DATA OX FRIABLE MATERIAL 363 



(assuming the specific gravity of the calcite to be 2.7) 1,185 ounces per 

 cubic foot as a general estimate, while in particular cases of denser bog- 

 Kme we find it 1,373 to 1,760, and at Zukey Lake, not far from Ann 

 Arbor, 1,630 ounces per cubic foot dry. 



Two lumps from the "shell marl" furnished by Ward from Monroe 

 County, New York, to Tufts College gave by the paraffin method 1,140 

 and 1,390 ounces respectively per cubic foot dry. 



A specimen from the British Oligocene, one surface of which is covered 

 with the little snail Melania turritissima, on a compact cream-colored 

 base, from Hempstead, and presumably of the Bembridge marls under 

 the Hempstead beds, as described in Geikie's Geology, fairly firm, gave 

 1,890 ounces dry and 2,200 ounces wet. 



A dry impure peat gave 680 ounces, the clay beneath it 840 ounces per 

 cubic foot, the same as the Eio Grande silt cited by E. Wesley Shaw. 14 



In the United States Water Supply Paper 234, page 81, it is estimated 

 that dry river mud weighs from 800 to 2,000 ounces per cubic foot. As 

 this involves shrinkage in drying, if the water were expelled by the gen- 

 eration of gas the rock might for a time be even lighter. 



New Data from Tests on glacial Brick-clays 



In view of the facts above cited, the difficulty of handling friable sam- 

 ples, the consequent scarcity of data regarding them, and the difficulty 

 of distinguishing mineral specific gravity and volume weight in refer- 

 ences, it seems quite possible that J. Barrell, 15 who has been followed by 

 Bowie, while he has nearly correct figures for the igneous rock, which 

 may probably apply to crystallines generally, has not allowed enough for 

 the porosity of other rocks, and hence has too high a weight for the aver- 

 age Cenozoic rock of probably nearly 20 per cent for the first mile or so 

 of rocks, especially those above sea-bottom and relatively undisturbed. 



It occurred to me that, to get a fair idea of what the average weight of 

 recently formed sediments might be, glacial clays offered special advan- 

 tages. Being not closely assorted, they might indeed be a little more 

 closely packed than the average sediment, . since close sorting tends to 



14 Science, vol. 38, 1913, p. 554. 



15 Journal of Geology, vol. 22, p. 219, gives the following figures: 



Gin 



Average igneous rock 2 . 89 



Shale 2 . 69 



Sandstone 2 . 67 



Limestone 2 . 76 



Cenozoic rocks 



Porosity. 



W 



'eight with pores half 

 full, tons per cubic 

 meter. 



0.2 







2.8 



8 







2.51 



15 







2 . 35 



5 







2 . <S4 

 2 . 67 



