and easy to see, also, that one may 

 expect to find, for sediments which are not too coarsely packed and com- 

 pressed, a weight about 2 / 7 to 4 / 7 that of the mineral substance, not 

 far from two tons per cubic meter. This will be less if the pores are filled 

 with gas, and it is not inconceivable that oil and gas might be detected 

 by an anomaly of negative character — that is, an abnormally low attrac- 

 tion of gravity above. G-. F. Becker 16 thought he noted some parallelism 

 between magnetic anomalies and oil. The anomalies due to oil and gas 

 would, however, be slight. 



It has been suggested that the abnormally low effect of gravity on the 

 coastal plain of the United States and in the Ganges Valley in India was 

 due to a lightness of the recent sediment which had not been fully al- 

 lowed for. Let us go into this a little. 



The force of gravity at various stations is not observed to be exactly 

 that which would be due to the attraction of the mass of the earth as a 

 rotating spheroid with the "mass that it has. This is due, in part anyway, 

 to the immediate effect of the attraction of the masses near by, just as we 

 find the mass of the earth by finding the effect on a pendulum of a big- 

 lead ball near by. 



Now Bowie and Hayford have compared the observed force of gravity 

 with that obtained, first taking no account of the local masses, only of the 

 elevation of the station from the center of the earth. This is equivalent 

 to assuming that the elevations of landmasses are in their effect on gravity 



iK U. S. Geol. Survey Bulletin 40. 



