071 the Organic Composition of Cluilk and Chalk Marl. :307 



portion of the sand ot the Libyan 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*. 



11. In the variouscountriesof the earth in which occur white 

 and eartliy, as well as coloured and compact rocks, composed 

 of microscopic calcareous animalcules, tiie genera and species 

 of these animalcules present so striking an agreement with 

 those of the white chalk of Riigen, that tliey may well be 

 deemed characteristic of one and the same period of geolo- 

 gical formation. It cannot be asserted for a certainty that 

 the same forms have been observed any where elsef- 



12. In the beds subjacent to and more ancient than tlie 

 chalk, namely, in those of the Oolite or Jura limestone for- 

 mation, 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:}:, though resting on no sufficient foundation, and 

 therefore justly held in slight regard by modern geologists, 

 yet now deserves every attention, since it cleaily 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 lime- 

 stone. If similar phaenomena appear also in the Jura lime- 

 stone 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 con- 

 siderable portion of the solid mass of the earth, to strengthen 

 our natural senses by artificial means, in order to obtain a di- 

 stinct knowledge of the extent to which organic life may have 

 contributed to its production. 



14. The extreme minuteness of the chalk animalcules is stri- 

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

 multitudes of them are still present, and may be applied with- 

 out 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 



* On these very interesting and not easily developed relations, I hope, 

 at a future day, to be able to make a more special communication. 



f If I have apj)lied the same name in some cases both to animalcules of 

 the chalk and to forms existing in the present sea-sand, or in recent fossil 

 beds, it has arisen partly from my being unacquainted with the original forms 

 of the latter, and partly from my desire not to create unnecessary perplex- 

 ity by the adoption of new names. It should be observed that they are di- 

 stinguished by marks of interrogation. All those which I could really com- 

 pare were dijferent. 



X By Linnaeus in 1745 and 1748, and BufTon in 1/49. 



X 2 



