106 P.G.H. Boswell—Quantitative Methods in Stratigraphy. 
II. Prrronogicat Meruops. 
The usual petrological methods of treating sediments by panning, 
and the use of heavy liquids, divide samples into two important and 
convenient crops of densities respectively above and below a mean of 
about 2°8. Of the liquids in common use, bromoform, when it can 
be obtained, appears to be most convenient. Operations with it are 
clean, and, what is of great importance when hundreds of samples 
are being examined, very rapid, the time taken in washing with 
benzene, and drying, being a minimum. Its mobility is a valuable 
asset, but has given rise to the objection that separations are rendered 
less complete on account of convection currents which are set up in 
it by slight differences in temperature. If necessary, care can be 
taken to avoid these, and in the writer’s experience separation is 
quite as good, if not better, than with the more viscous aqueous 
heavy solutions, where the process takes longer and the grains move 
less freely. The only real objection to the use of bromoform is its 
loss by evaporation during separation, and while the washings (in 
benzene) are being concentrated. ‘Theoretically, bromoform may be 
used over and over again without end, but practically there is a slow 
and steady loss. This may be obviated to some extent by the use of 
separating funnels stoppered at the top, but they are usually too small 
to accommodate a sufficiently large quantity of sediment, the sand, 
etc., tends to hang round the sides, and the subsequent washing is often 
troublesome. An ordinary funnel fitted with a glass stopper or a rubber 
tube and clip leaves little to be desired; but with bromoform a large 
surface of liquid is exposed to evaporation. The funnel may therefore 
be covered by a clock glass. Mercury potassium iodide is the most 
suitable of aqueous solutions, crystallizing out less rapidly and being 
less viscous than Klein’s solution (cadmium boro-tungstate). It is 
very poisonous and has a corrosive action upon the skin, but these 
objections are not serious with clean working, for the liquid need 
not be touched. It should, however, be kept from air, and in 
contact with alittle mercury. While not as convenient as bromoform, 
for it is less mobile, and necessitates washing with water, and 
consequent loss of time in washing and drying, it is the best substitute 
when that liquid is procurable with difficulty (as in 1914-15). It 
may be quickly prepared in the laboratory from mercuric and 
potassium iodides, and, like other aqueous solutions, may be used 
and recovered repeatedly without appreciable loss. For the further 
separation of the heavy crop ( > 2-8) methylene iodide, of density 
about 3°33, is very convenient. The sediment is washed with benzene 
and the separation is therefore rapid. Its expense is an objection, 
but small quantities only need be employed for the (usually small) 
heavy crops. 
The portion of density > 2°8 contains the heavy detrital minerals 
which are of greatest interest, beauty, and value from a strati- 
graphical point of view. Further separations may be made from this 
crop by electromagnetic and electrostatie methods, by the use of 
heavier liquids, and by hand-picking. 
Apart from the included rock-fragments and compound grains, 
sediments exhibit considerable variation in petrology. Most of the 
