198 Mr. H. C. Sorby on the Organic Origin 



optical properties, are exactly similar to the Coccoliths of the 

 Atlantic mud. When made to turn round, they both are seen 

 to be concave on one side and convex on the other, as shown by 

 the oblique side view of an unusually large one from the chalk, 

 fig. 5 (magnified 800 linear) ; and they give the same kind of 

 well-defined black cross with polarized light. Hence we must 

 abandon the idea of their being "peculiar to the chalk," and 

 may possibly be rather led to conclude that they are character- 

 istic of deep-ocean deposits. Many of those in the chalk have a 

 decided granular character, as shown in fig. 2 (magnified 800 

 linear). The rings, instead of being simple, are, as it were, made 

 up of separate beads; and the centre is also of a compound 

 granular character, with various modifications. Judging from 

 Ehrenberg^s drawings, and from what he says at p. 136 of his 

 paper on Chalk and Chalk-marl, he appears to look upon this 

 granular structure as their universal character, and concludes 

 that their minute constituent granules were derived from decom- 

 posed Foraminifera, and were afterwards arranged into crystal- 

 loids by means of some unknown ci'ystalloidal force. However, 

 as already stated, some show no such granular structure, but 

 are precisely similar to those in the Atlantic mud ; and the 

 granular constitution of the others admits of a very simple ex- 

 planation. As is well known, when shells become fossil, they 

 often acquire a crystalline texture ; and, in fact, this occurs in 

 the recent dead shells found in the mud of the Mediterranean, 

 described by Marcel de Serres and Figuier*. I have also suc- 

 ceeded, beyond all expectation, in producing artificially the same 

 change in recent shells by keeping them for a month or two in 

 a dilute solution of caustic potash, at a temperature of about 

 145° C. (293° F.), which, by dissolving the organic matter, per- 

 mits the carbonate of lime to crystallize according to a new 

 arrangement ; and not only do shells consisting of aragonite 

 undergo this change, but also sometimes those made of calcitef, 

 though, in the case of fossils, it has often only occurred in those 

 composed of aragonite. If such a molecular re-arrangement 

 were to take place in the Coccoliths of the Atlantic mud, they 

 would become almost exactly like the granular specimens found 

 in the chalk ; and I shall be much surprised if I do not succeed 

 in imitating them by such artificial means as I have just de- 

 scribed. 



* Annates des Sciences Nat. 3 ser. 1847, vii. 21 ; Comjites Rendus, 1846, 

 xxii. 1050; Neues Jahrbuch fiir Mineralogie, 1848, S73; Edinburgh New 

 Phil. Journ. 1847, xlii. 381. 



t See Rose's second treatise on Carbonate of Lime, Abhandlungeii d. 

 k. Ak. d. Wiss. zu Berlin, 1858, 63, since confirmed and extended by my 

 own exj eriments. 



