396 Organic Composition of Chalk and Chalk-Marl. 



the inorganic in the Egyptian pebbles {Cailloux d' Egypt c) 

 is throughout more jieifect than it is in many flints, although 

 the constituent elements of both kinds of stone are very pro- 

 bably quite the same. 



On the 2)'>'incij)al Organic Calcareous Forms 'which compose 

 the mass of all Chalk. 



From what has been already stated, it is evident that the 

 production of the calcareous mass of the chalk is not to be 

 attributed, as formerly conceived, to the larger organic bodies, 

 but to the minuter, and in the greatest measure to such as 

 are invisible, consisting of eight genera of Polythalamia with 

 twenty-five species, and excluding all such as may be distin- 

 guished by the naked eye, that is, exceeding ^':pth of a line in 

 magnitude ; the latter, however, are comparatively rare. It 

 is possible that several other, and perhaps many species of the 

 same genera, may yet be discovered in the chalk, as well as 

 other genera, since the investigations hitherto made could 

 only be applied to a minimum of its substance ; yet, as these 

 were conducted by me on chalk from many regions, it does 

 not appear probable that other sections of the animal king- 

 dom will be found to have taken so great a share in the form- 

 ation of chalk as the Polythalamia, the principal prevailing 

 forms of which I have indicated. 



From the preceding it is also apparent that the chalk rocks 

 of all countries agi'ee in their constituent organic forms not 

 only according to the zoological class, but also in genera, and 

 for the most part in species likewise ; this character being not 

 confined to the white tender writing chalk of Europe, but ex- 

 tending also to the compact limestone rocks of the North of 

 Africa and the West of Asia. Particularly striking is the 

 characteristic persistence of single forms through all these 

 different and widely-separated countries. Thus in all of them 

 are to be found Rotalia globidosa, with Textulaj-ia globtdosoy 

 T. aciculata P, and T. striata, as well as Planulina tmgida, 

 thus giving a common character to all these rock formations ; 

 and this character becomes the more important, when we con- 

 sider that these forms are the most numerous, and in fact are 

 the chief constituents of the chalk*. 



If now the question be asked whether the forms which occur 

 in such masses in chalk belong to it exclusively, and are hence 

 to be considered characteristic of that formation, I am almost 

 disposed to reply in the affirmative. The analogous forms 



* The Polythalamian forms which Mr. Lonsdale noticed in the English 

 chalk in 1837 as visible to the naked eye, and amounting to 1000 in one 

 pound of the chalk, and which, with Mr. LycU, he has named Lentkulina 



