SUM. MARY 



369 



is, excess of gravity — observed in certain small Precambrian areas, that 

 the rocks near the surface weigh over 2,670 ounces per cubic feet, seems 

 entirely pertinent. 



My suggestion 17 that the Lake Superior basin is in isostatic equilib- 

 rium, and that the Huron Mountain granite, with a weight of some 2,650 

 ounces per cubic foot, is balanced by some 20,000 feet of Keweenawan 

 traps underlying Lake Superior, with a weight of 2,850 ounces per cubic 

 foot, implies a deviation of gravity toward the lake which really seems to 

 exist. 18 



Summary 



The mean weight of wet sedimentary rock probably varies from less 

 than two tons per cubic meter, equivalent to 2,000 ounces per cubic foot, 

 upward, as the water is squeezed out, and there is a good deal of connate 

 water to be squeezed out, as Johnson has remarked. Glacial brick-clays 

 may serve as a good average. AVeighing a sample with its natural mois- 

 ture, then dipping in paraffin and weighing again and when immersed in 

 water, is a rapid way of making tests, which agrees with other methods. 

 More data as to volume weights are desirable, although quite a number 

 are assembled. 



The suggestion is confirmed that the abnormally low gravity in the 

 Coastal Plain of the United States and. in the Ganges Valley in India is 

 due to a lightness of the recent and upper sediment which has not been 

 fully allowed for, and the" suggestion is made that as gas- and oil-filled 

 strata are yet lighter than water-filled, they also may be marked by 

 anomalies of gravity. 



17 Michigan Geological Survey, 1906, p. 485. 



18 Plate 18, Coast and Geodetic Survey Publication 40. 



XXIV — Bull. Geol. Soc. Am., Vol. 33, 1921 



