250 



THE GEOLOGIST. 



degrees of fineness, is required. (See figures.) This sieve may be of 

 block-tin or zinc, the latter is preferable. The best consists of two 

 cylinders about three inches across, and rather more high, one fitting 

 v/ithin the other (a and e) ; each having its lower edge in-turned for 

 o.bout an eighth of an inch (I and f), so that, by pressing down the 

 internal cylinder, a circular plate of perforated zinc f c and dj, made 

 to fit the interior of the larger cylinder, is firmly held between the 

 bottom rims of the tv/o cylinders. There should be seven or eight 

 ■jjcrforated plates of different degrees of fineness ; such sieves may be ob- 

 tained of Mr. Snow, zinc-worker, 50 Millbank-street, Westminster, at 

 a cost of OS. each. In using the sieve, especially in wet siftings, care 

 cliouid be taken to keep the apparatus wiped clean and dry, so that 

 none of the fine particles of the different materials should become mixed 

 Avith each other; and special attention is required to prevent the fine 

 sand getting between the two cylinders, Vv^hich would render it very 

 difficult to separate them. 



The muds and sands obtained by dredgings and soundings, as well as 

 clays, kc, brought up by tlie anchor, maj^ be prepared in a manner 

 similar to that recommended for the manipulation of other clays. 



Picked specimens from clays and sands may be sometimes further 

 cleaned by being boiled in a weak solution of potash, in the proportion 

 of about one part of licpior potasscc, v/ith nineteen parts of water, or more 

 of the potash if required, according to the coarseness and foulness of the 

 specimens, up to one part in four, in a fiask over a spirit or gas damp for 

 ten minutes or more. 



_ To prepare chalk for microscopical examination, if only small quan- 

 tities are to be treated, the best plan is to select a piece of soft white 

 chalk, — that which has been kept some time in a cabinet is most easily 

 worked, especially chalk from the interior of an Ananchyte or Galcrite, 

 and to wash it with a moderately soft nail-brush in a hand-basin half 

 full of w^ater, keeping the chalk and brush just under the surface, so 

 that the loosened particles siiould all fall in the water. To prevent 

 the chalk from being worn into longitudinal furrows, the part under 

 the brush should be constantly moved round. It should also be looked 

 at occasionally with a hand-magnifier, so that any larger specimens of 

 bryozoa and foraminifera may not be destroyed by rough brushing, but 

 be more carefully picked out with a needle or penknife, or separated by 

 the careful use of the brush. When a sufficient quantity has been 

 brushed down, the water should be stirred and the chalky portion 

 poured away, or water may be allo^ved to run into and overflow the 

 basin, until the water remains clear over the sediment. The latter should 

 then be placed in a round-bottomed tea- cup or gallipot, and rubbed gently 

 with the finger in water, until, by further vrashing, some more of the 

 chalk has been removed, when the residue should be carefully dried in 

 an oven, or otherwise, without the least disturbance. AYhcn dry it 

 may be sifted and picked; but if it bo still chalky," further mani- 

 pulation by gentle rubbing in water will be necessary, especially if the 

 r.aulk-dust has been put by for some time after having been prepared. 

 On a larger scale, chalk may be prepared by being broken up in a 

 mortar, or roughly ground with a brick, stone, or roller, on a pavement, 

 and then washed m water, the larger fragments being excepted. 



