354 A. C. LANE- — WEIGHT OF SEDIMENTARY ROCKS 



ity, while it is not hard to turn them into the commoner form, in English 

 engineering books, of pounds per cubic foot, by dividing by 16. 



Beside the pores, Becker has remarked 3 that an appreciable volume 

 may be cracks. Yet there are many practical and theoretical problems in 

 which it is the volume of the rock and the proportions per unit of volume 

 that we should know. It may be the practical question of how much 

 there is in a given vein. It may be the theoretical one of the rate of 

 growth of a delta .at the mouth of a river bearing a certain weight of 

 sediment to the sea each year. In particular we have the question, im- 

 portant in discussion of isostasy, as to what weight and attraction there 

 may be in a certain thickness of rock. 



General engineering Data ox porous and pragmental Rooks 



AVe find some data in engineering handbooks like Trautwine, Peele's 

 Mining Engineer's Handbook, 4 Lefax, Macdonald, etcetera. Warren J. 

 Mead has also an article in Economic Geology, volume III, number 4, 

 1908, with an abstract in Lefax, sheet 7-355. 



If a sedimentary rock is considered as a fragmental rock or clastic 

 rock — an igneous rock all broken to pieces— we find that we may expect 

 an increase of volume in the pile of pieces of about 50 per cent — that is, 

 a porosity of 33 per cent and a decrease in weight per volume to about 

 two-thirds. If its original weight was 2.7 tons per cubic meter, 2,700 

 ounces per cubic foot, its weight dry would be about 1.800 ounces per 

 cubic foot, or 2,200 ounces if filled with water. 



A. B. Macdonald gives, reducing to ounces per cubic foot : 



In place. Broken. 



Trap 2.992 1.712 



Granite 2,720 1,552 



Sandstone 2.416 1,376 



Earth and clay, 50 per cent more than undisturbed. 



It is important in molding sands to have a high porosity to let the 

 gases escape, and, as Ries 5 and Rosen's figures show, they approached 

 Moore's theoretical maximum — that is, 34 tests of molding sands give: 



Weight in tons per 

 cubic meter. 



f A_ 



Gm Porosity. Wef. Dry. 



Average 2.63 41 1.96' 1.55 



Maximum 2 . 73 45 .9 



Minimum 2 . 54 28 



8 G. F. Becker : Proceedings of the Wash. Acad, of Sci., vol. vii. 1905, pp. 2T4-JT.". 

 See. also, his note in the journal of the same academy, vol. iv, September 19, 1014, pp. 

 4L , 9-44: , >. 



1 Robert Peele. Wiley & Sons, 1918. 



