Maryland Geological Survey 121 



reached much sooner with the coarse than with the finer sizes. As the 

 coarse material was, besides, usually less abundant the coarsest size was 

 not generally shaken more than a minute, while the finest, that is, that on 

 the 200-mesh sieve, when it was abundant required sometimes more than 

 half an hour. The amount of shaking that each size received depended 

 on the abundance of the material of that size, the sizes being successively 

 removed from the nest of sieves, in the order of their fineness, while the 

 finest was continued until observation, with the hand lens, of the material 

 passed showed that predominantly elongated grains were coming through. 

 The sieves were shaken by hand. The Department of Agriculture uses a 

 mechanical shaker in which the sieves are left for about three minutes. 

 Thoulet's principle is to continue shaking until a considerable shaking 

 passes only a negligible amount of material, 1 as it would require an 

 excessive length of time to produce an absolutely complete separation 

 of the finer sizes. But his limit, which is also only approximate, agrees 

 quite closely with the present, since, when dominantly elongated grains 

 come through, the rate of separation is very slow. The products of sieving 

 are weighed and put aside for study. 



Finally the " very fine sands " are separated according to their specific 

 gravity by means of Thoulet's solution, of a density slightly greater 

 than 2.7. The most serious defect of this separation in the rocks studied 

 was due to the glauconite. Fresh glauconite is lighter than all the feld- 

 spar and quartz, so that it remains in the light portion and can subse- 

 quently be in turn separated by its density. But in all the glauconitic 

 rocks considered in the following the greater part of the glauconite sank 

 with the " heavies " and was made up of grains ranging in density in 

 many cases from less than 2.7 -f- to higher than 3.00. This is doubtless 

 due to weathering effects. An exact determination of the amount of 

 glauconite by weight was therefore impossible, and even the fairly close 

 approximations that were obtainable with a solution of specific gravity 

 of 3.00 and the electro-magnet to be mentioned below, are not quite com- 

 parable on account of the difference in density of the lots from different 



1 Thoulet, J., Precis d'analyse (op. cit.), p. 64. 



