CHALK WITH BELEMNI FELLA QUADRATA. 359 



(2) By powdering tl e rock and washing in water the brown grains 

 are readily separated from the white mud in which they are em- 

 bedded. This mud has been ascertained by Dr. (x. J. Hinde to 

 consist mainly of coccoliths, discoliths, and rhabdoliths. ' The 

 greyish brown sand which is left behind includes, in order of 

 abundance, foraminifera wholly or partly filled with a brown 

 material, hexagonal and rounded prisms broken from the shells of 

 Inocerami, angular fragments of a clear amber-coloured material of 

 waxy lustre, and little oval pellets. Rotated under crossed nicols 

 many of the foraminifera show faint double refraction, a perfect 

 black cross being centred on each chamber. In those which have 

 been filled with the brown material, the terminations only of the 

 arms of the black cross are visible in the clear layer representing the 

 test, the brown mineral itself giving no figure. Some of the chambers 

 are empty, as is shown by their containing a bubble of air. Others 

 are partly filled with the doubly refracting mineral, presumably 

 crystaUine carbonate of lime, and partly with the brown material, the 

 boundary between the two being ragged and ill-defined. The hexa- 

 gonal and rounded prisms also present in part the optical properties 

 of crystalline carbonate of lime, but have been to a great extent con- 

 verted into the brown opaque material which fills so many of the 

 foraminifera. The replacement, however, has been still more 

 irregular owing to the absence of the confinement afforded by the 

 cell-walls. Lastly, some of the transparent amber- coloured fragments 

 show faint double refraction, which may, however, be merely a 

 strain-phenomenon. Many of the brown grains, and especially the 

 foraminifera, have a lustre resembling that of varnish. This lustre 

 is a marked feature in all the phosphatic chalks which I have 

 examined. 



(3) Acetic acid dissolves about two-thirds of the rock, but leaves 

 nearly the whole of the phosphate of lime, with a considerable 

 proportion also of carbonate of lime. The residue has a marked 

 brown colour, and is seen under the microscope to consist of those 

 foraminifera and prisms of Inoceramus-she]l that have been filled 

 with or preserved in the brown material, which may be concluded 

 therefore to be the phosphate of lime. The chambers are still 

 surrounded by a clear layer or test, and a few show in this layer the 

 terminations of the arms of a black cross. The contents give no re- 

 action with polarized light. If the action of the acid is watched 

 under the microscope, the carbonic acid can be seen escaping from 

 the foraminifera by occasional vents, and not from the whole 

 surface of the test. Similarly the prisms are attacked unequally, 

 the action being confined to a spot here and there, and the final 

 result being to give the prism an eroded appearance. A foraminifer 

 and a prism, treated separately under the microscope with concen- 

 trated nitric acid, dissolved with eff'ervescence, and the solution gave 

 a copious yellow precipitate with ammonium molybdate, thus con- 

 firming the view that the residue from acetic acid consists largely 

 of phosphate of lime. 



The angular chips of amber-coloured material also survive the 



