332 H. A. Baker — Loose Arenaceous Sediments. 



since errors in the determination of c are accumulative, the c of one 

 stage of the analysis becoming the a of the next. 



In the earlier stages of an analysis, when washing ofE the very 

 finest material, the method outlined above is not practicable. Wide 

 and heavy tubes, such as cannot be suspended from the arm of a 

 laboratory balance, are usually then in use. Moreover, much of the 

 material removed consists of clay or lithomarge, hydrated silicate of 

 alumina, and consequently the figure 2"65 for its density is inad- 

 missible. The writer determined the mean densities of a number of 

 Lower Tertiary sands of different types, and found that in sand 

 containing a fair percentage- weight of the clay grade the average 

 density dropped rather markedly. A quartz sand containing 4 per 

 cent by weight of clay has an average density of 2'60, and the average 

 density decreases below this figure at about •0015 for every 1 per cent 

 weight increase of the clay grade. A quartz sand with 20 per cent 

 by weight of clay has an average densit}^ of 2"574. The Lower 

 Tertiary sands, when washed free from clay, had the same density 

 as quartz (2"65). 



Another method, which avoids loss of time in the drying of residues, 

 is as follows : On the completion of a stage of the elutriation process 

 the residue is run out from the tube into a glass flask filled to a mark 

 on the neck with water. The weight of water required to fill the 

 flask to the mark is known. When the sediment is in the flask 

 the water displaced by it is removed, so that the water stands at the 

 mark once more. A weighing is then taken. The difference between 

 weight of sediment with water to mark and weight of water to mark 

 gives the weight of the sediment in ivater. The weight of this 

 sediment, when dry, is then calculated from the formula : — 



Weight (gms.) = . . X weight in water (gms.) 



where d = density of the material. 



As already stated, if the clay material has been removed from the 

 sample, the density of arenaceous deposits may safely be taken as 

 2' 65. The above formula assumes the density of the water to be 

 1, but the error involved is negligible in practice. 



This method is particularly useful when elutriating arenaceous 

 sediments after the clayey material has been removed. It is then 

 very convenient to calculate the initial weight of sediment from the 

 formula given above. If an initial weight of 40' 15 gm. be taken 

 this will weigh exactly 25 gm. in water, and the successive weights 

 in water obtained afterwards have only to be multiplied by four in 

 order to give percentages. 



(To be continued.) 



