of the so-called ' Crystalloids' of the Chalk. 197 



izing action, that I veiy much doubt its calcareous nature. The 

 individual Coccoliths, when on the spheres, or, still better by far, 

 when detached, each give an extremely well-defined black cross; 

 and their depolarizing action is much too powerful to allow us 

 to suppose that this is due to the same arrangement of the car- 

 bonate of lime as in the shell of Globigerina?, and that the 

 Coccoliths are the commencement of calcification. At the same 

 time it is not impossible that they might come off from the cells 

 before general calcification took place; and I have found some 

 shells of Foraminifera which showed imperfectly-defined oval 

 bodies, giving black crosses with polarized light, thus proving 

 that such a radiate arrangement of the carbonate of lime as that 

 in Coccoliths does occasionally, though rarely, occur in the shell 

 of Foraminifera. With respect to the 

 individual Coccoliths, their optical cha- 

 racters prove that they have an extremely 

 fine radiating crystalline structure, as if 

 they had grown by the deposition of car- 

 bonate of lime on an elongated central 

 nucleus, in accordance with the oval 

 ringed structure shown in fig. 1 (magnified 800 linear). 



In order to obtain a satisfactory knowledge of chalk, we should 

 commence with the study of thin sections of the harder varieties. 

 I am not aware that any one but myself has employed this me- 

 thod of research, biit I have by this means succeeded in proving 

 most completely that entire Foraminifera are comparatively rare, 

 and make up only quite a small proportion of the whole. More 

 or less detached and broken cells are, however, very numerous, 

 so much so that in some cases they are almost in contact through- 

 out the whole mass, and it is only the spaces between them that 

 are filled with fine granular matter, which in some other speci- 

 mens constitutes nearly the whole rock. In general, however, 

 the constitution of chalk is intermediate between these two ex- 

 tremes. The nature of the granular matter is best learned by 

 an examination of those vei'y soft specimens which have not been 

 much altered since deposition. When seen in water, under a 

 bit of thin glass, with a power of from 400 to 800 linear, it is 

 easy to perceive that a considerable part is made up of the de- 

 composed tissue of Foraminifera. There are often also small 

 well-defined groups of radiating crystals, similar to those named 

 by Ehrenberg ' Krystaldrusen,^ and figured on pi. xxv. b. 12-15 

 of his ' Microgeologie ;' the nucleus is sometimes a minute 

 fragment of the decomposed tissue of Foraminifera ; and there 

 can be no doubt respecting their crystalline and inorganic origin. 

 They, however, differ entirely from the well-defined oval bodies 

 hitherto described as chalk- crystalloids. These, in form and 



