THE DEPOSITS OE THE SEA-BOTTOM. 



87 



where they intersect the 16 lines thus produced, we may by means of these directions construct the 

 position of the threads, as it has been done in the figure below. 



The letters «, b, r, and d indicate the positions of the corresponding crosses in the preceding 

 figure. It will be seen that the central ring is built of a system of threads of calcite, somewhat 

 spirally twisted to the right, while the black crosses are twisted to the left. On the sides of the ring 

 the direction of the threads changes only little, and consequently the branches of the cross are here 

 very thick, while, on the contrary, the directions at the ends of the ring change quickly, and the cross 

 becomes here much thinner, but is, in return, more definitely bordered. Those same systems of black 

 ' crosses would also arise, if the threads had everywhere directions at right angles to those drawn here, 

 that is to say, if they formed spirals twisted to the left, and situated about tangentially ; but the 

 shiftings of colour, when a gypsum plate is interposed, show this latter possibility to be excluded. To 

 get an idea of the position of the threads in other dimensions, I have tried to get a picture of a 

 Coccolith seen from the edge in polarised light ; but here the impression is highly confused, as several 

 parts of the rings are seen over each other extinguishing the light at different times. We may draw 



the conclusion that the threads are not all arranged in one plane, as 



b C 



the Coccolith is not dark in any position whatever, but is light almost 



to the same degree during the whole turning. Also the marginal plate 

 appears illuminated in all positions, which, however, can only be ex- 

 plained by its receiving a reflection of light from the central ring, as 

 we cannot imagine any position of threads of calcite, by which the 

 marginal plate can appeal thus illuminated, while it is dark in all posi- 

 tions, when viewed from the flat side. 



For the sake of comparison I have also examined some speci- 

 mens of common chalk, and in this are found numerous genuine Cocco- 

 liths, although of a somewhat different structure from the present ones. 



The common chalk I have examined, was taken from a boring that takes place at the Grondal 

 bridge near Copenhagen; the Coccoliths appear here in different depths as oblong, disk-shaped bodies 

 of about the same size as the above mentioned ones, but wanting the marginal plate, so that conse- 

 quently the central ring and the central sphere are much larger than in those. The central ring is 

 comparatively thin, but shows otherwise the same phenomena in polarised light as those we have 

 described. The central sphere, on the contrary, is not single refracting, but consists of a granulary 

 aggregate OI small crystals of calcite extinguishing the light in different directions. If these Cocco- 

 liths have originally had marginal plates, they must accordingly have had the double size of the 

 present ones; but this, I think, cannot be taken for granted. Besides these Coccoliths may also, in 

 the species of chalk I have examined, be found common, globular, spherulitic bodies, which I do not 

 take to be genuine. Coccoliths, though they have earlier been described as such; they are, however, 

 only found in small numbers. If we would draw into the examination other species of calcareous 

 stones from the earlier geological periods , we should no doubt be able to get a clear view of the 

 way in which the Coccoliths in the process of time are transformed; it would, however, lead too far 

 from the object of the present account, and such an examination would, moreover, be rather extensive. 



