88 ON THE ORGANIC COMPOSITION OF CHALK. 



course of formation by living Bryozoa, which, though very small, resem- 

 bling grains of sand, are yet, for the most part, larger than the chalk 

 animalcules, and a large proportion of the sand of the Lybian Desert 

 has been proved to consist of such grains. It is only in Nubia above 

 Syene that the desert sand becomes a pure detritus of granite. * 



1 1 . In the various countries of the earth in which occur white and 

 earth)', as well as coloured and compact rocks, composed of microscopic 

 calcareous animalcules, the genera and species of these animalcules 

 present so striking an agreement with those of the white chalk of 

 Riigen, that they may well be deemed characteristic of one and the 

 same period of geological formation. It cannot be asserted for a cer- 

 tainty, that the same forms have been observed anywhere else. 



12. In the beds adjacent to, and more ancient than the chalk, 

 namely, in those of the Oolite and Jura limestone formation, we have 

 also clear evidence of the existence of other microscopic Polythalamia. 

 These, however, are such as have not hitherto been found anywhere in 

 the chalk. 



13. The early assertion that all limestone was the produce of 

 animals,f though resting on no sufficient foundation, and therefore 

 justly held in slight regard by modern geologists, yet now deserves 

 every attention, since it clearly appears that a limestone formation 

 widely extended on the surface of the earth is composed of microscopic 

 animals, visibly converted in a gradual manner into inorganic chalk and 

 compact limestone. If similar phenomena appear also in the Jura 

 limestone formation, and should become still further confirmed, these 

 considerations (combined with the long-known existence of coarser 

 corals and shells in both formations), tend to show how necessary it is, 

 when examining the composition of any considerable portion of the 

 solid mass of the earth, to strengthen our natural senses by artificial 

 means, in order to obtain a distinct knowledge of the extent to which 

 organic life may have contributed to its production. 



14. The extreme minuteness of the chalk animalcules is strikingly 

 proved by this, that even in the finest levigated whiting, multitudes 

 of them are still present, and may be applied, without suffering change, 

 to the most varied technical purposes ; thus in the chalk coating 

 given to painted chambers, paper, or even glazed visiting cards (when 

 not coated with white lead alone), may be seen a pretty mosaic of well- 



* On these very interesting and not easily-developed relations, I hope, at a future 

 day, to be able to make a more special communication, 

 t By Linnaeus in 1745 and 1748, and Buffon in 1749. 



